ROLE OF SOILS AND SEDIMENT IN WATER
POLLUTION CONTROL
PART 1
REACTIONS OF NITROGENOUS AND PHOSPHATIC COMPOUNDS
WITH SOILS AND GEOLOGIC STRATA
MARCH 1968
U.S. DEPARTMENT OF THE INTERIOR
FEDERAL WATER POLLUTION CONTROL ADMINISTRATION
SOUTHEAST WATER LABORATORY
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ROLE OF SOILS AND SEDIMENT IN WATER
POLLUTION CONTROL
PART 1
REACTIONS OF NITROGENOUS AND PHOSPHATIC COMPOUNDS
WITH SOILS AND GEOLOGIC STRATA
MARCH 1968
U.S. DEPARTMENT OF THE INTERIOR
FEDERAL WATER POLLUTION CONTROL ADMINISTRATION
SOUTHEAST WATER LABORATORY
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FOREWORD
Nitrogen and phosphorus pollutants are causing great concern in the area of water pollu-
tion control. Levels of nitrogen and phosphorus-bearing compounds in waste effluents are
often high, and the effluent, when introduced directly into watercourses, lowers the water
quality. A waste treatment method is needed to reduce nitrogenous andphosphatic compounds
in these effluents to innocuous levels, prior to introduction into watercourses.
Soil and geological strata could serve as such a waste treatment and disposal system.
The soil and underlying geological strata may be thought of as a three dimensional, aniso-
tropic, chromatographic column capable of adsorbing and removing solutes from a percolat-
ing solution. If the interactions between different nitrogen and phosphorus forms in the efflu-
ent and soils and geological strata are known, it is possible to predict the chromatographic
behavior of the “soil waste treatment system.” Using such information, it would be possible
to set-forth the criteria for selecting sites of “soil waste treatment systems”; similarly, it
would be possible to indicate how those soils, lacking the necessary critical properties, can
be modified for use as “soil waste treatment systems.”
Reported herein is a literature review relating to: (1) the reactions, nature, fate, and
behavior of nitrogenous and phosphatic compounds in soil and (2) the quantitative and qualita-
tive aspects of nitrogenous and phosphatic compounds in raw and treated sewage.
A synopsis of the nature and properties of soils and a glossary of soil science terms
are included, so that the reader can better understand and comprehend the subject matter
and gain a better appreciation of the impact of soils and sediments on water pollution control.
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ABSTRACT
Literature on the reaction, nature, fate, and behavior of nitrogenous and phosphatic
compounds in soils and geologic strata have been reviewed in relation to the use of soil and
geologic strata as a media for municipal and industrial disposal. The report consists of
four sections:
Nature and properties of soils . Principles of soil science are discussed in terms of
soil forming processes; the physico-chemical, mineralogical, and biological properties;
and their interrelationships.
Chemical character of sewage . The quantitative and qualitative aspects of nitrogen and
phosphorus in both raw sewage and treated effluent are indicated.
Behavior of nitrogen in soils . The mobility of inorganic and organic nitrogen in soils
is discussed, as well as the reaction mechanisms by which inorganic forms can be removed
from percolating solutions and fixed; the factors affecting fixation, subsequent release, and
degradation are also discussed.
Behavior of phosphorus in soils . The immobile nature of phosphorus in soils compared
to nitrogen and the generally high phosphorus fixation power of soil are discussed; seven
factors affecting the fixation of phosphorus by soils are discussed. The fixation mechanisms
for both inorganic and organic forms of phosphorus are treated in detail. A glossary of soil
science terms is included. (KEY WORDS: waste treatment and disposal system; fixation
mechanisms; adsorption mechanisms; movement in soils; sewage, N & P levels; soil pro-
perties; advanced waste treatment; ammonium; nitrate; phosphate.)
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TABLE OF CONTENTS
Introduction 1
Nature and Properties of Soils 2
Soil Formation 2
Mineralogical Character of Soils 5
1:1 Type Clay Minerals (Kaolin Group) 5
2:1 Type Clay Minerals 8
Montmorillonite Group 8
Mica Group 11
2:2 Type Clay Minerals (Chlorite Group) 11
Chain Structures 12
Physico-Chemical Properties of Soil Constituents 14
Surface Chemical Phenomena 15
Ion Exchange 16
Cation Exchange (Cationic Adsorption) 16
Anion Exchange (Anionic Adsorption) 19
Electrokinetic Properties 19
Nature of Clay-Water Systems 20
Physical Properties of Soils 21
Particle-Size Distribution 21
Particle Density 22
Bulk Density 22
Pore Space 22
Structure 22
Plasticity 24
Shrinkage and Cohesion 24
Swelling 24
Flocculation 26
Biological Properties 26
The Chemical Character of Sewage 29
Behavior of Nitrogen in Soils 31
Movement of Nitrogenous Compounds in Soils 32
Inorganic Nitrogen Compounds 32
Organic Nitrogen Compounds 34
Reactions of Various Forms of Nitrogen in Soils 35
Ammonia
Ammonium 37
Urea 40
Behavior of Phosphorus in Soils 41
Movement of Phosphorus Compounds in Soils 41
Factors Affecting Phosphorus Fixation by Soils 43
Soil-Type Effects 43
Particle-Size Effects 44
pH Effect 45
Effect of Reducing Conditions 46
Effect of Temperature 46
Effect of Organic Matter 46
Effect of Phosphorus Concentration and
Time of Reaction on Fixation 46
1
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TABLE OF CONTENTS (Cont)
Reactions of Phosphorus Compounds in Soils 47
Inorganic Phosphorus Fixation Mechanisms 47
Adsorption 47
Isomorphic Rep]acement 49
Double Decomposition Reaction Involving
Solubility Relations 51
Organic Phosphorus Fixation Mechanisms 54
Bibliography
Acknowledgments
11
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LIST OF FIGURES
Figure No . Title Page No .
1 Schematic Representation of the Concept of Soil Formation 3
2 Diagram of a Hypothetical Soil Profile Having all the 4
Principal Horizons.
3(A) Diagrammatic sketch showing (a) single octahedral unit and 6
(b) the sheet structure of the octahedral units.
3(B) Diagrammatic sketch showing (a) single silica tetrahedron and 6
(b) the sheet structure of silica tetrahedrons arranged in an
hexagonal network.
4 Diagrammatic Sketch of the Structure of Kaolinite 7
5 Diagrammatic Sketch of the Structure of Hydrated Halloysite 8
(Endellite).
6 Diagrammatic Sketch of the Structure of Montmorillonite 11
7 Diagrammatic Sketch of the Structure of Vermiculite 12
8 Diagrammatic Sketch of the Structure of Muscovite 14
9 Diagrammatic Sketch of the Structure of Chlorite 15
10 Schematic Presentation of the Structure of Attapulgite 16
11 Particle-Size Effect on the Relative Magnitude of Selected 23
Soil Properties
12 The Effect of Soil Texture on the Physico-Chemical 27
Properties of Soil
13 Form of Phosphorus Fixation as a Function of Soil pH. 45
111
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LIST OF TABLES
Table No. Title Page No .
1 Classification of Clays 7
2 Approximate Chemical Formulas for Various Clay 10
Minerals
3 Major Characteristics of Main Groups of Crystalline 13
Clay Minerals.
4 Methods of Determining Electrokinetic Potential 20
5 Water of Plasticity (in per cent by weight) 25
6 Free-Swelling Data for Various Clay Minerals 25
(in percent)
7 Selected Physical Properties of Certain Soil 29
Constituents
8 The Phosphorus Adsorbing Power of Three New Jersey 42
Soils, Limed and Unlimed, Expressed as Pounds of
20 Percent Super Phosphate per Acre Furrow Slice.
iv
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INTRODUCTION
The population explosion and intense industrialization place a severe demand on society
to treat and dispose of its water-borne wastes. Water and waterways can no longer be used
for the disposal of untreated wastes. Research is being carried out on advanced methods
of waste treatment including various physical- chemical techniques as (1) adsorption (activated
carbon), (2) electrodialysis, (3) evaporation, (4) freezing, (5) foaming, (6) oxidation, (7) re-
verse osmosis, (8) flocculation, (9) solvent extraction, and (10) total ion exchange.
Advanced waste treatment methods and other currently used processes generally pro-
duce a waste effluent containing low levels of contaminants. However, the concentrations of
these contaminants, when discharged into water courses, may still be sufficient to impair
water quality, especially by promoting the growth of algae. In this respect, nitrogen and
phosphorus have important effects, being nutrients used by algae.
One way to alleviate this problem is to pass the waste through soil and underlying geo-
logical strata to reduce the nitrogen and phosphorus contaminants to innocuous levels and
to re-introduce the “treated” effluent into the hydrologic cycle.
For the purpose at hand, soil and geological strata may be considered as three-
dimensional, anisotropic, chromatographic columns having the capability to selectively re-
move solutes from a percolating solution.
A vast amount of research (mainly agricultural) has been done on the identification and
characterization of nitrogen and phosphorus reactions in soils. There is a complete dichot-
omy of purpose in the application of this research information. For agronornic purposes,
the research information is used to maximize the nitrogen and phosphorus availability for
plant nutrition. While, in the case at hand, the need is to maximize the unavailability of
the various forms of nitrogen and phosphorus and to prevent their re-entry into water
courses.
The purpose of this discourse is to determine mainly from research in soil science:
(1) the extent to which nitrogen- and phosphorus-bearing compounds are retained by soils:
1
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(2) whether this retention is independent of the chemical nature or form of the nitrogen and
phosphorus compounds; (3) if dependent, what chemical forms are most effectively removed
from percolating solutions; and (4) what form, if present in the waste effluent, would pass
through the “soil waste treatment column” into ground water and eventually into water
courses. If certain chemical forms are more effectively removed or retained, conceivably
one or more steps in sewage waste treatment might be designed to concentrate the consti-
tutents which can be effectively removed by soil and to minimize or eliminate undesirable
chemical forms.
To aid the thscussion of the reactions and interactions of nitrogen and phosphorus in
soils, the first section is concerned with the formation, nature, and properties of the soil
system and soil constituents. To aid in a better understanding of this material, a glossary
of terms commonly used in soil science is included.
Most of the emphasis in the following sections is on soils. But, it should be realized
that the mineral constitutents in soils are also found in sedimentary, metamorphic, and
igneous rocks. Soils and strata will exhibit similar physical-chemical properties with re-
spect to removing solutes from percolating solutions. The major difference in their ability
to remove nitrogen- and phosphorus-bearing compounds is not due to difference in proper-
ties, but rather to differences in intensity and capacity factors of chemical reactions and
the permeability of the substrate.
NATURE AND PROPERTIES OF SOILS
Soil Formation
A schematic representation of the sequence of soil formation can be seen in Figure 1.
The country rock is broken up by physical and chemical weathering processes (Reiche,
1945). The physical and chemical characteristics of the original country rock are modi-
fied or destroyed leaving a residue. The residue or regolith is formed as indicated, by the
weathering of the country rock in or through transportation and deposition by water,
wind, or ice. From the upper portion of this heterogenous mass, a soil is developed.
As weathering proceeds, the soluble constituents in the regolith are leached by the
drainage water, leaving secondary and detrital minerals as products (discussed in the
following section). In addition, living organisms prosper, organic matter accumulates, and
weathering shifts from strictly physical- chemical to biochemical and biocolloidal weathering.
2
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Upper Layer —4 Parent Material Soil
Country Rock Regolith
Physical and chemical weathering Biochemical weathering
(Role of organic matter in soil (Role of organic matter - additions and
formation is negligible) decay dominant)
Figure 1. Schematic Representation of the Concept of Soil Formation
As microorganisms and higher plants prosper on the upper surface of the regolith,
organic matter accumulates on and in the surface layer. At this point actual soil formation
starts: a differential layering appears which can be ascribed largely to the action of living
organisms. In fact, a “true” soil in the agricultural sense cannot be formed without the
presence and decay of organic matter.
As weathering proceeds, horizontal layering (horizons) develops within the soil, and a
“profile” which is characteristic and unique is developed (Figure 2). The “A” horizon is
characterized by maximum leaching. The “B” horizon contains the maximum deposition and
accumulation of clay from above. The A and B horizons together are called the solurn.
The C horizon or parent material has not been subjected appreciably to soil formation
(Figure 2).
Forces of weathering represent a number of factors which largely control the kind of
soil that finally develops. These factors are termed soil-forming factors and are:
1. Climate (particularly temperature and precipitation)
2. Living organisms (especially native vegetation)
3. Parent material (texture and structure, chemical and mineralogical composition)
4. Topography and relief
5. Time
Of these five factors, climate is the dominant one in soil formation because of its in-
fluence on the rate of physical and chemical weathering and its interactions with vegetation.
Vegetative cover native to a region modifies climatic influence. Topography, through its
influence on drainage, hastens or delays the climatic forces.
3
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The 01 and 02 horizons are composed of organic matter, and rest on
the A 1 horizon of unplowed soils. They are found on most unburned
soils under forest vegetation and usually are absent from soils devel-
oped under grasses.
TheA horizon is the zone of maximum biological activity in the mineral
portion of the soil. In dry regions the A horizon isa zone In which
organic matter accumulates. The same is true in humid regions where,
in addition, it frequently has a clay Content lower than that of the B
horizon as a result of weathering and trsnsportation of clay by water.
The B horizon includes the lower layers that have been influenced by
the factors of toil formation. In soils developing on fresh geological
formations no B horizon may be discernible. The same may be true
occasionaity in mature soils found in dry regions. Usually the B horizon
is differentiated from the A horizon above and the C horizon below by
color. In addition, there may be differences in structure, in content of
c(ey, iron, aluminum, and organic matter, or a combination of all. In
dry regions the B horizon frequently is distinguished from the A hori-
zon by structure; the B horizon has a blocky or prismatic structure that
is not shared by the A and C horizons. Also the organic matter content
is lower in the B then in the A horizon, In cool, humid regions the B
horizon is a zone of accumuietion of clay, sesquioxldes, and organic
matter moved downward in varying proportions from the A horizon.
In warm, humid regions the B horizon often Is relatively thick and high
in content of clay and sesquioxidea, showing but little evidence of accu-
mulation from the A horizon.
The C horizon is a layer of unconsolidated material that is affected
relatively little by the action of organisms. The C horiton is presumed
to represent the parent material, i.e., the type of material from which
at least part of the overlying horizons have developed.
The El layer is any stratum that is essentially unaffected by organisms
acting in the overlying horizons and at the same time, is different from
she material from which the overlying layers were formed. The El hori-
zon is not a part of the soil profile, but it may be of significance to the
overlying soil, as when the B horizon consists of consolidated rock.
The O horitons consist of the original form of the vegetative matter
(visible to the naked eyel corresponds to the L (litter) and some
F (fermentation) layers in forest toil designations, and to the
horizon formerly called Aoo.
The horizons consist of organic matter (both plant and animal mat-
ter) which cannot be recognized with the naked eye. The 02
corresponds to the H (humus) end some F (fermentation) layers
in forest soil designations and to the horizons formerly called Ao.
The A 1 horizon is dark colored, and has a relatively high content of
organic matter mixed with the mineral matter. Usually it is rela-
tively thin in soils developed under forest vegetation and thick in
soils developed under grasses. Availability of plant nutrients is
relatively high as a result of the surface deposition of plant resi-
dues and the activities of the soil population.
The A 2 horizon is lighter in color than the A 1 horizon and usually is
lighter in color than the underlying horizon. It is the region of
maximum low of clay, iron, aluminum, or all three. This hori-
zon usually is absent from or weakly developed in soils of dry
regions, but may be strongly developed in soils of humid regions,
particularly those occurring in cool climates under forest vegeta-
tion.
The A and B horizons merge through the transitional A 3 and B 1 hori-
zons. If the B horizon is absent, the A 3 is transitional to the C
horizon.
The B 2 horizon is the layer in which the maximum “B” properties are
found.
The B 3 horizon is transitional to the C horizon.
The C 5 and Ccs horizons are layers of accumulated calcium carbonate
and calcium sulfate found characteristically in some soils ot dry
regions. Commonly the C c 5 horizon is below the Cca horizon.
Sometimes these accumulations are found in horizons other than
the C, in which case the symbol is changed from C to B or A, as
the case may b , and the subscripts remain the same,
The G horizon isa layer of intense reduction, It is characterized by the
presence of ferrous iron and by a gray to olive color that com-
monly changes to brown on exposure to air.
Figure 2. Diagram of a Hypothetical Soil Profile Having all the Principal Horizons.
(After Black, 1957, and using new Nomenclature From Glossary of
Soil Science Terms)
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The influence of parent material is manifested through an effect on drainage and on the
chemical and mineralogical composition, and it may control, to some degree, the natural
vegetation present.
Soils are grouped on the basis of their characteristics (see glossary). Zonal soils are
determined primarily by the climate in which the development occurs. Intrazonal soils are
those which reflect the influence of local conditions such as poor drainage, alkali salts, or
some local parameter. Azonal soils are those without profile characteristics.
Mineralogical Character of Soils
Minerals in soils can be classified into two groups: (1) Primary minerals (inherited
from the country rock or regolith and persisting more or less unchanged) and (2) Secondary
minerals (formed by low-temperature reactions and inherited from the regolith or formed
in soils by weathering).
Primary minerals in soils include quartz, feldspars, micas, amphiboles, pyroxenes,
olivine, carbonates, and sulfur-bearing minerals (gypsum and pyrite). Secondary minerals
include various clay minerals (kaolinite, illite, vermiculite, montmorillonite, and chlorite),
iron oxides (hematite, limonite, and magnetite) and the titanium oxides (rutile and anatase).
The clay minerals are layer-lattice alumino- silicates; the layers are made up of tetra-
hedral sheets of silicon oxide and octahedral sheets of aluminum oxide and hydroxide.
The terms, tetrahedral and octahedral, mean that the sheets are made up of four-sided and
eight-sided molecular building blocks, respectively. The tetrahedral sheet (Figure 3) is
composed of individual tetrahedra in which the cation silicon (and, in certain cases, alumi-
num) is surrounded by four oxygen anions; the octahedral sheet is composed of octahedra
where aluminum (in certain cases magnesium, manganese and other cations of similar size)
is surrounded by six oxygen and/or hydroxyl anions.
There are four general structural types among the clay minerals: three layer-lattice
structures (1:1 type, 2:1 type, and 2:2 type) and one with a chain structure (Table 1).
1:1 Type Clay Minerals (Kaolin group )
These minerals are hydrous alumino-silicates. Kaolinite (most common member of
this group) consists of a single silica tetrahedral sheet and a single alumina octahedral
sheet; the two are tied together into a common sheet by shared oxygen to form the kaolin
unit layer (see Figure 4). The unit layers are stacked and this constant periodicity makes
up a clay crystal.
5
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Figure 3. - Diagrammatic sketch showing (a) single octahedral unit and
(b) the sheet structure of the octahedral units.
Diagrammatic sketch showing (a) single silica tetrahedron
and (b) the sheet structure of silica tetrahedrons arranged
in an hexagonal network. *(Fronl APPLIED CLAY MINERALOGY
by Ralph E. Grim. Copyright (c) 1962, McGraw-Hill, Inc. Used
by permission of McGraw-Hill Book Company).
1’
and = Oxygens 0 and • = Silicons
Figure 3. * -
(a) (b)
and ( = Hydroxyls • Aluminums, magnesiums, etc.
(a)
(b)
6
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Table 1. Classification of Clays
A. Two-layer clays
1. Equidimensional, kaolin
2. Elongate, halloysite group
B. Three-layer types
1. Expanding lattice (along the c axis)
a. Equidimensional
(1) Montrnorillonite group, full expansion
(2) Vermiculite, limited expansion
b. Elongate
(1) Nontronite, saponite, hectorite
2. Nonexpanding lattice
a. Illite group
C. Regular mixed-layer types (four-layer clays)
1. Chlorite group
D. Chain structure types
1. Attapulgite, sepiolite
(After Grim, 1953)
6 OH
Hydrogen bond
C OX S 714A
Ooxygens 40+2(OH)
o Hydroxyts 4 Al
• Aluminums
• 0 Silicons 6 OH
Figure 4. Diagrammatic Sketch of the Structure of Kaolinite
(After Grim, 1962)
b axis
7
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Halloysite (Figure 5) is an elongate mineral that has two forms. One form is the
same as kaolinite and the other contains an added 2 H 2 0.
H ydrogert
I I I bonding
0 0 0 0
Hydrogen
O.25 A I bonding
O. 0. Q 0. O 6
c-AXES 4 0+2 (OH)
b-AXIS —
Figure 5. Diagrammatic Sketch of the Structure of Hydrated Halloysite (Endellite)
(From Firman E. Bear: CHEMISTRY OF THE SOIL, New York,
Reinhold Publishing Corporation, 1964)
2:1 Type Clay Minerals
Montmorillonite group . The mineral, montmorillonite, is composed of units consisting
of two silica tetrahedral sheets with a central octahedral sheet (see Figure 6). The bonds
between the units are weak, and water and other polar molecules can penetrate between these
units and cause expansion (see Figure 6); thus, the “montmorillonites” are called the “ex-
pansible” (or swelling) clay minerals and are strong adsorbents. This expansion is perpen-
dicular to the plane of the silicate sheets. The charge deficiency, due to isomorphic sub-
÷3
stitution (i.e., Al for Si in the tetrahedral layer) is balanced on the outside of the
crystals by such inorganic cations as sodium, calcium, magnesium, and other cations.
Approximate formulas for members of the montmorillonite group are shown in Table 2.
Vermiculite (Figure 7), also an “expansi ble” clay mineral, consists of layers made up
of two silica tetrahedral sheets with a central magnesium-iron octahedral sheet separated
by two layers of water molecules (Table 2). There is less randomness in stacking of the
layers in vermiculite than in montmorillonite (Grim, 1953).
8
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PAGE NOT
AVAILABLE
DIGITALLY
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A
60
4 Si
4 0+2(OH)
4 Al
14A
A (9.6- 130A
Or More 40+2(OH)
4 Si
coxis 60
o Oxygens l.4ydroxyis • Aluminum,iron. magnesium
O ondSSiiicon,occasionoily aluminum
Figure 6. Diagrammatic Sketch of the Structure of Montmorillonite (After Grim, 1962)
Mica Group . Illite is the general term (Grim et al., 1937) for mica-like clay minerals.
The basic structural unit is a layer composed of two silica tetrahedral sheets and a central
octahedral sheet. This unit is similar to that of montmorillonite except that the charge
deficiency is balanced by potassium ions which act as a bridge between the unit layers bind-
ing them together and preventing expansion or swelling (Figure 8). Illite differs from the
primary mineral muscovite in the following ways: (1) less substitution of aluminum for
silicon in the tetrahedral layer; (2) potassium ions which normally bind the unit layers to-
gether in the case of muscovite may be partially replaced by other cations (e.g., hydrogen);
(3) contains more structural water; (4) some randomness in the stacking of the silicate
layer; and (5) particle size is generally smaller.
2:2 Type Clay Minerals (Chlorite group )
True chlorite consists of alternate mica layers (two silica tetrahedral sheets and one
octahedral sheet) and brucite layers (such layers may be either magnesium, aluminum, or
iron hydroxides; or a combination of all three) as shown in Figure 9.
Exchangeable Cations
n H 2 0
11
-------
14 A,
( 10—154
or more)
c-AXIS
Ii
n-3 HO
Mg
(055Mg x 1 1 )
3 H 2 0
60
Al + 3 S
40+2 tOf-I
6 ç l.F ,Fe.Mq)
Tn :?ahedr 1
40+2 (OH)
Al • 3 S
60
Figure 7. Diagrammatic Sketch of the Structure of Vermiculite
(From Firman E. Bear: CHEMISTRY OF THE SOIL,
New York, Reinhold Publishing Corporation, 1964)
Vermiculite-chlorite series: Structurally, this type of mineral has a unit layer of two
silica tetrahedral sheets and an octahedral sheet and, in addition, in the interlamellar
position hydroxy-aluminum ions or polymers of yet undefined composition.
Chain Structures
In general, these minerals consist of a double silica chain linked together by octa-
hedral groups of oxygens and hydroxyls containing magnesium and aluminum. The linked
chains form a kind of double-ribbed sheet. The ribbed sheets are held together by alumi-
num and/or magnesium ions in octahedral coordination between the apices (oxygens) of
successive sheets (Figure 10). Chains of water molecules run parallel to the c axis and
fill the interstices between the amphibole chains (Grim, 1953).
The characteristics of the main groups of crystalline clay minerals are summarized
in Table 3.
QOD
0
b-AXIS
12
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Table 3. Major Characteristics of Main Groups of Crystalline Clay Minerals
Two-layer Clays
1:1
Three-layer
2:1
Clays
Expanding lattice
Non- expanding lattice
Group members
Equidimensional:
illite, biotite
muscovite
Equidimensional:
kaolinite, dickite,
nacrite
montmorillonite,
sauconite, beidellite
vermiculite
Elongate:
halloysite
Elongate:
nontronite, saponite,
hectorite.
Structure
Rigid lattice, unit
cell composed of one
silica tetrahedra
layer for each
alumina octahedra
layer
Rigid lattice, Unit
cell composed of 1
silica tetrahedra
layer, 1 alumina
octahedra layer, 1
silica tetrahedra
layer. K bonds
plates together
Lattice expanding and
contracting with water
content or dielectric.
Unit cell composed of
silica tetrahedra
layer, 1 alumina octa-
hedra layer, 1 silica
tetrahedra layer.
1
Swelling and
shrinking
Very little
Little
Much
Basic empirical
formula
2Si0 2 A1 2 0 3 2H 2 0
4Si0 2 A1 2 0 3 nH 2 O nK
4Si0 2 -A1 2 0 3 -nH 2 O
Si0 2 R 2 0 3
1.95 - 2.02
2.16 - 3.20
4.18 - 6.00
Ionic (isomorphic)
None
Al for Si
Al for Si
substitution in
Mg, Fe, Mn, Ti for Al
lattice
Absorptive capacity
Small
Intermediate
Large
for inorganic and
organic ions, water,
gases, vapors.
Negative charge on
None
2(muscovite.
1.4 - 0.9 Verm.
layer per Unit cell
paragonte, biotite)
1.2 or more illites
Mont. 1.0 - 0.6
Cation exchange
3-15 me 100g.
10-40 me 100 g.
80-150 me 100 g.
capacity (charge)
(kaolinite &
halloysite 2H 2 0)
halloysite 4H 2 0
10-40 me 100 g.
Stabilization
Low
Intermediate
Great
aqueous suspension
Heat of wetting
1 - 2 cal. g.
Around 4 cal. g.
10-20 cal ./g.
Specific Surface
7 - 30 m g.
65 - 100 m. 2 , g.
600 - 800 m. 2 g.
Basal spacing, re-
peat thickness with
7.2 .
10 X.
Montmorillonite 17 X.
Vermiculite 14
ethylene glycol
(After Kohnke, 1963)
13
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*
1K
A 60
3Si + Al
40 +2(OH)
caxls lO.OA 4A 1
40 +2(OH)
3 Si+ lAP
60
1K
o Oxygens. 0 Hydroxyls.SAluminum, Potasslum
Oand • Silicons (one fourth replaCed by aluminums)
Figure 8. Diagrammatic Sketch of the Structure of Muscovite
(After Grim, 1962)
Physico- chemical Properties of Soil Constituents
The properties of soils are due principally to the properties of the colloidal (inorganic
and organic) fraction. In essence, the properties of colloids depend on their surface proper -
ties. In the inorganic colloidal fraction, the clay minerals plus the amorphous and crystal-
line oxides and hydroxides are the principal components that determine the properties of
soil systems. In the organic fraction, the humus constitutes the most important colloidal
constituent.
For clarification, the two major classes of hydrous colloids are hydrophobic and hydro-
philic colloids. In the past, a hydrophobic colloid was defined as a colloid which exhibited
an aversion to water; a hydrophilic colloid displays an affinity for water. However, now
hydrophilic colloids are called macromolecular colloids or polyelectrolyte solutions. Their
colloidal properties are due to a liquid dispersion of large-size and small-size molecules;
these colloids are considered as two-phase systems characterized by large interfacial
areas.
b axis
14
-------
c axis
4
144
4— — - b ads
00* ,;er HydroXytS • Magrieslurn,lron
oond • Silicon,occosloflally aluminum
• Magresium, some replacement by aluminium and iron
60
3Si + 141
40 + 2(OH)
6(4+3. Fe +3 Mg 2 )
Tn octahedral
40 +2(OH)
3 Si + I Al
60
6 OH
6(41 + 3 Fe+ 3 Fe+ 2
Mg+ 2 )
6 OH
Figure 9. Diagrammatic Sketch of the Structure of Chlorite
(After Grim, 1962)
Practical differences between the two colloids are that hydrophobic colloids or sols are
very sensitive to the addition of salts, while macromolecular sols are insensitive toward
salts, and in the true sense of the word do not flocculate as do hydrophobic sols.
Surface Chemical Phenomena
Two major phenomena, adsorption and the electrokinetic behavior, influence the surface
properties of the colloids. The reader is referred to the article by Bailey and White (1964)
for a more detailed explanation of the phenomena of adsorption and for a discussion of
those factors affecting the adsorption of organic materials by soils and soil constituents.
Colloidal materials act as adsorbent for gases, liquids, and suspended solids in liquids.
In general. there are two forms of adsorption: ion exchange and molecular adsorption.
Molecular adsorption includes adsorption of polar organic molecules and adsorption of water
(or wastewaters) by clay minerals.
I \ > •/ \ -‘A
/ h
15
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o H 2 0 () Hydroxyl 0 Mg or Al
0 OH 2 (J Oxygen I Silicon
Figure 10. Schematic Presentation of the Structure of Attapulgite
(From APPLIED CLAY MINERALOGY by Ralph E. Grim.
Copyright (c) 1962, McGraw-Hill, Inc. Used by Permission
of McGraw-Hill Book Company)
Additional information on the nature of complexes between clay minerals and organic
molecules is included in works by Bradley (1945), MacEwan (1946), Grim (1953), Mac Ewan
(1960). and Eitel (1964) and in the section on swelling properties of clays and soil systems.
Ion Exchange . Ion exchange is probably the most important property of the soil col-
loidal system because the ions affect most of the other properties of the colloidal system .
CLay minerals and other colloidal materials can undergo two types of ion exchange--cationic
and anionic adsorption or exchange.
Cation Exchange (Cationic Adsorption) . Cation exchange capacities of colloids are
due principally to three causes (Grim, 1953, 1962):
÷3 .±4.
(1) Isomorphic substitution in the lattice (Al for Si in the tetrahedral layer and
÷3 .
Mg for Al in the octahedral sheet). Eighty percent of the cation exchange
capacity of the clay minerals, montmorillonite and vermiculite, is due to this cause.
b= 18.OA
16
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(2) Broken bonds around the edges of the silica-alumina units. Particle size affects
the number of broken bonds, and cation exchange capacity increases with decreas-
ing particle size. Broken bonds are the major factor responsible for the exchange
capacity of the kaolin group and are important for the illite, chlorite , and attapulgite-
sepiolite groups. For the clay minerals, montrnorillonite and vermiculite, broken
bonds contribute approximately 20% (thus a relatively small proportion) of the cation
exchange capacity.
(3) The hydrogen of exposed hydroxyls, around the edges of the unit layers, may be
exchanged by a cation (M).
O OH O 9M
OH’ NOH OH 7 OH
The position and proximity of exchangeable cations on the mineral surface vary among
mineral types and ions. In clay minerals whose exchange capacity results from broken
bonds, the exchangeable cations are held around the edges of the crystals, [ (although the
work of Weiss (1964) on kaolinite raises some question about this generally accepted fact .
In those minerals whose exchange capacity results mainly from isomorphic substitution, the
cations are held mainly on the interlamellar surfaces. In any system, the positions of all
the exchangeable cations with respect to the silicate surface is not the same, and even the
relative positions of all the same sort of cations are not the same; certain cations are in
closer proximity to the clay surface than the others will be. Marshall (1964), indicates
that the percent ionization of the cation varies with the particular clay mineral, the amount
of water, the nature of the cation (valency and ionic size), and the relative concentration
of the cations. Different cations are held to a given clay mineral with different bonding
energies, the magnitudes of which depend in part on the position of the adsorbed cation on
the clay mineral surface.
The rate of cation exchange varies with the clay mineral type, the concentration of the
cations, and the nature and concentration of anions. Reaction for kaolinite is essentially
instantaneous; the rate is much slower for rnontmorillonite and attapulgite and even much
slower for illite. One reason for the slower rate of reaction is that, in the case of three-
layer clay minerals, diffusion into the interlamellar spaces is the rate-limiting step, while
in the case of kaolinite the exchange occurs at the edges.
17
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There is no universal lyotropic replaceability series. The series varies depending on
the experimental conditions, cations involved, and the nature of the clay mineral. Cation
replaceability is of vital importance, and it influences to a great extent the macro- physical
properties of the system.
Five factors have been found (Grim, 1953) to be important in the replaceability of
cations.
(1) Effect of cation concentration--increased concentration of the replacing cation causes
greater exchange by that cation. This would be expected since cation exchange
is a stoichiometric reaction and the law of mass action would be expected to hold
true.
(2) Population of exchange positions- - replaceability depends in part on the nature of
the complementary ions populating the remainder of the exchange sites and also
on the degree to which the replaced ions saturate the exchange spots.
(3) Nature of the anion in the replacing solutions affects replaceability of the cation.
(4) The nature of the ion--the valence, ionic size, and the polarizability of the ion
influence the ease by which ions are replaced. In general, the higher the valence
of the ion, the greater its replacing power, and the more difficult it is to displace
when present on the clay.
(5) Nature of clay mineral- - a separate replaceability series exists for the various
types of clay minerals.
The type and amount of clay minerals present, the nature of the ion present (ionic
size, valence, etc.), and the population properties of the ions on the exchange sites de-
termine and govern (the properties of clays and thus) the properties of soils.
Since the kinds and amounts of clay minerals and the quantity and nature of organic
matter vary among soils, the cation exchange capacity varies among soils. A rough cor-
relation exists between the cation exchange capacity, texture, and organic matter content.
As the soil texture becomes finer (sands — barns clay loams),the exchange capacity
increases. Finer textured soils contain more clay and generally more organic matter than
coarser- textured soils.
18
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Knowledge of the exchange capacity of a soil is not sufficient to predict the properties
of that soil; in addition, the percent base saturation of the colloidal complex of the soil
must be known. Thus, the amount of exchangeable base (Ca, Mg, K, etc.) and exchangeable
acidity must be known. A correlation exists between percentage base saturation and pH
of a soil. As the percent base saturation is reduced, the pH is reduced in a similar pro-
portion. As a rule of thumb for soils with a pH range of 5 to 6 in humid-temperate
regions, a 5% change in base saturation occurs for each 0.10 change in pH.
Anion Exchange (Anionic Adsorption) . Relatively little is known about the anion
exchange by soil clays. Most of the information has been on the interaction of the phos-
phate anion with soils and soil constituents. Other important anions are sulfate, chloride,
fluoride, molybdate, and silicate. Data indicate that an average ratio of cation- to anion-
exchange capacity is about 0.5 for kaolinite, 2.3 for illite, and 6.7 for montmorillonite
(Grim, 1962).
Electrokinetic Properties . The electrokinetic properties of a mineral arise from:
(1) the release of ions from the surface of a solid particle into a liquid; (2) the adsorp-
tion of ions from a liquid: (3) the adsorption of polar molecules on the surface of the
particle; and (4) substitution of atoms of differing valence in the lattice.
In the case of clay-water systems, property (4) is the predominant factor affecting
charge density on the silicate surface. This charge density at the surface produces a
difference in electrical potential between the surface and the bulk solution. To maintain
electrical neutrality of the system, the surface charge is balanced by an exactly opposite
charge in the liquid phase. These adsorbed ions form a diffuse cloud at a finite and vari-
able distance from the solid interface. These ions in the outer layer participate in exchange
reactions and are called counter-ions.
Together, the adsorbed or fixed layer on the solid surface and the outer layer of ions
in the liquid phase are known as the electrical double layer. Detailed information on the
electrical double layer is available in works by Maclnnes (1939), Verwey and Overbeek
(1948), van Olphen (1963), Marshall (1964), and Sennett and Oliver (1965).
The four general methods of determining the electrokinetic potential of a colloidal sys-
tem are: electro- osmosis, electropho resis, sedimentation potential, and streaming potential.
The property to be measured and the nature of the mobile phase are given in Table 4.
Using the proper assumptions, it is possible to calculate (using one of the four electro-
kinetic phenomena) the electrokinetic potential or zeta potential. Zeta potential (van Olphen,
1963) is the electric potential in the double layer at the interface between a particle which
moves in an electric field and the surrounding liquid; the more negative the zeta potential
19
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the greater the degree of repulsion or, in other words, the more disperse is the system.
It should be realized that the zeta potential is not the true potential at the surface of the
particle, but is the potential at the shear plane between the bulk liquid and the envelope of
water which moves with the particle and therefore represents the electrical potential at an
unknown distance from the surface in the double layer.
The double layer theory has been used to explain the stability of colloidal dispersions
(dispersive or flocculated in nature) and nature of clay-water systems.
Nature of Clay-Water Systems
Since the nature of the low-temperature water (water held by clays up to a temperature
of 100_1500 C) and the factors that control its character, in essence, determine such physi-
cal properties as plasticity, cohesion, bonding, compaction, suspension, and shear strength,
a brief statement concerning the nature of this system is relevant to the discussion at hand.
For an extended treatment and/or review the reader is referred to such works as Grim
(1953, 1962); Low (1961); and Eitel (1964).
Table 4. Methods of Determining Electrokinetic Potential
Name of Procedure Property Measured Mobile Phase
Electro-osmosis Velocity Liquid
pressure None
Electrophoresis Particle Particle
mobility
Sedimentation Potential Potential Liquid
Streaming Potential
(After Sennett and Oliver, 1965)
Although there is a great diversity of opinion concerning many aspects of the clay-
water system, there is complete agreement on one fact--that the water in close proximity
to the silicate surface has properties different from those of pure water. One major con-
tention concerns the density of the adsorbed water. Tscapek (1934), as cited by Grim (1953),
found that the density of adsorbed water was greater than that of bulk water (density of pure
water = 1). DeWit and Arens (1950) obtained density values less than one. Low and co-
workers (Hemwall and Low, 1956; Anderson and Low, 1958; Low and Anderson, 1958a,
1958b; Low, 1960a, 1960b; Kolaian and Low, 1962; Low, 1962; Miller and Low, 1963; Oster
20
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and Low, 1964) made an exhaustive study of the nature of water in clay-water systems.
Their results clearly indicated that water close to the clay surface has a lower density
and a higher viscosity than that of pure water and exhibits non-Newtonion behavior at low
hydraulic gradients. Low (1960b) proposed that the water close to the clay surface has a
more ordered structure than that of pure water, i.e., a quasi-crystalline structure. More
discussion on this hypothesis is in the review article by Low (1961).
PHYSICAL PROPERTIES OF SOILS
The physical properties of soils include particle-size distribution, particle density, bulk
density, pore space, structure, plasticity, shrinkage and cohesion, swelling, and flocculation.
Particle- Size Distribution
Physically, a mineral soil is a porous aggregate of inorganic particles with an admix-
ture of decaying organic matter. Inorganic particles vary widely in size from those easily
discernible with the naked eye (stone, gravel, and sand) to those requiring the use of the
electron microscope to discern their morphology.
The various particle-size groups are called “separates.” The two most widely accepted
methods for determining the particle-size distribution are the pipette method and the Bouy-
oucos hydrometer method (Bayer, 1948; Black et al. 1964).
To define the proportion of the various sizes present, the term “texture” is used. The
various textural or soil classes are in the glossary.
The physical nature of the three major separates (sand, silt, and clay) varies dras-
tically. Sand particles, when not coated with clay or silt, exhibit essentially no plasticity
and cohesion. As a consequence of the large size of the spaces between the particles, a
high percolation rate, a low water-holding capacity, and aeration is accelerated.
Clay particles exhibit nearly the opposite properties of sand: High plasticity, cohesion,
high water-holding capacity, slow water and air movement. Clay, defined as a particle
size, has an equivalent spherical diameter less than 2 microns (0.002 mm).
The silt separate possesses some plasticity, cohesion, and adsorption, but to a much
lesser degree than does the clay separate.
21
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The effects of particle size on such properties as surface—area, adsorptive capacity,
swelling, plasticity and cohesion, and heat of wetting are shown in Figure 11. These prop-
erties increase exponentially as the particle size decreases.
Particle Density
The particle density (mass/unit volume of soil solids) of an average arable surface
soil ranges from 2.60 and 2.75, this is the range for such predominant minerals as quartz,
feldspars, and the colloidal alumino-silicates; the average is 2.65. Organic matter markedly
reduces the average value due to its lower particle density; surface arable soils exhibit a
higher organic matter content and may have a particle density of 2.4 or below.
Bulk Density
Bulk density (mass/unit volume of dry soil) includes the total soil space (space occupied
by solids and pore space combined). Therefore, the bulk density of any soil is determined
by the density and arrangement of the particles and reflects the status of the soil structure.
Bulk densities of clay, clay loam, and silt loam surface soils range from 1.00 to 1.60
grams/cm 3 ; sandy barns vary from 1.20 to 1.80 grams/cm 3 .
Pore Space
Pore space is that part of the soil occupied by air and water. The amount of pore
space is determined largely by the structural condition of a soil, i.e., the interaction of
texture, compaction, and aggregation. Pore space measurements therefore indicate how
porous a soil is to water and air. The size (and degree of interconnection) of the individual
spaces rather than the combined volume are of major importance.
Structure
Soil structure refers to the gross over-all aggregation or arrangement of soil solids.
Soil structure often varies from horizon to horizon. Since soil structure influences such
soil properties as heat transfer, water movement, aeration, bulk density, and porosity, and
since the soil structure may vary from horizon to horizon, these properties can be expected
to vary within a soil profile as well as between soils of different textural classes.
22
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SURFACE AREA
MAGNITUDE
Figure 11. Particle Size Effect on the Relative Magnitude of Selected
Soil Properties (After Lyon, Buckman, and Brady, 1952)
Plasticity
Plasticity, or pliability, is exhibited by gels. There is no standard test for deter-
mining the property of plasticity. Using water of plasticity (percent, of water, determined
on an oven-dry basis at 105° C, necessary to develop the optimum plasticity of soil) to
assess plasticity. differences in plasticity as a function of clay type can be seen in Table 5.
Montmorillonite would be the most plastic; the minerals, kaolinite, illite, and halloysite
exhibit less plasticity (chlorite would also be expected to exhibit similar behavior). Al-
though no data are given, hydrous oxides and hydroxides would be expected to exhibit much
lower plasticity and cohesion than the montmorillonite type of clay. Each clay mineral
group can be expected to show a range of values because particle size, exchangeable cations,
chemical composition, and crystallinity of the clay mineral exert an influence. In soils, the
presence of non-clay materials, soluble salts, plus significant amounts of coarse particles
and organic matter can temper the influence of the predominant clay species. In the case
of montmorillonite, the water of plasticity value would be higher when the predominant
species is sodium rather than calcium, magnesium, potassium, or hydrogen (the viscous
condition increases as the value increases).
ADSORPTION CAPACITY
(molecular and ionic)
SWELL ING
PLASTICITY and COHESION
HEAT of
WETTING
SAND SILT CLAY COLLOIDAL
CLAY
23
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Table 5. Water of Plasticity
(in percent by weight)
Kao linite
8.9
56.3
Illite
17 - 38.5
Halloysite
33 - 50
Attapulgite
93
Montmorillonite
83 -
250
(After White, 1947)
Shrinkage and Cohesion
These two properties are closely related to the property of plasticity.
When the water content of a clay gel is reduced, shrinkage occurs, and various struc-
tural units are developed. This shrinkage results in splitting of the soil mass into aggre-
gates accompanied by a cohesion of the particles. Probably the tendency of the clay parti-
cles to stick together is due, in part, to the mutual attraction of the clay particles for
water molecules held in between the particles. The clay mineral, montmorillonite, shrinks
and cracks to a much greater degree than does kaolinite.
Swelling
The extent to which a clay mineral adsorbs water is determined primarily by the clay
type. As can be seen in Table 6, the decreasing order of swelling is montmorillonite>
mite > halloysite > kaolinite (free swelling is defined as the increase in volume of the
substrate upon the addition of a certain volume of water). For montmorillonite clays, the
swelling decreases greatly with the exchange of sodium for other univalent ions or for di-
valent or trivalent ions. Bayer and Winterkorn (1935) found that for Wyoming bentonite
(montmorillonite type of clay), swelling decreased in the sequence, sodium > lithium >
potassium > calcium > magnesium > hydrogen.
24
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Table 6. Free-swelling Data for Various Clay Minerals (in percent)
Ca-Montmorillonite 45 - 145*
Na-Montmorillonite 1,400 - 1,600
Na-Hectorite 1,600 - 2,000
Illite 15 — 60*
Kaolinite 5 - 60*
*Range of values due to variation in source area of clay mineral.
(After Mielenz and King, 1951)
The magnitude of swelling varies also with the variation in layer charge, particle-size
distribution, electrolyte content of the aqueous phase, void size, and distribution.
The property of swelling directly affects the permeability of soils; the greater the de-
gree of swelling, the lower the permeability of soils.
Flocculation
This term is applied to the coagulation of clay sols in the suspended state upon the
addition of an electrolyte. Upon coalescence, these floccules settle to the bottom due to
their combined weight. This same coalescing action apparently takes place in the soil, but
much less rapidly, of course.
Although the coalescing capacities of various cations vary with the type of colloid,
there are empirical rules to predict the behavior of the various cationic species; the most
important is the Schulze- Hardy rule (van Olphen, 1963). It states that the coalescing power
of indifferent electrolytes (i.e., electrolytes which do not react with the soil particle) is
primarily determined by the valence of the ion, having a charge opposite to that of the
particle charge: the higher the ion valence, the greater the flocculating power. Soils in
the humid region are usually dominated with calcium, aluminum, and hydrogen (hydrogen
acts as a divalent or trivalent ion, probably due to the presence of aluminum) and are gen-
erally coagulated. In contrast, high concentrations of adsorbed sodium causes a soil to be-
come dispersed and sticky. Such conditions are found in certain sodic soils. The nature
25
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and concentration of the adsorbed cation thus can, for the same type of colloid present, in-
fluence the physical properties of the soil (e.g., permeability to water flow); a soil in a
dispersed state is less permeable than one in the coagulated state.
In summary, the properties of soils as influenced by texture are shown in Figure 12.
These are generalizations and indicate trends, some exceptions, e.g., the permeability to
water of sandy soils having an indurated horizon, may arise.
BIOLOGICAL PROPERTIES
The soil is a living dynamic system. The microflora and fauna present in soil include
actinomycetes, bacteria, algae, fungi, nematodes, protozoa, earthworms, and rodents. Bio-
chemical degradation of these organisms and plant tissue annually supplies large quantities
of organic residue to the soil. Since organic matter is derived from the metabolic products
and tissue from various sources, the chemical composition of organic matter and humus
(partially decomposed organic matter) is extremely complex; various components of organic
matter have largely defied identification. However, since most organic matter originates
with tissues of higher plants, it would be expected that such constituents as carbohydrates,
fats, waxes, tannins, lignins, and proteins are present.
There are two significant multi-step biochemical cycles in the soil: the carbon cycle
and the nitrogen cycle. In the carbon cycle, complex carbon-containing sources are degraded
or completely oxidized to carbon dioxide by microorganisms; the organisms use carbon as
an energy source.
In the nitrogen cycle, nitrogenous compounds are degraded or decomposed essentially
in three steps:
(1) Aminization of proteins
(2) Ammonification
(3) Nitrification
The aminization reaction is:
Protein + Microbial enzymatic - Complex amino compounds ± CO 2 +
action Energy + Other products
The amino compounds may be either incorporated into microbial cell tissue or changed to
simpler products through the ammonification reaction.
26
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Soil Property Soil Texture
Sand Silt Clay
Sand, Loamy Sand, Loam, Silt Loam, Sandy Clay, Silty
Sandy Loam Silt, Sandy Clay Clay, Clay
Loam, Clay loam,
Silty Clay loam
Surface A ea
Molecular adsorp-
tion
C. E. C.
S we 1 line
Plastic itv
Heat of wetting
Water holding
ca pa city *
Infiltration*
Per c 0 Ia t ion *
Permeability *
External drain-
age *
Aeration*
Organic matter
content *
Structure*
Bulk density*
Porosity (total)*
*COlligative properties
p
4
r
,
,
Figure 12. The Effect of Soil Texture on the Physico-Chemical Properties of Soil
27
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The enzymatic reaction called ammonification can be represented as:
CR - NH 2 + ItOH enzymatic ROH + NH 3 + Energy
hydrolysis
ammo
combination
Of most interest, however, is the process of nitrification. Nitrogen in soils (added as
a fertilizer amendment or present in soil organic matter) can be transformed to ammonia
and then to the nitrate-nitrogen form. The retention power of soils for ammonium and
nitrate forms of nitrogen differs markedly .
Nitrification is a two-step process and requires two different autotropic nitrobacteria.
The transformation can be represented by the following two reactions:
nitrosomonas
2NH 3 ÷30 2 2NO÷2H 2 0+4H +Energy
nitrosococcus
bacteria
nitrobacter
2 NO 2 + 2 NO 3 + Energy
The nitrate may disappear as a result of microbial reduction of nitrate through nitrite to
N 2 and subsequent loss by volatilization.
Humus possesses more pronounced colloidal properties than do inorganic mineral frac-
tions. Humus is adsorbed onto clay surfaces; the resulting clay-organic complex has colli-
gative properties that differ in nature from either of the two constituents. As shown in Table
7, organic matter has a much higher cation exchange capacity than any clay mineral and has
a surface area comparable to the expanding or swelling clay minerals.
With respect to physical properties, humus and organic matter (1) reduces plasticity and
cohesion in soils, (2) increases water-holding capacity of the soil, and (3) encourages granula-
tion in soils.
28
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Table 7. Selected Physical Properties of Certain Soil Constituents
Physical Property
Soil Constituent Cation exchange Surface area,
capacity, Sq. meters per
Me. per 100 gram gram
Organic matter 200 to 400 500 to 800
Vermiculite 100 to 150 600 to 800
Montmorjflonjte 80 to 150 600 to 800
Dioctahedra.1 vermiculite 10 to 150 50 to 800
Illite 10 to 40 65 to 100
Chlorjate 10 to 40 25 to 40
Kaolinite 3 to 15 7 to 30
Oxides and hydroxides 2 to 6 100 to 800
(After Bailey and White, 1964)
THE CHEMICAL CHARACTER OF SEWAGE
This section presents a synopsis of existing information on the quantitative and the qualita-
tive chemical aspects of raw and treated sewage. This synopsis indicates what might be
expected in treated sewage and, thus, what components in sewage effluent might react or
interact with soil or soil constituents.
The chemical character of the treated waste effluent (with respect to both the nitrogenous
and phosphatic constituents) would be expected to vary, depending on the contributing com-
ponents in the raw sewage and the waste treatment method used.
Raw domestic sewage contains 15 to 35 parts per million (ppm) of nitrogen and 2 to 4
ppm of phosphorus (Sawyer, 1952). An average value for phosphorus, expressed as P 2 0 5 , in
raw sewage is 5.2 ppm: the range is 4.0 to 9.2 ppm P 2 0 5 (Rudolfs, 1947). However, this
value maybe much higher now due to increased use of detergents and water softeners; phos-
phorus is a major constituent of both substances. A value of 1.2 X pound per capita per
day has been estimated as the amount of phosphorus contributed by the use of synthetic
detergents alone. Stumm and Morgan (1962) stated that the phosphorus content of domestic
sewage was 3 to 4 times greater than that prior to the advent of synthetic detergents and that
the phosphorus content of sewage may continue to rise. They also stated that aerobic biological
treatment processes mineralize substantial fractions of bacteriological oxidizable substances,
but are not capable of eliminating more than 20-50% of the nitrogenous and phosphatic com-
pounds in sewage.
29
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Cleason and Loonam (1933) analyzed sewage influent to a northside Chicago activated
sludge plant and found that the total nitrogen level was 16,1 ppm (range 13-18 ppm); of this
concentration, ammonia-nitrogen was 7.4 ppm and organic nitrogen, 8.7 ppm.
The sewage treatment method used determines the nitrogenous and phosphatic content
of the treated waste effluent. Under plant-scale tests ,using a treatment sequence (fine screening,
high-rate trickling filter-secondary clarifier, low-rate trickling filter, and a final clari-
fier with 545 ppm of calcium oxide added to the final settling tank), the total nitrogen content
of the sewage could be reduced from 7.4 to 1.7 ppm (77% reduction) (Owen, 1953). Under
laboratory conditions (pH 10.9, settling time 1 hour, 540 ppm CaO), the total phosphorus con-
tent could be reduced from 6 to 0.3 ppm: 99% removal could be effected if settling time was
increased from 1 to 18 hours (Owen, 1953). Rudolfs (1947) indicated that 50 to 60% of the total
phosphorus couldbe removed with the use of settling alone, that filtration effected a 75 to 80%
removal and that the activated sludge process reduced the initial concentration by 80 to 90%.
Similar results have been recorded by Hurwitz et al. (1965). The authors found that in
a southwest Chicago treatment plant using the activated sludge process, the total P0 4 content
was reduced by 77% (from 16.1 to 3.75 ppm). With regard to nitrogen removal, Cleason and
Loonam (1933) found that in an activated sludge treatment plant on the north side of Chicago,
the total nitrogen content was reduced by 73% (16.2 to 4.3 ppm); ammonia-nitrogen was re-
duced by 67% (7.4 to 2.5 ppm); and organic nitrogen was reduced by 79% (8.7 to 1.8 ppm).
From a comprehensive study of the Lake Tahoe watershed, McGauhey et al. (1963) sug-
gested that the unit design factor for domestic water containing phosphate and total nitrogen
should be 8 and 45 ppm, respectively, of the elemental form present.
It should be emphasized that results expressed as percentages, although impressive, may
be misleading. It is necessary to know two quantities before percentage reduction can be
properly interpreted: (1) the initial concentrations in the influent and (2) these values must
be compared to tolerence levels for nitrogen and phosphorus, principally that level which pro-
motes algae growth and water quality problems.
Analytical data from seventeen Wisconsin lakes led Sawyer (1952) to conclude that water-
borne concentrations of soluble phosphorus in excess of 0.01 ppm and inorganic nitrogen in
excess of 0.30 ppm at the spring turnover time would produce algal blooms, constituting a
nuisance. Similar conclusions were reached by Sylvester (1961): a nitrate-nitrogen concen-
tration greater than 0.2 ppm and a soluble phosphorus concentration greater than 0.01 ppm
would support algal blooms and constitute a nuisance.
30
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Sawyer and Ferullo (1961). based on a laboratory study of nitrogen fixation in natural
waters, believed that: (1) sewage plant effluents contained excessive amounts of phosphorus:
(2) fertilization of aquatic regions by domestic wastes stimulated biological productivity: an
(3) excessive amounts of phosphorus could stimulate extensive blooms of nitrogen-fixing blue-
green algae.
Based on foregoing information, it appears that nitrogen and phosphorus concentrations
in treated sewage effluent exceed those levels where stimulation of algal growth occurs and
thus, the “nuisance potential” is exceeded, even when the most efficient conventional treat-
ment systems are used.
Various methods have been used to lower the soluble phosphate content in sewage treat-
ment plant effluents: however, the data usually do not show whether the insoluble phosphates
were removed. Rohlich (1961) reported that laboratory studies at the University of Wiscon-
sin and at the Madison treatment plant indicated that 96 to 99 of soluble phosphorus could
be removed by physical-chemical precipitation using additives (alum, ferrous sulphate, ferric
sulphate, or copper sulphate): alum appeared to be the most suitable coagulant. The “Pitcon
process” removed approximately 95 of both total and soluble phosphate, but had little effect
on total nitrogen: the major effect on nitrogen was the conversion of organic nitrogen to
ammonia. Neither processwas evaluated astoits economic merit. The useof ion exchangers
(Nesselson. 1958) or air stripping of ammonia (Kuhn, 1954) for nitrogen removal was found
to be economically prohibitive.
It appears that, at the present time, there is no single method that is both functional and
economically feasible for use in removing sufficient amounts of nitrogen and phosphorus from
sewage treatment-plant effluent, before disposal into a watercourse or into the hydrologic cycle.
With this in mind, the following section reviews the literature on the principles of soil
science and nitrogenous reactions, with respect to the use of soil as a waste treatment
method and a means for waste disposal.
BEHAVIOR OF NITROGEN IN SOILS
The leaching of nitrogen, phosphorus, and other nutrients in soils has been investigated
mainly by collecting and analyzing water from either tile drains or lysimeters. In the first
method, an areais selectedin whichthetile drain receives onlythe waterfromthe land under
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study. The flow is recorded periodically, and analyses of the drainage water indicate nutrient
and water losses. The advantage of this method is that a large area of soil may be studied
under normal field conditions.
However, the usual method has been the use of a lysimeter (see glossary and Kohnke
et al. (1940). In lysimetry, the leachate from either a disturbed or undisturbed soil column
is collected, the amount of water is determined, and the leachate is chemically analyzed for
the elements of concern. The advantages of using a lysimeter over a tile-drainage system
are: (1) variations in a large field are avoided and (2) the study is more easily controlled
and less expensive.
Kohnke et al. (1940) stated in their summary that soil texture, soil structure and exchange -
able cations influence the rate of percolation which, in turn, influences the magnitude of the
leaching losses. Percolationis more rapid (and of greater volume) through coarse-textured
soils than fine-textured ones. Percolation is more rapid when the soil has a porous crumb
structure and when the cation complex is staurated mainly with calcium and aluminum rather
than sodium (due to the dispersive action of sodium).
MOVEMENT OF NITROGENOUS COMPOUNDS IN SOILS
Inorganic Nitrogen Compounds
Most soil research on nitrogen has been on the leaching losses of nitrate -nitrogen (NO-N),
ammonium-nitrogen (NH -N), and nitrite-nitrogen (N0-N) from soils. However, some work
has beendone on the reactions of other nitrogenous forms with soils: these are discussed in
the next section.
The ammonium and nitrite forms of nitrogen constitute only a small portion of the total
nitrogen removed. Results of lysimeter studies in California and New York showed that nitrate
is the predominant form of nitrogen in percolating water --more than 99% of the soluble nitrogen
was in the nitrate form; although, under certain circumstances, appreciable amounts of
ammonia and organic nitrogen including urea can be found in the leachate (Chapmanet al.,
1949: Collison and Mensching, 1930). Less than i% was present as ammonia: and only a
trace was present as nitrite (Collison and Mensching, 1930). The explanation for the low nitrite
level was that only a small concentration was present in the soil, while the low loss of ammonium
was attributed to the fixation by the colloidal exchange complex.
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Usually all the nitrogen in leaching waters is in the nitrate form and the ammonium
form is either held by the exchange complex or oxidized to nitrates by microbal action and
thus, is lost (Kohnke et al., 1940). In peat soils, a large amountofammonium-nitrogen was
found in the leachate; this was attributed to the reducing power of the peat (Benson and
Barnette, 1939; Joffe, 1940; Bates and Tisdale, 1957; and Il’in, 1959). More recent work
(Smith, 1944; Broadbent and Chapman, 1950; and Shopkhoev, 1958) confirms earlier findings
that applied nitrogen present in the nitrate form (or converted to nitrate by microbial action)
is easily and generally completely removed from the soil profile by percolating waters.
Calvert (1962) found thatheavy application of water to the soil surface caused solutionof
applied nitrate fertilizers and redeposition at depths of 30 inches or more. Simakin and
Vasilenko (1965), found that nitrate, naturally in the soil (leached chernozem) or applied as
fertilizer (NH 4 NO 3 ), was almost completely leached from the top 15 centimeters into the
subsoil during the wthter and early spring, and was subsequently carried into ground water
wells, and into the Kuban River.
The nitrogen form present or applied to soil greatly affects the leachability of the material.
Benson and Barnette (1939) studied the leaching characteristics of four sandy soils, applying
various forms of nitrogenous fertilizer. Theyfoundthat all nitrogen applied as nitrate (NO 3 )
was leached out of the profile. One-third of the amount applied as (NH 4 ) 2 S0 4 and NH 4 NO 3
was leached. Very little (NH 4 ) 3 P0 4 was removed. Also, the four soils retained (N B 4 ) 2 CO 3
very efficiently until nitrification began. Urea was not leached at all when immediately assayed,
but after 1- and 4-day incubation periods, 35 and 60 o, respectively, were lost. Bates and
Tisdale (1957), by a multiple regression analysis of data from column leaching-studies of
eight coarse -textured North Carolina Coastal Plain soils, found a relationship between nitrate
movement, amount of percolating water, and soil porosity. The relationship was:
Y = 1121.93 + 64.66X 1 - - 5.83X 3
Where
Y mean movement of nitrate in centimeters
X 1 = porosity index of soil
X 3 = porosity index times amount of water added
The regression accounted for 87.6 of the treatment sum of squares and was highly signifi-
cant. Other factors found to influence nitrate movement included: (1) the degree of soil
aggregation (soil structure-related factor): (2) the nature of the accompanying ion; and
(3) the form in which the nitrate was present.
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Loss of nitrate by leaching is proportional to the nitrate concentration and loss of water.
Leaching of nitrate-nitrogen may be very intense in irrigated agriculture where most of the
applied nitrate enters the ground water.
In field work on four drainage systems and two soil types in the San Joaquin Valley of
California, Johnson et al. (1965) recorded the applications of fertilizer and irrigation water
and determined the amount of tile effluent drainage from a number of plots. They analyzed the
drainage water for total nitrogen and phosphorus as well as for nitrate -nitrogen and ammonium -
nitrogen. Most of the nitrogen in the effluent was in the nitrate form. However, up to 7.1 ppm of
nitrogen (in the form of ammonium) were lost immediately after application of nitrate; also,
nitrogen as organic nitrogen and nitrite in concentrations up to 1.3 and 0.01 ppm, respectively,
were lost. Onaweightedaveragebasis (average weighted on the basis of discharge), the con-
centration of nitrate was 21.1 ppm; the range was 1.8 to 62.4 ppm. The authors indicated that
the chemical composition of the drainage effluent would be influenced by: (1) hydraulic con-
ductivity of soil; (2) salinity; (3) cation exchange capacity of soil; (4) stratification: and
(5) physical arrangement of the drainage system.
Organic Nitrogen Compounds
Movementorleachability of anorganic compound is afunction of two parameters: (1) ease
of adsorption/resistance to desorption and (2) water solubility.
As indicated by Bailey and White (1964), the amount of adsorption is determined by the
interaction of such micro-factors as: (1) nature of adsorbent; (2) nature of adsorbate; (3) soil
reaction; (4) nature of cationic species on soil exchange complex; (5) soil moisture content;
and (6) temperature of soil.
The second parameter, water solubility of a compound, is influenced by the rate at which
water passes through the soil and by the concentration of the solute in question.
Organic nitrogen containing herbicides of interest include s-triazines, substituted ureas,
carbamates, acetamides, anilides, and amides. Over a wide range of soil and environmental
conditions, the two most common s-triazines, simazine and atrazine, as well as the substituted
ureas, fenuron, monuron, diuron, and neburon, are not very mobile in soils; they are generally
not leached past a 4-6 inch depth (Hill et al., 1955; Abel, 1957; Upchurch and Pierce, 1957 and
1958; Sheets, 1958; Montgomery et al., 1958 and 1959: Subra and Guillemot, 1959; Stroube
and Bondarenko, 1960; Ashton, 1961; Dewey, 1961; Burnside et al., 1961 and 1963; Rodgers,
1962; Stroube, 1962; Geissbilhler et aL, 1963; and Wiese and Davis, 1964). However, the
behavior of such organics is difficult to predict in continuously water-laden soils.
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Herbicides such as dipropalin (N, N-dipropyl- 2, 6 -dinitro-p-toluidine), diphenamid (N, N-
dimethyl -2, 2 -diphenylacetamide) and diphenatrile (diphenylacetonitrne) exhibit a slightly
greater mobility (Pieczarka, 1961), while CDAA ( 2 -chloro-N,N-diallylacetarnide) did not
leach past 3 inches with 2 inches of rainfall (Helgeson and Anderson, 1955) However, more
rainfall would probably leach COAA to greater depths.
REACTIONS OF VARIOUS FORMS OF NITROGEN IN SOILS
Reactions of various nitrogenous forms with soils bears directly on the problem of re-
tention or movement through soils and determines whether nitrogenous sewage components
are held in the soil.
Ammonia
One form of nitrogen which might be present in sewage waste effluents is the molecule
ammonia. Obviously, reactions of ammonia (NH 3 ) cannot be divorced from the reactions of
the ammonium ion (NH) in soils. For clarity, a distinction should be made between these
different forms of nitrogen. Ammonia is an electrically neutral, but highly polar molecule.
The ammonium ion possesses an electrostatic charge and therefore exhibits properties
similar to those of other cations in solution. The ammonia molecule is pyramidal in con-
figuration.
In the literature, the terms “ammonia” and “ammonium” have been used inter changeably.
Under alkaline conditions, the concentration of ammonia may be relatively high in comparison
with that of the ammonium ion. A one-molar solution of ammonia in water at 25°C is only
0.00426 M with respect to the ammonium ion.
From the standpoint of physical chemistry, the ammonia molecule can be adsorbed
chemically and/or physically (Bailey and White, 1964). Chemical adsorption would be expected
where hydrogen is present as an ion or as a constituent of hydrated aluminum ions. In the
case of reaction with hydrogen ions associated with a clay mineral or organic matter, the
ammonium ion would be formed since ammonia is a very strong base and would accept the
proton and form NH, the conjugate acid. If this occurs, the ammonium ion becomes the
exchangeable ion on the mineral’s exchange complex.
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Physical adsorption would be expected to occur by a hydrogen bond between a hydrogen
atom of the ammonia molecule and an oxygen on the clay surface (NH 0). Since ammonia
can form “hydrates” with various salts, such as CaC1 2 . 2NH 3 , similar to genuine hydrates,
the ammonia molecule may form such hydrates with exchangeable bases on the cation exchange
complex of the soil systems.
Both chemical and physical adsorption of ammonia by such soil constituents as clay
minerals and organic matter has been observed experimentally. Buswell and Dudenbostel
(1941) observed that ammonia was chemically adsorbed by a hydrogen-saturated clay; the
ammonia reacted with hydrogen-saturated layer-lattice silicates to form an ammonium-
saturated clay. Jenny et al. (1945) and Mortland (1955) both showed that ammonia could be
physically adsorbed by clays and clay minerals. This type of adsorption was established by
showing (1) the easy reversibility of adsorption and (2) the low energy of adsorption. Work-
ing with a soil clay (Yolo clay) which had been hydrogen-saturated (actually an H-Al clay
system), Jenny et al. (1945) demonstrated that ammonia adsorption occurred simultaneously
by both chemical and physical adsorption. The nature of the substrate affected the magnitude
of ammonia adsorption. Mortland (1955), using a vapor-phase adsorption technique and the
Brunauer, Emmett, Teller equation, found that ammonia adsorption by three soil components
decreased in the following order: muck, bentonite, and kaolinite (cation exchange capacity
also decreased in this order). The heat of adsorption of ammonia by bentonite was determined
by using a thermodynamic relationship (Classius -Clapyron equation) and sorption isotherms.
The surface sites of adsorption on bentonite were markedly heterogeneous since the heats
of adsorption varied according to the amount of surface covered.
By combining X-ray diffraction, infrared spectroscopy, calorimetry, and ND 3 and D 2 0
isotopic exchange techniques, Mortland et al. (1963) demonstrated that: (1) chemisorbed
NH 3 was present as NH ions rather than as the molecular form and (2) ammonia (as NH )
was chemisorbed by base-saturated clay mineral samples; the reaction was between the NH 3
and residual water on the interlamellar surfaces. These authors speculated that certain ob-
served spectral features of NH 3 could be attributed to the amination reaction (“hydrate” for-
mation) or to physically-adsorbed species.
Much of the ammonia sorbed by various kinds of organic matter is in a non-exchangeable
form and is resistant to decomposition. Broadbent et al. (1960) found that an ammonia-organic
matter combination was so stable chemically that it was not broken down by refluxing with
6 N HC1 for 16 hours. Ammonia fixation was found to be a linear function over the range from
pH 4-9. Thus it would be necessary to use basic waste effluents to maximize adsorptionby
soil organic matter .
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Several factors can affect the loss of adsorbed ammonia from soil. Volatilization occurs
when the ammonia concentration is high and the ability of the soil to react with the ammonia
is low. The volatilization would be high especially from mucky, humic, and peat soils because
ammonia is produced from organic matter. Because water retains ammonia, a low moisture
content reduces the amount of ammonia dissolved in the water and promotes volatilization.
Of greater importance than the water content, however, are the cation exchange capacity
and pH of the soil. These properties affect the ability of the soil to react chemically with
ammonia.
Under the pH conditions normally found in soils (pH 4 - 9), ammonia probably is chemically
adsorbed by clays in an exchangeable form and by the organic matter in the alkaline range.
Using manometric techniques, Brown and Bartholomew (1963) studied the effects of the inter-
actions of moisture content, ammonia concentration, and cationic species on the adsorption of
ammonia by bentonite and halloysite. The adsorption isotherms indicated that, when the
ammonia vapor pressure was less than 60 mm. Hg, dry clays adsorbed more ammonia than
comparable moist clays. At a vapor pressure in excess of 60 mm. Hg, the greater the water
content the greater the amount of ammonia adsorbed. The decreasing order of cationic
influence on magnitude of adsorption was aluminum to calcium to potassium. Evidence was
also presented that site competition occurred.
In an earlier study using a similar technique, Brown and Bartholomew (1962) found that
at a pH of 7, synthetic gibbsite did not chemically adsorb ammonia. They reasoned that.
since Al(OH) 3 is basic in nature, a hydroxyl group and not a hydrogen group, would be expected
to be dissociated. Also, potassium- and ammonium-saturated systems adsorbed less ammonia
than did bentonite saturated with such ions as hydrogen, aluminum, calcium, or magnesium.
Apparently, certain ions decrease or prevent the adsorption of ammonia, a property of im-
portance in a soil column waste-effluent system .
Ammonium
Previously, it was noted that ammonia may react with acid soil systems and become
chemically adsorbed by either an amination reaction (hydrate formation) or proton association.
Because ammonium is near the bottom of the lyotropic series, it is very easily replaced.
Hence, the ammonium ion in a readily exchangeable form, would be readily leached from a
soil upon exchange with other ions in the percolating soil (or waste) solution. Some, however,
are not readily exchanged and are “fixed.” The phenomenon of ammonium fixation has been
reviewed by Gieseking (1949), Reitemeler (1951) and Grim (1953).
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Ammonium fixation commonly occurs in soils and has even occurred in rocks (Stevenson,
1959). Conditions such as drainage, vegetative cover, and the extent of leaching have little
effect on the fixed ammonium content of soils (Stevenson and Dhariwal, 1959).
The following 15 factors affect and determine ammonium fixation and release:
(1) Amount of clay-size particles (Stevenson, 1959). In general, the more clay present,
the greater the fixation of ammonium.
(2) Types of clay minerals present. The mica-type clay minerals (vermiculite, mica-
vermiculite, illite, and montmorillonite) are responsible for ammonium fixation; also
for potassium fixation--the same mechanism is generally believed to apply to both
potassium and ammonium. (Volk, 1934; Stanford and Pierre, 1947; Joffe and Levine,
1947; Bower, 1950; Barshad, 1950; Allison, Kefauver, and Roller, 1953; and Doetsch,
Roller, and Allison, 1953).
(3) Magnitude of layer lattice cha.rge of clay minerals (Barshad, 1954a). The higher the
layer charge, the greater the percentage of ammonium ion fixed.
(4) Particle size (Barshad, 1954b). Ammonium fixation decreases as particle size in-
creases.
(5) Presence of exchangeable hydrogen on the exchange complex, (Barshad, 1954a).
Ammonium fixation is inversely related to the content of exchangeable hydrogen.
(6) Nature of the replacing cation (Barshad, 1954a). The ease of replacement of the
fixed ammonium is barium > lithium> sodium > magnesium > calcium.
(7) Nature of interlayer potassium present (Barshad, 1954a). Ammonium fixation is
greater if interlayer potassium is fixed rather than native in origin.
(8) Origin of layer-lattice charge. Minerals having an unbalanced charge predominantly
in the tetrahedral layer possibly fix ammonium more effectively than minerals with
an unbalanced charge predominantly inthe octahedral layer (Wear and White, 1951).
On the other hand, the magnitude of the layer charge- -not the site of origin- -may be
the governing factor in both ammonium and potassium fixation. (Barshad, 1954b).
(9) Competition for adsorption site. Potassium, either in the crystal lattice or in the
reacting solution, blocks the fixation of ammonium. Minute amounts of potassium
or ammonium in the solution prevent the release of fixed ammonium from soils.
(Stanford and Pierre, 1945; Barshad, 1954a; Hanway and Scott, 1956; Scott, Hanway,
and Edwards, 1958; Edwards, 1961; and Leggett and Moodie, 1963).
(10) Moisture status. Bentonite fixes ammonium on drying, but not while moist (Page
and Bayer, 1940; Joffe and Levine, 1947; Allison, Kefauver, and Roller, 1953; and
Allison, Roller, and Doetsch, 1953). Walsh and Murdock (1960) foundtwice as much
fixation after soil was frozen as when it was moist and un-frozen.
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(11) pH of the soil system. Scott, Hanway, and Edwards (1958) found that more ammonium
could be removed from vermiculite with a sodium -extracting solution with decreasing
pH. A linear relationship was found.
(12) Surface vs. subsurface soils. Greater fixation occurs in the subsoil than in surface
horizons (Allison, Roller, and Doetsch, 1953: Rich, 1960: and Walsh and Murdock,
1960). Allison, Roller, and Doetsch (1953) found that the subsoil fixed 5 to 10 times
more ammonium than the surface soils.
(13) Presence and extent of development of aluminum interlayers. Rich (1960) found that
ammonium fixation decreased with increasing development of aluminum interlayers.
Aluminum interlayers decreased the cation exchange capacity of minerals and thus
decreased the sites available for ammonium fixation. Also, they prevent closure of
the lattice layers of vermiculite and hence, prevent entrapment of ammonium.
(14) Strength of ammonium fixation as a function of the nature of the clay mineral. The
order of the strength of fixation was found to be vermiculite > illite > montmoril -
lonitic soil (Allison, Kefauver, and Roller, 1953).
(15) Nitrification. Fixation of ammonium greatly reduces nitrification of ammonium by
bacteria(Allison, Kefauver, andRoller, l953andAllison, Roller, andDoetsch, 1953).
Such a reduction is of great importance since the product of nitrification is nitrate,
which is highly leachable.
The mechanism by which ammonium (also potassium) is fixed while other cations (e.g.,
lithium, sodium, calcium, and magnesium) are not, is based on: (1) ionic size of the dehy-
drated ions; (2) the expandability and contractibility of the mica-like or 2:1 type clay minerals:
and (3) geometric arrangement of the oxygen ions at the silicate surface. The major tenets
of the theory of Page and Bayer (1940) are as follows:
(1) Exposed interlamellar surfaces of micaceous minerals consist of sheets of hexa-
gonally-arranged oxygen ions. Radoslovich (1960, 1961, 1962a, 1962b) proved that
the opening is not hexagonal but actually is ditrigonal, the opening within the “hexa-
gon” being 2.8 angstroms in diameter.
(2) Dehydration of clay prompts contraction of the layers; ions lose water of hydration
and approach the unhydrated ionic diameter (ammonium ion diameter, 2.96 A;potas-
0
sium ion diameter, 2.66 A).
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(3) ions having a diameter close to those of the “hexagonal” openings fit into the open-
ings in adjacent silicate layers. Further contraction locks-in the ions against fur-
ther rehydration. Larger ions do not fit into the openings; hence, these cations are
loosely held between the layers and are accessible for rehydration. Small cations
enter the openings but make insu.fficient contact with adjacent layers to effect strong
binding. Therefore, ammonium is fixed or retained by the 2:1 type clay mineral
such that it does not leach through the soil and enter the sub-surface or ground
water. Ammonium is held in such a tenacious manner by the 2:1 type clay mineral
that it is very difficult for the ion to be desorbed and moved out of the profile.
Urea
Little is known about the retention of urea in soils. Chemically, urea (NH 2 CONH 2 ) is a
weak base; K 1.5 X
Chin and Kroontje (1962) found that a urea solution (50-1,000 ppm), adsorbed by soil,
obeyed the Freundlich adsorption isotherm (adsorption magnitude related to amount of solute
in equilibrium with the adsorbent). The adsorption of urea by soils or soil cellulose mixture
(representative of organic matter) was 2 to 3 times greater during dry than wet conditions
and varied between soils. All the urea was desorbed by 50 ml of water from Yolo loam
(a high montmorillonite-content soil), Norfolk fine silt loam, and Norfolk sand; while, Sauna
clay and Tatum silt loam (mainly dioctahedral vermiculite and kaolinite) required 2,500 and
1,000 ml of water, respectively, to desorb equivalent amounts. The difference in desorption
ease was attributed to a difference in adsorption mechanisms. Physical adsorption was the
postulated mechanism for the first three soils and chemical adsorption for the last two.
Adsorption of urea by organic matter was attributed to both physical adsorption and complex
formation. The cation exchange capacity, pH, and clay content, were not found to be signifi-
cant in urea sorption. The authors concluded that soils in general have a weak affinity for
urea.
In contrast, Hourigan and Seay (1957) reported that in Kentucky soils urea was fixed by
soil colloids and that leaching of urea was not observed under field conditions.
Mitsui and Takotoh (1963) used infrared spectroscopy to study the interactions between
urea and soil constituents. The authors used a wide range of adsorbents including five paddy
soils, kaolinite, bentonite, and allophane; both anionic and cationic exchange resins, and humic
acid. The N-H stretching band at 3500 cm shifted to 3400 cm . Urea adsorption was
attributed to hydrogen bonding to such functional groups as Si-OH (of amorphous clays); the
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OH at the broken edge of the clay; and COOH, OH, and C = 0 (functional groups in organic
matter). However, a later infrared study by Mortland (1966) clearly indicates that urea is
principally adsorbed not by hydrogen bonding of the NH group to the clay surface but either
by protonation of the carbonyl group (C = 0) or by interaction of the carboi y1 group with the
exchangeable cation (i.e., an ion-dipole type electrostatic bond).
Urea is hydrolyzed to ammonia in soils by the enzyme, urease (Conrad, 1940a, 1940b;
Simpson and Melsted, 1963). Rate of hydrolysis varies between soils (Benson and Barnette,
1939; Simpson and Meisted, 1963) and is a temperature-dependent reaction.
BEHAVIOR OF PHOSPHORUS IN SOILS
The average total phosphorus concentration in soils varies from 0.007 to 0.11%; but,
generallyitislessthan 0.05% exceptin youngsoils (Dahnke eta!., 1964). Expressedin another
manner, soils usually contain 0.10 to 0.25% P 2 0 5 and hardly ever more than 0.5%.
For purposes of waste disposal, information on the actual quantity of phosphorus that
soils are capable of fixing is of paramount importance. The very large phosphorus fixation
capacities of some soils can be seen in Table 8. For example, to satisfy the phosphorus-
fixing power of the unlimed Collington soils, nearly 47 tons of superphosphate containing 20%
P 2 0 5 would be required. One Coastal Plain soil was reported to have a phosphate-fixing capa-
city of 125 tons of 20% superphosphate per acre-furrow-slice (Bear and Toth, 1942).
For agricultural purposes, land having a high phosphorus-fixation power poses a great
problem. However, f or use as a waste effluent soil treatment system, the greater the fixation
capacity the more efficient and the longer lasting the “column” would be. It also should be
remembered that the amount stated above applied only to the top 6 inches (acre-furrow-slice).
The lower soil horizons also have a large fixation capacity; but, as will be pointed out, the
greatest phosphorus fixation capacity appears to be in the A horizon.
Movement of Phosphorus Compounds in Soils
In contrast to the high mobility of nitrate-nitrogen in soils, most phosphorus-bearing
compounds react vigorously with the soil and very little passes through the soil profile into
the ground water.
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Table 8. The Phosphorus Absorbing Power of Three New Jersey Soils. Limed and
Unlimed Expressed as Pounds of 20 Percent Super Phosphate per Acre
Furrow Slice (A.F.S.)
P -Fixing Power,
Lbs. 20% Super Phosphate
Soil Treatment pH Per A.F.S .
Sassafras No lime 3.6 28,400
Sassafras lime 6.5 13,916
Collington No lime 3.2 93,720
Collington lime 6.5 50,268
Dutchess No lime 3.8 68,728
Dutchess lime 6.5 44,020
(After Toth and Bear, 1947)
Results of a two-year study of the nitrogen and phosphorus content of tile drainage from
two soils in the San Joaquin Valley, California, following fertilizer application and intensive
agriculture, indicated that the percentage of applied phosphorus lost varied from 1 to 17%;
but the concentration ranged from 0.053 to 0.23 ppm (on a weighted average in accordance
with discharge (Johnson et al., 1965). The authors stated that concentrations of 0.079 ppm
phosphorus and 25.1 ppm nitrogen (on a weighted average in accordance with discharge) would
promote the growth of algae, as evidenced by growth in the drainage ditches downstream from
each tile drainage system.
The lysimetry work of Morgan and Jacobson (1942) provided further evidence thatphos-
phorus was essentially not leached from soils. In this study, fertilizer was added to a soil in
an amount equivalent to 3,005 pounds of phosphorus per acre over an 11-year period. The
average leachmg loss through an 18-inch soil depth was 0.1 pound of phosphorus per acre per
year. In sixteen other sets of lysimeters receiving phosphorus in amounts ranging from 481
to 973 pounds per acre during an 11-year period, only one set lost a measurable amount of
phosphorus. Similarly, only a trace of phosphorus was lost during lysimetry studies on Dun-
kirk silty clay loam and the Volusia silt loam for a 10-year period (Bizzell and Lyon, 1927).
Kohnke et al. (1940), in their summary of lysimeter work of 250 years, indicated that only
trace amounts of phosphorus had ever been found in the percolate. Furthermore, the authors
stated that only in light-textured soils which have been heavily fertilized would a small amount
of phosphorus percolate through the profile.
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It should be pointed out that, especially inthe case of phosphorus, the analytical methods
presently used to determine the concentration of phosphorus are more sensitive than those
used in the first part of this century. But, for present purposes, relative amounts are suffi-
cient to demonstrate the general principles of phosphorus retention by soils.
Factors Affecting Phosphorus Fixation by Soils
Several factors play a distinct and important role in phosphorus fixation: type of soil,
particle size, pH, reduction potential, temperature, organic content, and reaction time. Each
factor will be discussed separately.
Soil-Type Effects
Since soils are formed by different combinations of soil-forming factors and thus, differ
in physico-chemical properties, it would be expected that soils would differ in their ability to
fix phosphorus. Also, as expected, the total amount of phosphorus present as well as the total
organic and inorganic phosphorus contents differ among soils. Williams (1959) in a study of
the influences of parent material and drainage on soil-phosphorus relationships, found that
soils formed from basic igneous rocks had a higher content of total and inorganic phosphorus
than soils formed on slate, granite, and sandstone. In addition, phosphate adsorption capacity
was greatest in soils formed from basic igneous rocks. In the same study it was found that,
irrespective of the parent material, the following were associated with poorly-drained soils,
in comparison to well-drained soils:
(1) Lower total phosphorus level;
(2) Much lower total organic phosphorus level;
(3) Lower ratio of organic to total phosphorus:
(4) Higher acid- soluble phosphorus content and higher percentage of inorganic phosphorus
extractable by acid:
(5) Lower a.mmonium flouride-soluble inorganic phosphorus content;
(6) Lower acid-oxalate-soluble aluminum, but similar or higher acid-oxalate -soluble-
iron content: and
(7) A much lower ammoniurn-soluble aluminum content.
Soil profiles differ in adsorption characteristics. The great soil groups, Podzol, Serozem,
Chernozem, and Yellow earth adsorbed phosphorus uniformly throughout the profile. While,
in the case of Krasnozem soils, phosphorus was adsorbed in the upper layers only (Rachinskii,
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1954). Phosphorus (present in the water soluble form) could be eluted from the four great
soil groups mentioned; but not from the Krasnozem soil; a 1 sodium hydroxide solution was
necessary to elute the phosphorus from this soil. Brown (1952) found that soils formed from
basaltic parent material exhibited a much higher phosphorus fixation capacity than Silurian-
derived soils.
The phosphorus fixation capacity (the extracted electrodialyzed clay fractions) from A 1 ,
A 2 , and B horizons of two Coastal Plain soils (Bladen and Norfolk series) was determined by
Hester (1937). Results from the study showed that, as compared to the initial phosphorus con-
tent, the A horizon had an eightfold increase after reaction with solutions containing varying
concentrations of phosphorus; the A 2 horizon had an approximate fivefold increase: while the
phosphorus content of the B horizon did not change appreciably. Such a study indicates that
most of the applied phosphorus (from solutions such as waste effluents) is fixed in the A or the
surface horizon .
In a study of Rumanian soils, Serbanescu and Blaunaru (1963) found that degraded pod-
zols and podzolized brown soils werelowin mobile aluminum phosphate, while leached Cher-.
nozem soils contained the various forms of phosphorus in well-balanced proportions. In the
Steppe Chernozems, phosphorus occurred mainly as calcium -aluminum-phosphates.
Particle Size Effects
The effects of particle size on phosphorus adsorption depend on: (1) the soil separate in
which maximum phosphorus fixation occurs and (2) the fraction in which most of the inorganic
or organic phosphorus occurs.
The importance of the above can be seen in the works of Kanwar (1956). A lateritic-
podzol soil with gravelly sandy-loam texture, containing io% clay, had a phosphorus retention
capacity of 1,500 ppm P0 4 ; of this, only 2% of the total adsorption was attributable to the clay
particle-size fraction. In the case of a red-brown earth of loamy texture having a 62% clay
content, 80% of the phosphorus adsorbed was retained by the clay fraction. This suggests that
adsorption may not strictly depend on the amount of clay, but on the physico-chemical nature
of the constituents comprising the clay fraction.
In soils derived from basic igneous rocks, slates, sandstones, and granites, most of the
organic phosphorus was in the clay fraction; while 20-60% of the inorganic phosphorus in the
topsoils occurred in the sand fraction (Williams, 1959).
44
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pH Effect
The pH of the soil system is a dominating factor in phosphorus fixation. The products of
phosphorus, produced by reactions with the soil, are determined by the soil pH or soil reaction.
In an acid environment (considering solubility products), a predominance of aluminum and
iron phosphates would be expected; while in an alkaline environment, calcium phosphates
would be expected to predominate. A minimum of phosphorus fixation occurs when the soil is
“neutral”, pH 6 to 7. Phosphorus fixation as a function of pH is shown in Figure 13.
Most of the discussion on the role of pH on phosphorus fixation will be deferred to the
section on “reaction mechanisms.” The soil reaction influences: (1) the amounts of iron,
aluminum, and manganese in the soil solution; (2) the nature of iron-, aluminum-, and
manganese-containing minerals; (3) the available calcium present; and (4) the amount and
extent of organic decomposition.
Fixation by Fixation mainly
hydrous oxides \ / as calcium
z of aluminum, iron \ / phosphates.
2 and mango ness, and
by soluble forms of
aluminum, iron ond
manganese.
I I I I I
4.0 5.0 6.0 7.0 8.0 9.0 10.0
Soil pH
Figure 13. Form of Phosphorus Fixation as a Function of Soil pH. Above
Illustrates Average Conditions and no Inference Should be Made
to any Particular Soil
45
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Effect of Reducing Conditions
Water-saturated soils in the presence of decomposing organic matter cause reducing
(anaerobic) conditions. Oxidizing or reducing conditions can be determined by eH measure-
ments (the determination of the redox-potential).
Reducing conditions in soils increase phosphorus fixation especially in soils high in
phosphorus (Popvic, 1958). This has been attributed to the greater solubility of iron
(Fe 3 + e-’--Fe 2 ) after reduction occurs and the pH is lowered. Mack(1959) found that par-
tially anaerobic conditions increased the proportion of acid-soluble phosphorus fraction, while
aerobic conditions (oxidizing conditions) decreased the acid-soluble aluminum-phosphate frac -
tion and increased the alkali-soluble phosphorus fraction.
Effect of Temperature
Mack (1959) found that an increase in temperature increased the amount of water-soluble
phosphorus that could be leached from a soil system. He noted also that a greater amount of
phosphorus could beleachedfrom a soil thathad been frozenthan onethathad notbeenfrozen.
Effect of Organic Matter
From a scant amount of research, it appears that organic matter and derivatives of
organic matter may retard or enhance phosphate fixation. A study of the retention of phos-
phate ions by Fe(OH) 3 , CaCO 3 , and clays from solutions containing H 3 P0 4 , ammonium nitro-
humate, ammonium humate and sodium humate indicated that, in general, humates retarded
phosphate fixation (Hashimoto and Okuda, 1962). However, in certain cases (e.g., Fe(OH) 3 ),
phosphorus fixation was enhanced.
Effect of Phosphorus Concentration and Time of Reaction on Fixation
Sen Gupta and Cornfield (1963) found that in soils, having either a high (16.5 ) or low
(0.15%) calcium carbonate percentage, the phosphorus fixation of added phosphorus increased
as the reaction progressed; usually the fixation decreased when increasing amounts were
applied. A somewhat different observation was made by Benku et al. (1963) on the adsorption
of P 32 - labeled KH 2 PO 4 by a gray-brown podzolic soil. Phosphorus fixation increased as the
time and concentration of the reaction solution increased.
46
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Reactions of Phosphorus Compounds in Soils
Inorganic Phosphorus Fixation Mechanisms
This discussion has shown that phosphorus becomes fixed in soils, but the mechanisms
of phosphorus fixation have been mentioned only briefly. Fixation is the process whereby
readily soluble compounds are changed into less soluble forms by reaction with inorganic or
organic components of the soil with the result that the compounds become restricted in their
mobility in the soil (Hemwall, 1957a).
Inorganic phosphorus in soils exists almost entirely as salts of orthophosphoric acid.
These salts can be classified generally as oxy-, fluoro- and hydroxy-phosphates of iron,
aluminum, calcium, titanium, magnesium, and manganese.
In all fixation reactions in which phosphorus is involved, one or more of the ionic forms
of orthophosphoric acid may be present. Buehrer (19 32), using ionization constants for H 3 P0 4 ,
calculated the relation of pH to the relative concentration of the undissociated H 3 P0 4 and the
three ionic species, H 2 P0 4 , HPO 2 , and PO, 3 . Over the pH range of 4.0 to 8.0 (this range
encompasses the majority of most agricultural soils) the principal ionic species in the soil
are H 2 PO and HPO 2 . The monovalent ion, H 2 PO , would be dominant below pH 6.7; above
this value the divalent ion, HPO 2 , would dominate. The trivalent form becomes more im-
portant than the monovalent ion above pH 9.0; however, even at pH 12, the concentration of
HPO 2 is still relatively greater than the concentration of PO 3 . Obviously, all phosphate
reaction systems will be influenced by the hydrogen-ion activity in the reaction system.
Research to date indicates that the reaction mechanisms responsiblefor phosphorus fix-
ation are of three general types:
(1) adsorption;
(2) isomorphic replacement; and
(3) double decomposition reaction involving solubility—product relationships.
Adsorption . The adsorption phenomenon is due to the attraction or repulsion between a
solid surface and, in this case, a solution. This attraction or repulsion is a result of inter-
action between fields of force emanating from the surface of the adsorbent and the ions of the
adsorbate. (Adsorption is discussed in the article by Bailey and White, 1964.) In this case,
the liquid phase corresponds to the soil solution; the solid phase corresponds to the clay min-
erals, oxides and hydroxides of iron and aluminum, and the humic component of the soil
organic matter.
Adsorption reactions may be classified generally into two types: (1) chemical adsorption
and (2) physical adsorption. Both types may be characterized by the Freundlich adsorption
isotherm or by the Langmuir adsorption isotherm.
47
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Adsorption has been suggested as one of the mechanisms responsible for the fixation of
phosphorus by soils, but the mechanism has not been clearly defined or described. In many
cases, investigators have called the uptake of phosphorus by the soil “adsorption”, whereas
no direct evidence has substantiated this. To ascertain unequivocally whether true adsorption
occurs, the problem should be studied by infrared and nuclear magnetic resonance spectros-
copy.
The amount of phosphorus taken up by soils is proportional to the concentration of phos-
phorus initially present. Davis (1935) and Kurtz et al. (1946) showed that the fixation of
phosphorus by soil could be described by the Freundlich adsorption isotherm, E-. kc -. Low
and Black (1950) and Mallons and Barkoff (1953) showed that fixation of phosphorus at lower
concentrations by kaolinite followed the Freundlich adsorption isotherm. Fisher (1922) pointed
out that agreement of data with the results of the Freundlich adsorption isotherm cannot be
considered as a criterion of adsorption. Kaithoff (1922) has shown that the Freundlich adsorp -
tion isotherm can represent exchange reactions when applied to ions, and it was pointed out
by Hsu and Rennie (1962a) that the Langmuir adsorption isotherm has certain limitations when
applied to ions in solution. Fixation of phosphorus by calcium carbonate and by heterogenous
soil systems has been shown to conform with the Langmuir adsorption isotherm; this has been
taken as absolute evidence of adsorption (Cole et al., 1953; Fried and Shapiro, 1956; Olson
and Watanabe, 1957; Rennie and McKercher, 1959; Weir and Soper, 1962; and Woodruff and
Kamprath, 1965).
The Langmuir adsorption isotherm was initially based on the assumptions that gas par-
ticles move in free space and that no force other than the attraction of solids interfered with
their kinetic movement. However, the kinetic movement of the phosphate ion in an aqueous
solution is affected by other ions. The Langmuir adsorption isotherm is valid only when the
interfering force, due to interacting ions, is negligible. In a phosphate-aluminum hydroxide
system, the activity of the aluminum ions (interfering force) is negligible above pH 5.
If phosphorus is adsorbed by the solid phase, or if other reactions occur, a reaction rate
study would indicate whether adsorption is the sole reaction mechanism. Hsu and Rennie
(1962a), in a study of the fixation of phosphorus by X-ray amorphous “aluminum hydroxide”
in the pH range of 3.8 to 7, found that the initial reaction was completed within 30 minutes,
and the authors attributed this to adsorption. However, this reaction was followed by a slow
decomposition-precipitation process. Low and Black (1947), in a study of phosphorus fixa-
tion by kaolinite, concluded that the short-time reaction (1 1/2 to 3 hours) and low concen-
tration (up to 0.0016 M) were probably adsorption reactions; other reactions dominated at
higher concentration (1.0 M) and after longer contact (8 to 11 days).
48
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Isomorphic Replacement . (1) Isomorphic replacement of hydroxyl or silicate anions
from the crystal--such a reaction could be called anion exchange; this reaction involves
silicate minerals such as the clay minerals and possibly the hydrated oxides of iron and
aluminum, where the phosphate ion would substitute for hydroxyl ions under normally acid
conditions (Mattson, 1930; Scarseth, 1935; Kelley and Midgley, 1943; Coleman, 1945; Low and
Black, 1947; Swenson et al., 1949). Evidence for this reaction to occur is based on the
following observations:
(a) Clay suspension and a phosphorus solution of similar pFI are mixed and an increase
in pH (due to increase in the hydroxyl ion concentration) accompanies the disappear-
ance of phosphorus ions from solution.
(b) Phosphate ions fixed by soils can be displaced to some extent by hydroxyl, fluoride,
silicate, and arsenate anions, thus implying anion exchange.
(c) Phosphorus fixation by soil colloids increases the amount of silicon in the super-
natant solution.
A general reaction, representing the fixation of phosphorus by minerals under acid con-
ditions and considering all of the fixing agents as having a replaceable OH, is as follows:
Colloid - Colloid
Micelle OH ± H 2 P0 4 Micelle H 2 P0 4 + OH
Solution Fixed
It seems possible that adsorbed phosphorus could be transformed subsequently to a
hydroxyl- or silicon-ion by isomorphic replacement in the crystal.
(2) Crystallization of new compounds. Low and Black (1947) showed that the fixation of
phosphorus by kaolinite involved an actual breakdown of the silicate mineral into aluminum
and silicate ions in accordance with the solubility-product principle. They also showed that
the phosphorus precipitated the aluminum, resulting in the formation of a basic aluminum
phosphate. By use of electron microscopy and X-ray diffraction techniques, Kittrick and
Jackson (1955) showed that kaolinite will “dissolve ” in the presence of phosphate at 90° C and
that an aluminum phosphate is formed, leaving no trace of kaolinite after a reaction period of
31 days. Reifenberg and Buchwold (1954) demonstrated that treatment of both soils and clay
minerals with excess phosphate resulted in lattice decomposition and formation of new
products.
Recent work on the identification of decomposition products of phosphorus-silicate-
minerals reaction products and the reaction of various fertilizers with soils has partially
indicated the nature of the reaction products. Using chemical, optical, and X-ray diffraction
methods, Haseman, et al. (1958) identified some of the crystalline products produced by in-
teraction of phosphate with illite, kaolinite, montmorillonite, goethite, and limonite. The
49
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ucts were members of an isomorphic series with the general composition [ (H, K, Na,
NH 4 ) 3 x (FeAlf P0 4 . nH 0 , where x is greater than zero but less than 3. Other products
were identified as members of the variscite-strengite isomorphic series--(Al, Fe)-
P0 4 . 21120 (x + y = 1).
Major initial reaction products of monoammonium and diammonium phosphate solutions
after 10 days in a fine sandy loam (pH 7.7; 6.6% CaCO 3 ; 6 ppm NaHCO 3 extractable phos-
phorus) appeared to be dicalcium phosphate dihydrate and hydroxyapatite (Beaton et al,
1963).
Some crystalline material of the variscite-strengite isomorphic series was found to be
the ultimate product of phosphorus applied to acid soils (Wright and Peech, 1960).
In the absence of K and NIH 4 , dissolution of gibbsite (aluminum hydroxide in an acidic
aqueous solution of 113 P0 4 and CaH 4 (P0 4 ) 2 precipitated a mixture of CaHPO 4 , CaA1 (HPO 4 ) 2
and amorphous material (Taylor et al., 1964); while reaction of goethite (Fe 2 0 3 . H 2 0) with
the same acidic solution of H 3 P0 4 and CaH 4 (P0 4 ) 2 precipitated H 4 CaFe 2 (P0 4 ) 2 and amor-
phous FePO 4 .
Reaction of KPO 3 with CaCO 3 at a pH of 6 to 9 produced mainly hydroxyapatite, but
Ca 8 H 2 (P0 4 ) 6 or CaHPO 4 2H 2 O were also formed, especially at pH 6 (Cecconi and. Ristori,
1964).
From a consideration of solubility-product relationships, it appears that fixation of
phosphorus should occur through precipitation with iron, aluminum, and calcium. In certain
cases, precipitation with manganese may also be important. However, the question remains
as to which reactions are the more important under differing environmental conditions of
soil acidity, initial phosphorus concentration, clay mineral type, etc.
Hemwall (1957a), after reviewing research on phosphorus fixation, concluded that:
(a) Phosphorus fixation in acid soil resulted primarily from the formation of aluminum
and iron compounds of the nature M (H 2 O) 3 (OH) 2 H 2 P0 4 ; M stands for the iron or alum i-
num cations. The sources are the aluminum- and iron-containing soil minerals
including clay minerals (ion source as an exchangeable cation, or from decomposi-
tion of alumino-silicates). Under certain conditions, the phosphate-reaction prod-
ucts form a precipitate; while under a different set of conditions, they are adsorbed
or formed as a result of a surface reaction. Whether the compound is formed on
the surface of a soil mineral or is precipitated, the compounds formed and the
mechanism of reaction appear to be essentially the same; that is, iron and alumni-
num compounds and clay minerals both fix phosphorus by the same method. The
50
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overall fixation of phosphorus by acid soils can be visualized by the following
general reaction:
[ M(Ho)A] +3 to +1 + A + H 2 PO - [ M(H 2 o) AH 2 PO ]
1 Precipitated or adsorbed
M 2 0 3 , M(OH) 3
Clay mineral and
exchange site
where M stands for the cations of iron or aluminum and A stands for oxide or
hydroxide.
(b) Phosphorus fixation in calcareous and alkaline soils is due to the formation of in-
soluble phosphate compounds in a solid solution series. Also, in certain instances,
phosphorus can be fixed by the Al 3 or Al(OH) 3 in the clay fraction of a1i aline soils,
the extent of fixation is affected by the other cations present.
More recent research results have verified Hernwall’s conclusions. Results indicate
that aluminum is more important than iron in phosphorus fixation (Leaver and Russell, 1957;
Sani and MacLean, 1965). Leaver and Russell studied the use of blocking agents to measure
the relative importance of iron and aluminum oxides in sorption of phosphorus by acid soils.
The “blocking agents” used included potassium ferrocyanide, selenious acid, quinalizarin,
oxine, cupferron, alizarin, and Aluminon. It was found that the power to fix phosphorus in
both the soils containing hydrated oxides and hydroxides was reduced with the “blocking
agents,”due to the formation of insoluble or un-ionized compounds containing iron and
aluminum ions. Interestingly, the soil and oxide and hydroxides “fix” more blocking agent
than was evidenced by the decrease in phosphorus fixation. The authors believed that it is not
possible to fix a sufficient amount of blocking agent to eliminate phosphorus fixation entirely.
Blanchet (1959a) showed that Fe(OH) 3 , and especially A1(OH) 3 , possessed a greater capa-
city to fix phosphate ions than CaCO 3 or the clay minerals, kaolinite and montmorillonite.
Using isotopic dilution, Blanchet (1959b) studied the reaction of a colloidal suspension of
Fe(OH) 3 in O.O1N sodium acetate with increasing concentrations of Na 2 HPO 4 . He used an
adsorption period of 5 months and an isotopic dilution period of 3 days. He concluded that
fixation of phosphorus is similar to adsorption and is not due to the precipitation of an iron
phosphate.
Double Decomposition Reaction Involving Solubility Relations . Hsu and Rennie (1962a)
derived a unified equation for the adsorption and precipitation of phosphate in an aluminum
system (the aluminum system consisted of X-ray amorphous “aluminum hydroxides”). The
equation was based on the assumptions that adsorption and precipitation result from the
51
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chemical union of aluminum with phosphate and that the adsorption equilibrium is related to
the solubilityof aluminum phosphate. The equation, as given below, was derived from: (1) the
solubility of aluminum phosphate [ A1(oH) 2 11 2 P0 4 ] , as defined by K 1 (Al 3 )(H 2 PO )(0H) 2
and (2) the adsorption of phosphate by Al (OH) 3 as described by R-OH + H 2 PO 4 RH 2 PO 4 + 0H;
where R is the radical of the Al(OH) 3 colloid, R-OH is the potential site for the adsorbing
phosphates, and R-H 2 P0 4 is the adsorbed phosphate. The equation is:
K 1 = ( R-OH ) + (Al 3 ) (H 2 PO ) (0H) 2
(R-H 2 Po 4 ) (0H) 3
The equation suggests that the presence of phosphorus in the soil solution (in a leachable
form) is governed by the total activity of ions that can f ix phosphate (i.e., the aluminum in the
system). Thus the activity of the aluminum in determining the solubility of aluminum phos-
phate should include both its activity on the surface and that in solution. It should be possible
to show that iron would react in a similar manner. In general, aluminum would probably be
more important in the fixation of phosphorus than iron because aluminum is more abundant
structurally in alumino-silicates and can thus become exchangeable under acid conditions due
to autotransformation of the layer-lattice silicate by the hydronium ion. This can be repre-
sented by the following:
VI IV \TI IV
K 2 (Al 4 ) (Si 6 A1 2 ) (OH) 4 020 + 3H 3 0LK 2 (A1 3 H 3 ) (Si 6 A1 2 ) (011)4 020 - - Al 3 + 31120
(Muscovite)
The aluminum ion can then either be exchanged for another ion on the cation exchange
complex or hydrate to [ Al(H 2 O) then hydrolyze to the monomer [ Al (OH)(H 2 0) 5 1 +2 +
H 3 0+, and afterwards polymerize. The exact nature of the resulting polymer varies accord-
ing to the pH (the ratio). Hsu and Rich (1960) postulated that [ Al 6 (OH) 12 (H 2 0) 1 J +6 is
formed and adsorbed by clays. Jackson (1963) discusses the alumino-hexa-hydronium ion,
its role in (1) soil acidity and liming, (2) cation exchange of soils and clays, and (3) the anion
retention by soils.
From a study on the availability and utilization of phosphates in Hawaiian soils, DeDatta
(1964) concluded that the tendency of a soil system to fix phosphorus depends, in decreasing
order of importance, on the presence of the following: amorphous hydrated oxid es, goethite,
gthbsite, kaolinite, and 2:1 clays. Addition of lime (decreasing soil acidity) to a pH of 5.5 re-
sulted in the precipitation of a large amount of active aluminum and in an increased phos-
phorus solubility.
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The work of Chang and Jackson (1958) indicated that chemical weathering in soils can
change the state or form of phosphorus in the soil. One effect of liming on the form of phos-
phorus in soil is to favor the formation of calcium phosphate and the release (through the re-
pression of aluminum and iron) of phosphate from the solid aluminum and iron phosphates.
The importance of aluminum over iron in phosphorus fixation was shown by Bromfield (1964).
Using a biological reduction method to remove iron, but not aluminum, the phosphorus fixation
capacity of three acid soils was not removed; whereas, when aluminum was removed, the
phosphorus sorptive capacity decreased.
Olsen and Watanabe (1957) found that phosphorus sorption was closely related to surface
area, that acid soils retained more phosphorus per unit surface than alkaline soils, and that
acid soils also held the phosphorus with a greater bonding strength than the alkaline- soils did.
On the basis of solubility product calculations, most investigators have assumed that in
the soils the ion form would be H 2 P0 4 and that it was thus fixed as this ion. The infrared
work of Gastuche et al. (1963) indicated that this is not always the case; the ionic form may
be a function of the substrate and the concentration of the phosphorus in a solution. Treat-
ment of iron and aluminum compounds with 0.O1M KH 2 PO 4 resulted in the fixation of the P0 4
anion; while reaction of aluminum compounds with 1 M KH 2 PO 4 resulted in the fixation of 112 P0 4 .
The effect of exchangeable aluminum in phosphorus fixation was further emphasized by
Coleman et al. (1960). Phosphorus sorbed by a subsoil was correlated with exchangeable
aluminum; the equation was: Y = 0.077 + 0.703X, wherex amount of exchangeable aluminum.
Removal of exchangeable aluminum by leaching with a KCI solution reduced phosphorus
sorption except in those soils where the level of exchangeable aluminum was low or where
the iron and aluminum oxide content was high. The amount of phosphorus was stoichiometrically
equal to the concentration of exchangeable aluminum under conditions where aluminum hy-
drolysis is favored.
The mechanism of lattice decomposition and resultant release of aluminum and the forma-
tion of aluminum phosphate was supported by the work of Tamini et al. (1964). Reaction of the
clay minerals, montmorillomte and kaolinite, with solutions of (NH 4 ) 2 HP0 4 at a low PH re-
sulted in diminished characteristic X-ray diffraction peaks for these two minerals. The
authors believed that the reaction product was taranakite (K 2 Al 6 [ o 4 ] 6 [ 01112 . 18 H 2 0).
Organic materials have been shown to influence phosphorus sorption on clay minerals and
soils (Fokin and Chistova, 1964). The authors believed that the fixation of phosphorus by
kaolinite and by soils coated with organic material is as follows: the initial sorption of phos-
phorus in low concentration isby exchange with a film of organo-mineral gel surrounding the
53
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particles comprising the mineral fraction of the soil; while at a high phosphorus concentra-
tion, the film becomes saturated with phosphorus; and at still higher concentrations, the
phosphorus diffuses through the film and reacts with the mineral components of the soil.
It does not appear in all cases that the aluminum content and the presence of aluminum-
containing minerals are the most important factors in phosphorus fixation. Demolon and
Muller (1954) from a study of phosphate (P0 4 ) diffusion, initially present as CaH 4 (P0 4 ) 2 , in
alumino- and ferrosilicates at various pH levels, found that there was less diffusion in the
iron system than in the alumino-silicate system. The experiment again pointed out the major
effect of acidity on phosphorus fixation. At pH 3 to 4.5, the phosphate precipitated in imme-
diate contact with the phosphorus granule; at pH 5 to 6.5, rapid migration without precipita-
tion occurred: while at a pH greater than 7, diffusion resulted in a series of concentric rings
of precipitated phosphate. Humic acid added to the gels kept the phosphate in a more water-
soluble form.
Organic Phosphorus Fixation Mechanisms
It is quite conceivable that some phosphorus in waste effluents might be present in the
organic form: therefore, information on whether organic phosphorus would be removed from
a percolating solution is important.
A portion of the total phosphorus in soils is organic in nature. Only a minor part of the
organic phosphorus of soils appears to be present as nucleic acids and phospholipids. Inositol
phosphates form the major part of the soil organic phosphorus thus far identified.
Of particular importance is the evidence cited by Bower (1949) that nucleic acids are
adsorbed by clay minerals. Such adsorption was greater on bentonite than on kaolinite, and
the magnitude increased with decreasing pH. One would expect that the greater adsorption
would be by bentonite because of its higher exchange capacity: also with increasing acidity,
the basicity of the compounds increases.
Further work on the nature and character of the adsorption of nucleic acids by clay
minerals was carried out by Goring and Bartholomew (1952). These authors showed that the
reaction was: (1) rapid, (2) temperature dependent, (3) pH dependent (increased adsorption
with decreasing pH), (4) cation dependent (greater adsorption when calcium or magnesium
was present on the exchange sites, rather than sodium or potassium), (5) reversible in nature,
(6) linearly related to the cation exchange capacity of the various clay fractions and (7)
nucleic acids at least partially adsorbed interlamellarly. Further, the authors concluded that
the nucleic acids were adsorbed by a cation exchange reaction and also through the ortho-
phosphate groups.
54
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Jackman and Black (1951) found that iron and aluminum salts of inositol phosphates
were very insoluble in an acidic solution above pH 2 to 3. Because the pH solubility curves
are very similar to the inorganic orthophosphates, organic compounds such as phytin and
their derivatives should be fixed in soils as insoluble salts of aluminum, iron, and calcium in
a manner similar to inorganic phosphates.
It appears that soil minerals other than clay minerals adsorb inositol phosphates. Ander-
son and Arlidge (1962) found that boehmite, ferric oxide gel, and a soil high in sesquioxides
all adsorbed (3 —glycerophosphate. In general, the extent of adsorption was related to the number
of phosphate groups in the molecule: the higher the number of phosphate groups in the mole-
cule, the greater was the degree of adsorption. There was also a relationship between pH,
nature of adsorbent, and extent of adsorption. Maximum adsorption of inositol hexaphosphate
was found in two distinctly acid media (pH 3 to 4) and in an acid concentration of about 0. iN.
Montmorillonite was most reactive atpH 4. The author attributed this to maximum content of
active aluminum. Ferric oxide gel removed a considerable amount of inositol phosphate from
a strongly acid solution, presumably partly by adsorption and partly by precipitation of in-
soluble salts or complexes of the ester. In a less acid medium (pH 3 - 4), the gel was rela-
tively ineffective in removing the organic phosphate. Boehmite was highly reactive over a
wide range of pH. From pH 4 down to a concentration of 4 with respect to HC1, greatest
adsorption occurred at pH of approximately 1. Boehmite adsorbed more inositol mono-, di-,
and tn-phosphates aswellas/3-glycerophosphate in 1 HC1 than in 0.1w HCI. X-ray diffract-
tion did not indicate the presence of an ordered clay-organic complex.
5 5/56
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SELECTED BIBLIOGRAPHY
Abel, A. L. The substituted urea herbicides. Chem. md. (London) . p. 1106 (1957)
Allison, F. E., M. Kefauver and E. M. Roller. Ammonium fixation in soils. Soil Sci. Soc.
Am. Proc . 17:107 (1953)
Allison, F. E., E. M. Roller and J. H. Doetsch. Ammonium fixation and availability in
vermiculite. Soil Sc 75:173 (1953)
Anderson, D. M. and P. F. Low. The density of water absorbed by lithium, sodium and
potassiurn-bentonite. Soil Sci. Soc. Am. Proc . 22:99 (1958)
Anderson, G. and E. Z. Arlidge. The adsorption of inositol phosphates and glycerophosphates
by soil clays, clay minerals and hydrated sesquioxides in acid media. 3. Soil Sci .
13:216 (1962)
Ashton, F. M. Movement of herbicides in soil with simulated furrow irrigation. Weeds 9:612
(1961)
Bailey, G. W. and J.L. White. Review of adsorption and desorption of organic pesticides by
soil colloids with implication concerning pesticide bioactivity. J. Agr. Food Chem .
12:324 (1964)
Barshad, I. Cation exchange in micaceous minerals.I. Replaceability of the interlayer
cations of vermiculite with ammonium or potassium. Soil Sci . 77:463 (1954a)
Barshad, I. Cation exchange in micaceous minerals. II. Replaceability of ammonium and
potassium from vermiculite, biotite, and montmorillonite. Soil Sci . 78:57 (1954b)
Bates, T. E. and S. L. Tisdale. The movement of nitrate-nitrogen through columns of
coarse-textured soil material. Soil Sci. Soc. Am. Proc . 2 1:525 (1957)
Bayer, L. D. and H. F. Winterkorn. Sorption of liquids by soil colloids. II. Surface behavior
in the hydration of clays, Soil Sci . 40:403 (1935)
Bear, F. E. and S. J. Toth. Phosphate fixation in soil and its practical control. md. Eng.
Chem . 34:49 (1942)
Beaton, 3. D., T. L. Charlton and R. Speer. Identification of soil-fertilizer reaction products
in a calcareous Saskatchewan soil by infrared adsorption analysis. Nature 197:1329
(1963)
Benko, V., J. Hampl, and L. Vnuk. Contribution to the study of fixation and liberation of
phosphorus in gray-brown podzolic and podzolized soils. Rostl. V’yroba 36:1206
(1963) ( Soils and Ferts . 27:1305)
Benson, N. and R. M. Barnette. Leaching studies with various sources of nitrogen. 3. Am.
Soc. Agron . 31:44 (1939)
Birk, L. A. Penetration of and persistence in soil of the herbicide 3-(p-chlorophenyl)-1,
1-dimethylurea (CMU). Can. J. Agr. Sci . 35:377 (1955)
57
-------
Bizzell, J. A. and T. L. Lyon. Composition of drainage waters from lysimeters at Cornell
University. Trans. Intern. Congr. Soil Sci. 1st, Madison, Wis . 11:342 (1927)
Black, C. A. Phosphate fixation by kaolinite and other clays as affected by pH, phosphate
concentration and time of contact. Soil Sci. Soc. Am. Proc . 7:123 (1943)
Black, C. A. Soil-Plant Relationships . Wiley, New York (1957)
Blanchet, R. Adsorption and isotopic dilution of phosphate ions in contact with ferric
hydroxide. C. R. Acad. Sci., Paris . 248:1028 (1959a) ( Soils and Ferts . 22:966)
Blanchet, R. The phosphorus -fixing capacity of various soil constitutents and estimation
of the reserves in the soil. C. R. Acad. Agric. Fr . 45:247 (1959b) ( Soils and Ferts .
22:2046)
Bower, C. A. Studies on the form and availability of soil organic phosphorus. Iowa Agr.
Exper. Sta. Res. Bull . 362 (1949)
Bower, C. A. Fixation of ammonium in difficultly exchangeable form under moist conditions
by some soils of semiarid regions. Soil Sci . 70:375 (1950)
Broadbent, F. E. and H. D. Chapman. A lysimeter investigation of gains, losses and balance
of salts and plant nutrients in an irrigated soil. Soil Sci. Soc. Am. Proc . 14:261
(1949)
Broadbent, F E., W. D. Burge and T. Nakashima. Factors influencing the reaction between
ammonia and soil organic matter. Trans. Intern. Congr. Soil Sci. 7th Madison, Wis .
2:509 (1960)
Bromfield, S. M. Relative contribution of iron and aluminum in phosphate sorption by acid
surface soils. Nature . 201:321 (1964)
Brown, J. M. and W. V. Bartholomew. Sorption of anhydrous ammonia by dry clay systems.
Soil Sci. Soc. Am. Proc . 26:2 58 (1962)
Brown, J. M. and W. V. Bartholomew. Sorption of gaseous ammonia by clay minerals as
influenced by sorbed aqueous vapor and exchangeable cations. Soil Sci. Soc. Am.
Proc . 27:160 (1963)
Brown, W. 0. A comparative laboratory study of phosphate relationships in some soils
derived from basaltic and Silurian parent materials. Res. Exp. Rec., Minist. Agr.,
N. Ireland 3:142 (1952) ( Soils and Ferts . 18:19 10)
Buehrer, T. F. The physico -chemical relationships of soil phosphates. Arizona Agr. Exp.
Sta. Tech. Bull . 42 (1932)
Burnside, 0. C., E. L. Schmidt and H. Behrens. Dissipation of simazine from the soil.
Weeds . 9:477 (1961)
Burnside, 0. C., C. R. Fenster and G. A. Wicks. Dissipation and leaching of monuron,
simazine and atrazine in Nebraska soils. Weeds . 11:209 (1963)
58
-------
Buswell, A. M. and B. F. Dudenbostel. Spectroscopic studies of base exchange materials.
J. Am. Chem. Soc . 63:2554 (1941)
Calvert, D. V. Adsorption of nitrate by corn as related to movement of nitrate and water in
soil. Diss. Abst . 23:778 (1962)
Cecconi, S. and G. Ristori. Investigation on the phosphate enrichment of soil. III. The
relationship between calcium carbonate and phosphate ions. Agrochimica . 8:362
(1964) ( Soils and Ferts . 28:705)
Chang, S. C. and M. L. Jackson. Soil phosphorus fractions in some representative soils.
J. Soil Sci . 9:109 (1958)
Chapman, H. D., C. F. Liebig and D. S. Rayner. A lysimeter investigation of nitrogen gains
and losses under varying systems of covercropping and fertilization, and a dis-
cussion of error sources. Hilgardia . 19:57 (1949)
Chin, W. T. and W. Kroontje. Mechanisms of urea adsorption by soils. Soil Sci. Soc. Am.
Proc . 26:479 (1962)
Chin, W. and W. Kroontje. Urea hydrolysis and subsequent loss of ammonia. Soil Sci. Soc.
Am. Proc . 27:316 (1963)
Cleason, G. H. and A. C. Loonam. The development of sewage. Sewage Work J . 5:1:6 1
(1933)
Cole, C. V., S. R. Olsen and C. 0. Scott. The nature of phosphate sorption by calcium
carbonate. Soil Sci. Soc. Am. Proc . 17:352 (1953)
Coleman, N. T., J. T. Thorup, and W. A. Jackson. Phosphate-sorption reactions that involve
exchangeable Al. Soil Sci . 90:1 (1960)
Coleman, R. The mechanism of phosphate fixation by montmorillonite and kaolinite clays.
Soil Sci. Soc. Am. Proc . 9:72 (1945)
Collison, R. C. and J. E. Mensching. Lysimetry investigations: 1. Nitrogen and water
relations of crops in legume and non -legume rotations. New York (Geneva State
Agr. Exp. Sta.) Tech. Bull . 166:3 (1930)
Conrad, J. P. Hydrolysis of urea by thermolabile catalysis. Soil Sci . 49:253 (1940a)
Conrad, J. P. The nature of the catalyst causing the hydrolysis of urea in soils. Soil Sci .
50:119 (1940b)
Cornell, Howland, Hayes and Merryfield. Preliminary report on a tertiary waste treatment
plant. South Tahoe Public Utility District (1963)
Dahnke, W. C., J. L. Malcolm, and M. E. Menendez. Phosphorus fractions in selected soil
profiles of El Salvador as related to their development. Soil Sci . 98:33 (1964)
Davis, L. E. Sorption of phosphates by non-calcareous Hawaiian soils. Soil Sci . 40:129
(1935)
59
-------
De Datta, S. K. Availability of phosphorus and utilization of phosphate fertilizers in some
great soil groups of Hawaii. Diss. Abst . 25:716 (1964)
Demolon, A. and J. Muller. Kinetic Study of the reactions of mono-calcium phosphate in
gels. Bull. Docum. Ass. mt. Fabr. Superph . 16:3 (1954) ( Soils and Ferts . 18:575)
Dewey, 0. R. Fate of simazine in soils. Proc. Brit. Weed Control Conf., 5th Brighton 1960.
p. 91 (1961)
De Wit, C. T. and P. L. Arens. Moisture content and density of some clay minerals and
some remarks on the hydration patterns of clay. Trans. 4th Intern. Cong. Soil Sci .
2:59 (1950)
Dutt, G. R. and P. F. Low. Relationship between the activation energies for deuterium oxide
diffusion and exchangeable ion conductance in clay systems. Soil Sci. 93:19 5 (1962a)
Dutt, G. R. and P. F. Low. Diffusion of alkali chlorides in clay-water systems. Soil Sci .
93:233 (1962b)
Edwards, A. P. Replaceability of fixed ammonium in clay minerals. Diss. Abst . 2 1:1752
(1961)
Eitel, W. Silicate Science, Volume I: Silicate Structures . Academic Press, New York
(1964)
Fisher, E. A. The phenomena of adsorption in soils: a criteria discussion of hypothesis
put forward. Trans. Faraday Soc . 17:305 (1922)
Fokin, A. D. and E. D. Chistova. Effect of humic substances on phosphate sorption by soils.
Agrokhimiya 11:41 (1964) ( Soils and Ferts . 28:2209)
Fried, M. and R. E. Shapiro. Phosphate supply pattern of various soils. Soil Sci. Soc. Am.
Proc . 20:471 (1956)
Gastuche, M. C., J. J. Fripiat and S. Sokoiski. Phosphorus fixation by amorphous and
crystalline hydroxides of iron and aluminum. Pedologie 13:155 (1963) ( Soils and
Ferts . 27:1932)
Geissbiihler, H., C. Haselbach and H. Aebi. The fate of N-(4-chlorophenoxy)-phenyl-NN-
dimethylurea (C -1983) in soils and plants. 1. Adsorption and leaching in different
soils. Weed Res . 3:140 (1963)
Gieseking, J. E. The clay minerals in soils. Adv. Agron . 1:159 (1949)
Goring, C. A. T. and W. V. Bartholomew. Adsorption of mononucleotides, nucleic acids
and nucleoproteins by clay. Soil Sci . 74:149 (1952)
Grim, R. E., R. H. Bray, and W. F. Bradley. The mica in argillaceous sediments. Am.
Mineral . 22:813 (1937)
Grim, R. E. Clay Mineralogy . McGraw-Hill, New York (1953)
Grim, R. E. Applied Clay Mineralogy . McGraw-Hill, New York (1962)
60
-------
Haas, A. Investigation of phosphorus fixation and mobilization on the soil types of Hungary.
Agrokem. Talajt . 8:331 ( 1959)(Soils and Ferts . 23:1009)
Hanway, J. J. and A. D. Scott. Ammonium fixation and release in certain Iowa soils. Soil
Sd. 82:379 (1956)
Haseman, J. F., E. H. Brown, and C. D. Whitt. Some reactions of phosphate with clays and
hydrous oxides of iron and aluminum. Soil Sci . 70:257 (1950)
Hashimoto, Y. and A. Okuda. Effect of humates and nitrohumates on phosphorus fixation in
soil. J. Soil Sci. Tokyo 33:275 (1962)
Harris, C. I. and G. F. Warren. Determination of phosphorus fixation capacity in organic
soils. Soil Sci. Soc. Am. Proc . 26:381 (1962)
Helgeson, E. A. and H. N. Anderson. Leaching of alpha-chloro-N, N-dialylacetamide
(CDAA) in Fargo clay. Proc. N. Central Weed Control Conf. Res. Rept . 12:183
(1955)
Hemwall, J. B. and P. F. Low. The hydrostatic repulsive force in clay swelling. Soil Sci .
82:135 (1956)
Hemwall, J. B. The fixation of phosphorus by soils. Adv. Agron . 9:95 (1957a)
Hemwall, J. B. The role of clay minerals in phosphorus fixation. Soil Sd . 83:101 (1957b)
Hester, J. B. The influence of phosphate fertilization upon the amphoteric properties of
Coastal Plainsoils. J. Am. Soc. Agron . 29:10 (1937)
Hill, G. D., J. W. McGahen, H. M. Baker, D. W. Finnerty and C. W. Bingeman. The fate of
substituted urea herbicides in agricultural soils. Agron. J . 47:93 (1955)
Houregan, W. R. and W. A. Seay. Soil chemistry of fertilizer urea. Kentucky Agr. Exp.
Six. Annual Rept. of Director . 64:37 (1957)
Hsu, P. H. and C. I. Rich. Aluminum fixation in a synthetic cation exchanger. Soil Sci. Soc.
Am. Proc . 24:2 1 (1960)
Hsu, P. H. and D. A. Rennie. Reactions of phosphate in aluminum systems. I. Adsorption of
phosphate by x-ray amorphous “aluminum hydroxide”. Can. J. Soil Sci . 42:197 (l962a).
Hsu, P. H. and D. A. Rennie. Reactions of phosphate in aluminum systems. H. Precipitation
of phosphate by exchangeable aluminum on a cation exchange resin. Can. J. Soil Sci .
42:210 (1962b)
Hurwitz, E. R. Beaudoir and W. Walter. Phosphates: Their fate in a sewage treatment plant-
waterway system. Water and Sweage Works 112:3 (1965)
I1’in, V. B. The vertical movement of nitrates in chestnut loamy soils of the Kulundin steppe.
Poch . 4:94 (1959)
Jackman, R. H. and C. A. Black. Solubility of iron, aluminum, calcium and magnesium inositol
phosphates at different pH values. Soil Sci . 72:179 (1951)
Jackson, M. L. Aluminum bonding in soils: A unifying principle in soil science. Soil Sci.
Soc. Am. Proc . 27:1 (1963)
61
-------
Jackson, M. L. Chemical composition of the soil, in Chemistry of the Soil, ed. F. E. Bear,
ACS Monograph Series, Reinhold, New York (1964)
Joffe, J. S. and A. K. Levine. Fixation of potassium in relation to exchange capacity of
soils. Soil Sci . 63:151 (1947)
Joffe, J. S. Lysimeter studies: The translocation of cations in the profile of a Gray-brown
podzolic soil. Soil Sci. Soc. Am. Proc . 21:309 (1957)
Jenny, H. Factors of Soil Formation . McGraw-Hill, New York (1941)
Jenny, H., A. S. Ayres and J. S. Hosking. Comparative behavior of ammonia and ammonium
salts in soils. Hilgardia . 16:427 (1945)
Johnson, W. R., F. Ittthadieh, R. M. Daum and A. F. Pillsbury. Nitrogen and phosphorus in
tile drainage effluents. Soil Sci. Soc. Am. Proc . 29:287 (1965)
Kanwar, J. S. Phosphate retention in some Australian soils. Soil Sci . 82:43 (1956)
Kelley, J. B. and A. R. Midgley. Phosphate fixation and exchange of phosphate and hydroxyl
ions. Soil Sci . 55:167 (1943)
Kittrick, J. A. and M. L. Jackson. Rate of phosphate reaction with soil minerals and electron
microscope observations on the reaction mechanism. Soil Sci. Soc. Am. Proc .
19:292 (1955)
Kolaian, J. H. and P. F. Low. Thermodynamic properties of water in suspensions of
montmorilonite. Proc. Nat. Conf. Clays Clay Minerals . 9:71 (1962)
Kohnke, H. R., R. Dreibelbis and J. M. Davidson. A survey and discussion of lysimeters
and a bibliography on their construction and performance. USDA Misc. Pubi. 372 .
(1940)
Kohnke, H. Soil Physics , Purdue University (1963)
Koithoff, J. F. Die elektroadsorption als rein chemische Eracheinung. Kolloid-Z . 30:35
(1922)
Krantz, B. A., A. J. Ohlrogge and G. D. Scarseth. Movement of nitrogen in soils. Soil Sci.
Soc. Am. Proc . 8:189 (1944)
Kuhn, P. A. Removal of ammonia nitrogen from sewage effluent. M. S. Thesis, University
of Wisconsin. (1954)
Kurtz, T., E. E. De Truk and R. H. Bray. Phosphate adsorption by illinois soils. Soil Sci .
61:111 (1946)
Leaver, J. P. and E. W. Russell. The reaction between phosphate and phosphate -fixing
soils. J.SoilSci . 8:113(1957)
Leggett, G. E. and C. D. Moodie. The release of fixed ammonium from soils by sodium as
affected by small amounts of potassium or ammonium. Soil Sci. Soc. Am. Proc .
27:645 (1963)
62
-------
Low, P. F. and C. A. Black. Phosphate -induced decomposition of kaolinite. Soil Sci. Soc.
Am. Proc . 12:180 (1947)
Low, P. F. and C. A. Black. Reactions of phosphate with kaolinite. Soil Sci . 70:2 73 (1950)
Low, P. F. and D. M. Anderson. The partial specific volume of water in bentonite suspension.
Soil Sci. Soc. Am. Proc . 22:22 (1958a)
Low, P. F. and D. M. Anderson. Osmotic pressure equations for determining thermodynamic
properties of soil water. Soil Sci . 86:251 (1958b)
Low, P. F. Influence of adsorbed water on electrochemistry of clays. Trans. Intern. Cong.
Soil Sci. , 7th. 2:328 (1960a)
Low, P. F. Viscosity of water in clay systems. Proc. Nat. Conf. Clays Clay Minerals .
8:170 (1960b)
Low, P. F. Physical chemistry of clay-water interaction. Adv. Agron . 13:269 (1961)
Low, P. F. Effect of quasi-crystalline water on rate processes involved in plant nutrition.
Soil Sci . 93:6 (1962)
Lyon, T. L., H. 0. Buckman, and N. C. Brady. The nature and properties of soils , 5th Ed.,
MacMillan, New York, p. 295. (1952)
MacEwan, D. M. C. Complexes of clays with organic compounds. I. Complex formation
between montmorillonite and halloysite and certain organic liquids. Trans. Faraday
Soc. 44:349 (1948)
Maclimes, D. A. The principles of electrochemistry . Reinhold, New York (1939)
Mack, A. R. The influence of temperature and moisture on soil phosphorus as characterized
by plant uptake and chemical fractionation. Diss. Abst . 20:18 (1959)
Marshall, C. E. The physical chemistry and mineralogy of soils , Vol. 1: Soil materials .
Wiley, New York. (1964)
Mattson, S., E. Alvsaker and E. Koutler-Anderson. Phosphate relationships of soil and
plant. IX. Phosphate solubility in pedocal and cadalfer soils. K. Lantbr. Hogsky.
Ann . 20:19 (1953) ( Soils and Ferts . 17:473)
Mattson, S. and E. Barkoff. Phosphate relationship of soil and plant. X. Phosphate adsorp-
tion by soils as a function of concentration. K. Lantbr. Hogsk. Ann . 20:71 (1953)
( Soils and Ferts . 17:474)
McGauhey, P. H., R. Eliassen, G. Rohlich, H. F. Ludwig and E. A. Pearson. Comprehensive
study on protection of water resources of Lake Tahoe Basin through controlled
waste disposal. Board of Directors, Lake Tahoe Council. pp. 157 (1963)
Mielenz, R. C. and M. E. King. Identification of clay minerals by staining tests. Am. Soc.
Testing Mater. Proc . 51:1213 (1951)
Mielenz, R. C. and M. E. King. Physical-chemical properties and engineering performances
of clays. Calif. Div. Mines Bull . 169:196 (1955)
63
-------
Miller, R. J. and P. F. Low. Threshold gradient for water flow in clay systems. Soil Sci.
Soc. Am. Proc . 27:605 (1963)
Mitsui, S. and H. Takatoh. Soil adsorption of urea. II. An infrared spectrophotometric
analysis for a mechanism of adsorption of urea. Soil Sci. and Plant Nutrition
9:3:19 (1963)
Montgomery, M., V. H. Freed and S. C. Fang. Some aspects of the behavior of simazine
in soil. Res. Prog. Rept. Western Weed Control Conf . p. 62 (1958)
Montgomery, M. and V. H. Freed. A comparison of the leaching behavior of simazine and
atrazine in Chehalis sandy loam. Res. Prog. Rept. Res. Committee Western Weed
Control Conf . (1959)
Morgan, M. F. and H. G. M. Jacobson. Soil and crop interrelations of various nitrogenous
fertilizers. Windsor lysimeter series B. Conn. (New Haven) Agr. Exp. Sta. Bull .
458. (1942)
Mortland, M. M. Adsorption of ammonia by clays and mucks. Soil Sci . 80:11 (1955)
Mortland, M. M., J. J. Fripiat, J. Chaussidon and J. Uytterhoeven. Interaction between
ammonia and the expanding lattices of montmorillonite and vermiculite. J. Phy.
Chem . 67:248 (1963)
Mortland, M. M. Urea complexes with montmorillonite: An infrared absorption study.
Clay Minerals 6:143 (1966)
Nesselson, E. J. Removal of inorganic nitrogen from sewage effluents. Ph.D. Thesis,
University of Wisconsin. (1954)
Olsen, S. R. and F. S. Watanabe. A method to determine a phosphorus adsorption maximum
of soils as measured by the Langmuir isotherm. Soil Sci. Soc. Am. Proc . 2 1:144
(1957)
Oster, J. D. and P. F. Low. Heat capacities of clay, clay-water mixtures. Soil Sci. Soc.
Am. Proc . 28:605 (1964)
Owens, L. D. Nitrogen movement and transformation in soils as evaluated by a lysimeter
study utilizing isotopic nitrogen. Soil Sci. Soc. Am. Proc . 24:372 (1960)
Owen, R. Removal of phosphorus from sewage plant effluent with lime. Sewage md. Wastes
25:5: 548 (1953)
Page, J. B. and L. D. Bayer. Ionic size in relation to the fixation of cations by colloidal clay.
Soil Sci. Soc. Am. Proc . 4:150 (1940)
Pieczarka, S. J. Leaching and decomposition studies with pre -emergent herbicides. Weed
Abst . p. 47 (1961)
Popovic, Z. Phosphorus fixation and oxidation -reduction conditions in soil. Zemi. Bilj .
7:333 (1958) ( Soils and Ferts . 22:2578)
64
-------
Rachinskii, V. V. The use of the radiochromatographic methods in studying the dynamics of
the sorption, movement and distribution of phosphate ions in soil. Diki. Akad.
Nauka . 95:849 (1954) ( Soils and Ferts . 18:32)
Radoslovich, E. W. The structure of muscovite KA1 2 (Si 3 A1) 010 (OH) 2 . Acta. Cryst .
13:919 (1960)
Radoslovich, E. W. Surface symmetry and cell dimension of layer-lattice silicates. Nature
19 1:67 (1961)
Radoslovich, E. W. and K. Norrish. The cell dimensions and symmetry of layer-lattice
silicates. I. Some structural considerations. Am. Mm . 47:599 (1962a)
Radoslovich, E. W. The cell dimensions and symmetry of layer-lattice silicates. II.
Regression relations. Am. Mm . 47:617 (1962b)
Ragland, J. L. and W. A. Seay. The effects of exchangeable calcium on the retention and
fixation of phosphorus by clay fractions of soil. Soil Sci. Soc. Am. Proc . 2 1:261
(1957)
Reiche, P. A survey of weathering processes and products . The University of New Mexico
Press, Albuquerque (1945)
Reifenberg, A. and S. Buchwold. The release of silica from soils by the orthophosphate
anion. J. Soil Sci . 5:106 (1954)
Reitemeier, R. F. Soil potassium. Adv. Agron . 3:113 (1951)
Rennie, D. A. and R. B. McKercher. Adsorption of phosphorus by four Saskatchewan soils.
Can. J. Soil Sci . 39:64 (1959)
Rich, C. I. Ammoniurn fixation by two Red-yellow podzolic soils as influenced by interlayer
Al in clay minerals. Trans. Intern. Cong. Soil Sci. 7th . 4:468 (1960)
Rodgers, E. G. Leaching of four triazines in three soils as influenced by varying frequencies
and rates of simulated rainfall. Proc. Southern Weed Control Conf . 15:268 (1962)
Rohlich, G. A. Chemical methods for the removal of nitrogen and phosphorus from sewage
plant effluents. Algae and Metropolitan Wastes . USPHS, SEC TR. W61-3:130 (1961)
Rudolfs, W. Phosphates in sewage and sludge treatment. Sewage Works J. 19:1:43 (1947)
Russell, G. C. and P. F. Low. Reaction of phosphate with kaolinite in dilute soluction. Soil
Sci. Soc. Am. Proc . 18:22 (1954)
Sani, C. R. and A. A. MacLean. Phosphorus retention capacities of some New Brunswick
soils and their relationship with soil properties. Can. J. Soil Sci . 45:15 (1965)
Sawyer, C. N. Some new aspects of phosphates in relation to lake fertilization. Sewage md.
Wastes . 24:6:768 (1952)
Sawyer, C. N. and A. F. Ferullo. Nitrogen fixation in natural waters under controlled
laboratory conditions. Algae and Metropolitan Wastes . USPHS, SEC TR. W61-3:100
(1961)
65
-------
Scarseth, G. D. The mechanism of phosphate retention by natural aluminosilicate colloids.
J. Am. Soc. Agron . 27:596 (1935)
Schoen, U. Identification of clay by phosphate fixation and cation exchange. Z. Pfl. Ern .hr.
Dung. Bodenk . 63:1 (1953)
Schwertmann, U. Selective cation adsorption of clay fraction in some soils derived from
sedimentary rocks. Z. P11. Ernãhr. DUng . Bodenk. 97.9 (1962)
Scott, A. D., J. J. Hauway and A. P. Edwards. Replaceability of ammonium in vermiculite
with acid solutions. Soil Sci. Soc. Am. Proc . 22:388 (1958)
Sen Gupta, M. B. and A. H. Cornfield. Phosphorus in calcareous soils. IV. Nature of and
factors influencing the fixation of added phosphate. J. Sci. Food Agr . 14:8 13
(1963)
Sennett. P. and J. P. Oliver. Colloidal dispersions, electrokinetic effects and the concept of
zeta potential. md. Eng. Chem . 57:8:33 (1965)
Serb nescu. I. and V. Blanaru. A study of the different forms of phosphorus in certain soils
of the Rumanian Peoples’ Republic and some aspects of phosphorus fixation. Am.
Inst. Cent. Cerc. Agr. 30A:63 (1963) ( Soils and Ferts . 27:1936)
Sheets, T. J. The comparative toxicities of four phenylurea herbicides in several soil types.
Weeds . 6:413 (1958)
Simakin, A. I. and L. I. Vasilenko. Dynamics of nitrates in leached chernozems of the Kuban
when fertilizers are applied. Agro. Khimiya 1:46 (1965) ( Soils and Ferts . 28:2201)
Simpson, D. M. H. and S. W. Meisted. Urea hydrolysis and transformation in some Illinois
soils. Soil Sci. Soc. Am. Proc . 27:48 (1963)
Smith, H. V. A lysimeter study of the nitrogen balance in irrigated soils. Ariz. Agr. Exp.
Sta. Tech. Bull . 102:257 (1944)
Smith, R. J., Jr. and W. B. Ennis, Jr. Studies on the downward movement of 2,4-D and
3-chloro-IPC in soils. Proc. Southern Weed Control Conf . 6:63 (1953)
Sohn, J. B. and M. Peech. Retention and fixation of ammonia by soils. Soil Sci . 85:1 (1958)
Stanford, G. and W. H. Pierre. The relation of potassium fixation to ammonium fixation.
Soil Sci. Soc. Am. Proc. 11:155 (1947)
Stevenson, F. J. On the presence of fixed ammonium in rocks. Science . 130:22 1 (1959)
Stevenson, F. J. and A. P. S. Dhariwal. Distribution of fixed ammonium in soils. Soil Sci.
Soc. Am. Proc . 23:121 (1959)
Stroube, E. W. and D. P. Bondarenko. Persistence and distribution of simazine applied in
the field, Proc. N. Central Weed Control Conf . 17:40 (1960)
Stroube, E. W. Movement and persistence of simazine and atrazine in soil and some related
studies. Diss. Abst . 22:3339 (1962)
66
-------
Strumm, W. and J. J. Morgan. Stream pollution by algal nutrients. Ann. Conf. San. Eng.
U. of Kansas . 12:16 (1962)
Subra, P. and J. Guillemot. The movement through the soil of a substituted urea herbicide.
Weed Abst . 9:401 (1959)
Swenson, R. M., C. V. Cole arid D. H. Sieling. Fixation of phosphate by iron and aluminum
and replacement by organic and inorganic ions. Soil Sci . 67:3 (1949)
Sylvester, R. 0. Nutrient content of drainage water from forested, urban and agricultural
areas. Algae and Metropolitan Wastes . USPHS, SEC TR. W61-3:80 (1961)
Tamini, Y. N., Y. Kanehiro and G. D. Sherman. Reactions of ammonium phosphate with
gibbsite and montmorillonitic and kaolinitic soils. Soil Sci . 98:249 (1964)
Taylor, A. W., E. L. Burney and E. C. Moreno. Precipitation of phosphate from calcium
phosphate solutions by iron oxide and aluminum hydroxide. Soil Sci. Soc. Am.
Proc . 28:49 (1964)
Toth, S. J. and F. E. Bear. Phosphorus-adsorbing capacities of some New Jersey soils.
Soil_Sci. 64:199 (1947)
Tscapek, W. The density of adsorbed water in soils. Z. Pflan. Dung u. Bodenk . 34:265 (1934)
Upchurch, R. P. and W. C. Pierce. The leaching of monuron from Lakeland sandy soil.
Part I. The effect of amount, intensity and frequency of simulated rainfall.
Weeds . 5:321 (1957)
Upchurch, R. P. and W. C. Pierce. The leaching of monuron from Lakeland sandy soil.
Part II. The effect of soil temperature, organic matter, soil moisture and amount
of herbicide. Weeds . 6:24 (1958)
van Olphen, H. An introduction to clay colloidal chemistry . Interscience. (1963)
Verwey, E. J. W. and J. Th. G. Overbeek. Theory of the stability of lyophobic colloids .
Elsevier, New York. (1948)
Volk, N. J. The fixation of potash in difficulty available forms in soil. Soil Sci . 37:2 67
(1934)
Walsh, L. M. and J. T. Murdock. Native fixed ammonium and fixation of applied ammonium
in several Wisconsin soils. Soil Sci . 89:183 (1960)
Wear, J. I. and J. L. White. Potassium fixation in clay minerals as related to crystal
structure. Soil Sci . 71:1 (1951)
Weir, C. C. and R. J. Soper. Adsorption and exchange studies of phosphorus in some Mani-
toba soils. Can. J. Soil Sci . 42:31 (1962)
White, W. A. The properties of clays. M. S. Thesis, University of Illinois. (1947)
Wiese, A. F. and R. G. Davis. Herbicide movement with various amounts of water. Weeds .
12:101 (1964)
67
-------
Williams, E. G., W. M. Scott and M. J. McDonald. Soil properties and phosphate sorption.
J. Sci. Food & Agr . 9:551 (1958)
Williams, E. G. Influence of parent material and drainage conditions on soil phosphorus
relationships. Agrochimica . 3:279 (1959)
Woodruff, J. R. and E. J. Kamprath. Phosphorus adsorption maximum as measured by the
Langmuir isotherm and its relationship to phosphorus availability. Soil Sci. Soc.
Am. Proc . 29:148 (1965)
Wright, B. C. and M. Peech. Characterization of phosphate reaction products in acid soils
by the application of solubility criteria. Soil Sci . 90:32 (1960)
68
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ACKNOWLEDGME NT
The author is indebted to Dr. C. A. Black, Agronomy Department, Iowa State University,
Dr. J. L. White and Dr. S. A. Barber, Agronomy Department, Purdue University, for their
constructive criticisms of the manuscript.
Thanks are due to Miss Geraldine Werdig for editorial assistance.
Acknowledgment is made to Mr. Walter L. Wilson, formerly of this Laboratory and now
of the Alaska Laboratory, for information made available to the author on the chemical char-
acter of sewage effluents.
69/70
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Glossary of Selected Soil Science Terms
A horizon — See soil horizon.
A
ARC soil — A soil with a distinctly developed profile, including A, B, and C
horizons.
AC soil — A soil having a profile containing only A and C horizons with no
clearly developed B horizon.
accelerated erosion — See erosion (ii)
acid soil — A soil with a preponderance of hydrogen ions, and probably of
aluminum, in proportion to hydroxyl ions. Specifically, Soil with a pH
value <7.0. For most practical purposes a soil with a pH value <6.6.
The pH values obtained vary greatly with the method used and, conse-
quently, thereisnounanimousagreementas to what constitutes an acid
soil. (The term is usually applied to the surface layer or to the root
zone unless specified otherwise.)
acidity, active — The activity of hydrogen ion in the aqueous phase of a soil.
It is measured and expressed as a pH value.
acidity, free — The titratable acidity in the aqueous phase of a soil. This
may be expressed in mill iequivalents per unit mass of soil or in other
suitable umts.
acidity, potential — The amount of exchangeable hydrogen ion in a soil that
can be rendered free or active in the soil solution by cation exchange.
Usually expressed in milliequivalents per unit mass of soil.
actinomycetes —A general term applied to a group of organisms interme-
diate between the bacteria and the true fungi that usually produce a
characteristic branched mycelium and sporulate by segmentation of
the entire mycelium or. more commonly, by segmentation of special
terminal hyphae. Any organism belonging to the order of Actinomy-
cetales.
adsorption complex — The group of substances in soil capable of adsorbing
other materials. Organic and inorganic colloidal substances form the
greater part of the adsorption complex; the noncolloidal materials, such
as silt and sand, exhibit adsorption but to a much lesser extent than
the colloidal materials.
aerate — To impregnate with a gas, usually air.
aeration, soil The process by which air iii the soil is replaced by air from
the atmosphere. In a well-aerated soil, the soil air is very similar
in composition to the atmosphere above the soil. Poorly aerated soils
usually contain a much higher percentage of carbon dioxide and a cor-
respondingly tower percentage of oxygen than the atmosphere above the
soil. The rate of aerationdepends largely onthe volume and continuity
of pores within the soil.
aeration porosity — See air porosity.
aerobic — (i) Having molecular oxygen as a part of the environment. (ii)
Growing only in the presence of molecular oxygen, as aerobic orga-
nisms.
(iii) Occurring only in the presence of molecular oxygen (said of cer-
tain chemical or biochemical processes such as aerobic decomposition) -
agronomy — A specialization of agriculture concerned with the theory and
practice of field-crop production and soil management. The scientific
management of land.
air-dry — (i) The state of dryness (of a soil) at equilibrium with the mois-
ture content in the surrounding atmosphere. The actual moisture con-
tent will depend upon the relative humidity and the temperature of the
surrounding atmosphere.
(ii) To allow to reach equilibrium in moisture content with the sur-
rounding atmosphere.
air porosity — The proportion of the bulk volume of soil that is filled with
air at any given time or under a given condition such as a specified
moisture tension. Usually the large pores; that is, those drained by a
tension of less than approximately 100 cm of water. See Moisture
Tension.
alkali soil — (i) A soil with ahigh degree of alkalinity (pH of 8.5 or higher)
or with a high exchangeable sodium content (t5c or more of the ex-
change capacity), or both. (ii) A soil that contains sufficient alkali
(sodium) to interfere with the growth of most crop plants. See saline-
alkali soil and sodic soil.
alkaline soil — Any soil that has a pH >7.0. See reaction, soil.
alkalization The process whereby the exchangeable sodium content of a
soil is increased.
Alluvial soil — Ii) A soil developing from recently deposited alluvium and
exhibiting essentially no horizon development or modification of the
recently deposited materials. (ii) When capitalized the term refers
to a great soil group of the azonal order consisting of soils with little
or no modification of the recent sediment in which they are forming
(indicated by absence of a B horizon).
alpha humus — A mixture of dark-colored organic subsiances of indefinite
chemical composition extracted from the soil by dilute alkali and pre-
cipitated by making the extract strongly acid.
amendment, soil (i) Analterationof the properties of a soil, and thereby
of the soil, by the addition of substances such as lime, gypsum. saw-
dust, etc., to the soil for the purpose of making the soil more suitable
fortheproductionofplants. (ii) Anysuch substance used for this pur-
pose. Strictly speaking, fertilizers constitute a special group of soil
amendments.
ammonification — The biochemical process whereby ammoniacal nitrogen
is released from nitrogen-containing organic compounds.
ammoniurn fixation — The adsorption or aosorption of ammonium ions by
the mineral or organic fractions of the soil in a manner that they are
relatively insoluble in water and relatively unexchangeable by the usual
methods of cation exchange.
anaerobic .- (I) The absence of molecular oxygen. (ii) Growing in the ab-
sence of molecular oxygen (such as anaerobic bacteria). (iii) Occur-
ring in the absence of molecular oxygen (as a biochemical process).
anion -exchange capacity — The sum total of exchangeable anions that a soil
can adsorb. Expressed as milliequivalents per 100 grams of soil (or
of other adsorbing material such as clay).
antibiotic — A substance produced by one species of organism that, is low
concentrations, wilt kill or inhibit growth of certain other organisms.
artificial manure — See compost. (In European usage may denote commer-
cial fertilizers.)
association, soil See soil association.
autochthonous inca — That portion of the microflora presumed td subsist
on the more resistant soil organic matter and little alfected by the ad-
dition of fresh organic materials. Contrast with zymogenous flora.
autotrophic — Capable of utilizing carbon dioxide or carbonates as the sole
source of carbon and obtaining energy for the reduction of carbon diox-
ide and for other life processes from the oxidation of inorganic tie-
ments or compounds such as iron, sulfur, hydrogen, amnioniurn, and
nitrites, or from radiant energy. Contrast with heterotrophic.
available nutrient — ThatporUon of any element or compound is the soil that
canbe readilvabsoroed and assimilated by growing plants. (“Available’
should not be confused with “exchangeable.”)
available water — The portion of water ins soil that can be readily aosorbed
by plant roofs. Considered by most workers to be that water held in
the soil against a pressure of up to approximately 15 bars. See field
capacity and moisture tension.
azonal soils Soils withoul distinct genetic horizons. A soil order.
B horizon — See soil horizon.
B
badland — A land type generally devoid of vegetation and broken by an intri -
cate maze of narrow ravines, sharp crests, and pinnacles resulting
from serious erosion of soft geologic materials. Most common in arid
or semiarid regions. A miscellaneous land type.
bar — A unit of pressure equal to one million dynes per square centimeter.
base -saturation percentage — The extent to which the adsorption complex
of a soil is saturated with exchangeable catioris other than hydrogen.
It is expressed as a percentage of the total cation-exchange capacity.
BC soil — A soil profile with B and C horizons but with little or no A hori-
zon.
Selected terms from the “Glossary of Soil Science Terms” originally pub-
lished in Soil Science Society of America Proceedings, Volume 29, p. 330-
351 (1965).
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bedrock — The solid rock underlying soils and the regolith in depths ranging
from zero (where exposed by erosion) to several hundred feet.
bench terrace — See terrace.
biological immobilization — See immobilization and biological interchange.
biological interchange — The interchange of elements between organic and
inorganic states in a soil or other substrate through the agency of bio-
logical activity. It results from biological decomposition of organic
compounds and the liberation of inorganic materials (mineralization):
and, from the utilization of inorgarnc materials in the synthesis of mi-
crobial tissue (immobilization). Both processes commonly proceed
continuously in soils.
biological mineralization — See mineralization and biological interchange.
biosequence A sequence of related soils that differ, one from the other,
primarily because of differences in kinds and numbers of soil orga-
nisms as a soil -forming factor.
bisect — A profile of plants and soil showing the vertical and lateral distri-
bution of roots and tops in their natural position.
Black Earth — A term used by some as synonymous with ‘Chernozem;” by
others (in Australia) to describe self-mulching black clays.
bleicherde The light-colored, leached A2 horizon of Podzol soils.
blocky soil structure — See soil structure.
blown-out-land Areas from which all or almost all of the soil and soil
material lisa been removed by wind erosion. Usually barren, shallow
depressions with a flat or irregular floor consisting of a more rests -
tard layer and (or an accumulation of pebbles, or a wet zone immedi-
ately above a water table. Usually unfit for crop production. A mis-
cellaneous land type.
blowout —A small area of blown-out land.
bog iron ore — Impure ferruginous deposits developed in bogs or swamps
by the chemical or biochemical oxidation of iron carried in solution.
Bog soil — A great soil group of the intrazonal order and hydromorphic
suborder. Includes muck and peat.
border-strip irrigation — See irrigation methods.
bottomland — See flood plain.
breccia —A rock composed of coarseangular fragmentscemer.ed together.
broad-base terrace — See terrace.
Brown Earths — Soils with a mull horizon but having no horizon of accuntu-
lationof clay or sesquioxtimn. (Generally used as a synonym for “Brown
Forest soils” but sometimes for similar soils acid in reaction.)
Brown Forest soils — A great soil group of the infrazonal order and calci -
morphic suborder, formed on calcium-rich parent materials under de -
ciduous forest, and possessing a high base status but lacking a pro-
nounced tiluvial horizon. (A much more narrow group than the European
Brown Forest or Braunerde.)
Brown Podzolic soils — A zonal great soil group similar to Podzols but
lacking the distinct A2 horizon characteristic of the Podrol group.
(Some American soil taxonomists prefer to class this soil as a kind
of Podzol and not as a distinct great soil group.)
Brown soils — A great soil group of the temperate to cool arid regions, com-
posed of soils with a brown surface and a light-colored transitional
subsurface horizon over calcium carbonate accumulation.
Brunizem - Synonymous with Prairie soils.
buffer compounds, soil — The clay, organic matter, and compounds such as
carbonates and phosphates which enable the soil to resist appreciable
change in pit.
bulk density, soil — The mass of dry soil per unit bulk volume. The bulk
volume is determined before drying to constant weight at lOSC.
bulk specific gravity — The ratio of the bulk density of a soil to the mans of
unit volume of water.
bulk volume — The volume, including the solids and the pores, 01 anarbi-
tracy soil mass.
Buried oit — Soil covered by an alluvial. loessal. or other deposit, usually
to a depth greater than the thickness of the solum.
C horizon — See soil horizon.
C
calcareous soil — Soil containing sufficient calcium carbonate (often with
magnesiumcarbonate)toeffervescevisiblywhen treated with cold 0.1ff
hydrochloric acid.
calciphytes Plants that require or tolerate considerable amounts of cal-
cium or, are associated with soils rich in calcium.
Caliche — (i) Alayernearthesurface, mnoreorless cemented by secondary
carbonates of calcium or magnesium precipitated from the soil solution.
Itmay occurasasoftthinsoilhorizon, as a hard thick bed just beneath
the solum, or as a surface layer exposed by erosion (see Croute cal -
caire). Not a geologic deposit. (ii) Alluvium cemented with sodium
nitrate, chloride, and/or other soluble salts in the nitrate deposits of
Chile and Peru.
capillary fringe — A zone just above the water table (zero gauge pressure)
that remains almost saturated. (The extentand the degree of definition
of the capillary fringe depends upon the size-distribution of pores.)
capillary potential — See soil water.
carbon cycle — The sequence of transformations whereby carbon dioxide is
fixed in living organisms by photosynthesis or by chemosynthesis, liber -
ated by respiration and by the death and decomposition of the fixing
organism, used by heterotrophic species, and ultimately returned to its
original state.
carbon-nitrogen ratio — The ratio of the weight of organic carbon to the
weight of total nitrogen in a soil or in organic material. It is obtained
by dividing the percentage of organic carbon (C) by the percentage of
total nitrogen (N).
category — Anyone of the ranks of the system of soil classification in which
soils are grouped on the basis of their characteristics.
catena — A sequence of soils of about the same age, derived from similar
parent material, and occurring under similar climatic conditions, but
having different characteristics due to variation in relief and in drainage.
See clinosequence and toposequence.
cation exchange — The interchange between a cation in solution and another
cation on the surface of any surface -active material such as clay colloid
or organic colloid.
cation -exchange capacity — The sum total of exchangeable callous that a
soil can adsorb. Sometimes called “total-exchange capacity,” “base-
exchange capacity,’ or ‘cation-adsorption capacity.” Expressed in
milhiequivalents per 100 grams of soil (or of other adsorbing material
such as clay).
cemented—Indurated; having ahard, brittle consistency because the parti-
cles are held together by cementing substances such as humus, calcium
carbonate, or the oxides of silicon, iron, and aluminum. The hardness
and brittleness persist even when wet.
check-basin irrigation — See irrigation methods.
chemically precipitated phosphorus — Relatively insoluble phosphorus corn -
pounds resulting from reactions between constituents in solution to
form chemically homogeneous particles of the solid phase. Examples
are: calcium and magnesium phosphates which are precipitated above
a pit of about 6.0 to 6.5 (if calcium and magnesium are present); and.
iron and aluminum phosphates which are precipitated below a pH of
about 5.8 to 6.1 at which many iron and aluminum compounds are sot -
able. A form of fixed phosphate. See chemisorbed phosphorus.
chemisorbed phosphorus — Phosphorus adsorbed or precipitated on the sur-
face of clay minerals or other crystalline materials as a result of the
attractive forces between the phosphate ion and constituents in the
surface of the solid phase.
Chernozem — A zonal great soil group consisting of soils with a thick, nearly
black or black, organic matter-rich A horizon high in exchangeable
calcium, underlain by a lighter colored transitional horizon above a
zone of calcium carbonate accumulation; occurs in a cool subhumid
climate under a vegetation of tall and midgrass prairie.
chert — See coarse fragments.
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Chestnut soil — A zonal great soil group consisting of soils with a moderately
thick, dark-brown A horizon over a lighter colored horizon that is above
a zone of calcium carbonate accumulation.
chisel, subsoil — A tillage implement with one or more cultivator-type feet
to which are attached strong knifelike units used to shatter or loosen
hard, compact layers, usually in the subsoil, to depths below normal
plow depth. See subsoiling.
chroma — The relative purity, strength, or saturation of a color; directly
related to the dominance of the determining wavelength of the light and
inversely related to grayness; one of the three variables of color. See
Munsell color system, hue, and value, color.
chronosequence — A sequence of related soils that differ, one from the other,
in certain properties primarily as a result of time as a soil-forming
factor.
class, soil — A group of soils having a definite range in a particular property
such as acidity, degree of slope, texture, structure, land-use capability,
degree of erosion, or drainage. See soil texture and soil structure.
classification, soil— The systematic arrangement of soils into groups or
categories on the basis of their characteristics. Broad groupings are
made on the basis of general characteristics and subdivisions on the
basis of more detailed differences in specific properties. Inthe United
States the following system has been used for a number of years (from
Soil Survey Staff, SCS, USDA. 1960. Soil classification: A compre-
hensive system— 7th approximation. p. 9. U. S. Government Printing
Office, Washington):
Order Suborder Great soil groups
Desert soils
Red Desert soils
S rozem
Brown soils
Reddish Brown soils
Chestnut soils
Reddish Chestnut soils
Chernozem soils
Prairie soils
Reddish Prairie soils
Degraded Chernozem
Noncalcic Brown or
Shantung Brown soils
Podzol soils
Gray Wooded, or
Gray Podzolic soils
Brown Podzolic soils
Gray-Brown Podzolic soils*
Red -Yellow Podzolic soils
6) Lateritic soils of Reddish -lirown Lateritic soils*
forested warm- Yellowish-Brown Lateritic soils
temperate and tropi- Laterite soils
cal regions
Intrazonal 1) Halomorphic (Saline
soils and alkali) soils of
imperfectly drained
arid regions and
littoral deposits
2) HydromorphiC soils Humic Gley soils (includes
of marshes, swamps. Wiesenboden)
seep areas, and flats Alpine Meadow soils
Bog soils
Half-Bog soils
Low-Humic Gley soils
Planosols
Ground-Water Podzol soils
Ground-Water Laterite soils
3) Calcimorphic soils Brown Forest soils (Braunerde)
Rendzina soils
The following scheme has been proposed for adoption as the official
system in the United States (from Soil Survey Staff, SCS, USDA, 1960.
Soil classification: A comprehensive system — 7lh approximation. p.13.
U. S. Government Printing Office, Washington):
Present order Approximate equivalents
1) Entisola Azonal soils, and some Low-Humic Gley soils
2) Vertisols Grumusols
3) Inceptisols Ando, Sot Brun Acide, some Brown Forest, Low-
Humic Gley, and Humic Gley soils
4) Aridisols Desert, Reddish Desert, Sierozem, Solonchak,
some Brown and Reddish Brown soils, and asso-
ciated Solonetz
5) Mollisols Chestnut, Chernozem, Bruntzem(Prairie),
Rendzina, some Brown, Brown Forest, and
associated Solonetz and Hurnic Gley soils
6) Spodosol Podzols, Brown Podzolic soils, and Ground-
Water Podzols
7) Alfisols Gray-Brown Pocizolic, Gray Wooded soils, Non-
caicic Brown soils, Degraded Chernozem, and
associated Planosols and some Half-Bog soils
8) Ultisols Red-Yellow Podzolic soils, Reddish-Brown
Lateritic soils of the USA, and associated
Planosols and Half-Bog soils
9) Oxisols Laterite soils, Latosols
10) Histosols Bog soils
clay — (i) A soil separate consisting of particles <0.002 mm in equivalent
diameter. See soil separates. (ii ) A textural class. See soil texture.
clayey — Containing large amounts of clay or having properties similar
to those of clay.
clay loam — A textural class. See soil texture.
clay mineral — (i) Naturally occurring inorganic crystalline material found
in soils andother earthy deposits,the particles being of clay size;that
is, <0.002 mm in diameter. (ii) Material as described under (i), but
not limited by particle size.
claypan — A dense, compact layer in the subsoil having a much higher clay
content than the overlying material, from which it is separated by a
sharply defined boundary; formed by downward movement of clay or
by synthesis of clay in place during soil formation. Claypans are usually
hard when dry, and plastic and sticky when wet. Also,they usually im-
pede the movement of water and air, and the growth of plant roots.
climatic index— A simple, single numerical value that expresses climatic
relationships: for example, the numerical value obtained in Transeau’s
precipitation- evaporation ratio.
climax — A plant community of the most advanced type capable of develop-
ment under, and in dynamic equilibrium with, the prevailing environ-
ment.
climosequence— A sequence of related soils that differ,onefromtheother,
in certain properties primarily as a result of the effect of climate as
a soil-forming factor.
clinosequence — A group of related soils that differ, one from the other, in
certain properties primarily as a result of the effect of the degree of
slope on which they were formed. See toposequence.
clod— A compact, coherent mass of soil ranging in size from 5 or 10 mm
to as much as 8 or 10 inches; produced artificially, usually by the
activity of man by plowing, digging, etc., especially when these opera-
tions are performed on soils that are either too wet or too dry for
normal tiilage operations.
Zonal soils 1) Soils of the cold zone Tundra soils
2) Light-colored soils
of arid regions
3) Dark-colored soils of
semiarid. subhumid,
and humid grass-
lands
4) Soils of the forest-
grassland transi-
tion
5) Light-colored
podzolized soils of
the timbered regions
Solonchak. or
Saline soils
Solonetz soils
Soloth soils
Azonal soils Lithosols
Regosols (includes Dry Sands)
Alluvial soils
New or recently modified great soil groups.
73
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coarse fragments — Rock or mineral particles >2.0mm in diameter. The
creep— Slow mass movement of soil andsoii materialdown relatively steep
following names are used for coarse fragments in soils.
slopes primarily under the influence of gravity, but facilitated by satu-
ration with water and by alternate freezing and thawing.
critical reaction— In biology that pHat which a biologicalprocessbecomes
Frsgmentst Descriptive terms applied to fragments that have:
Shape Material Diameters less Diameters from Diameters more
too slow to measure or at which organisms die.
than 3 inches 3 to 10 inches than 10 inches
rounded or all kinds of gravelly cobbly stony l
crotovi na — A former animal burrow in one soil horizon that has been filled
with organic matter or material from another horizon (also spelled
subrounded rock
“krotovini”).
irregular and chert cherty coarse cherty stony
croute calcaire — Hardened caliche, often found in thick masses or beds
angular other than angular angular stony
overlain by only a few inches of earth. See caliche.
chert cobbly
Lengths up to Lengths from Lengths over
crumb — A soft, porous, more or less rounded ped from ito 5mm indiameter.
See soil structure types and soil structure classes.
6 inches 6 to 15 inches 15 inches
thin and limestone, channery flaggy stony
crumb structure— A structural condition in which most of the pods are
crumbs. See soil structure types.
flat sandstone,
or schist
crushing strength— Theforce required to crush a mass of dry soilor, con-
slate slaty flaggy stony
shale ahaly flaggy stony
versely, the resistance of the dry soil mass to crushing. Expressed in
units of force per unit area (pressure).
crUst— A surface layer on soils, ranging in thickness from afew milli-
*yrom: Soil Sorvey &sff,SCS,USDA. 1951. Soil survey manual,U.S.Dept.
Agr. Handbook 18, p. 214. U. S. Government Printing Office, washington.
t The adjectives describing fragments are also applied to lands and soils
meters to perhaps as much as an inch, that is much more compact,
hard, and brittle, when dry, than the material immediately beneath it.
when they have significant amounts of such fragments. I “bouldery” is
sometimes used when stones are larger than 24 inches.
crystal— A homogeneous inorganic substance of definite chemical compo-
sition bounded by plane surfaces that form definite angles with each
coarse sand — See soil separates and soil texture,
other, thus giving the substance a regular geometrical form. See soil
mineral.
coarse sandy loam — See soil texture.
crystal lattice — See lattice structure.
coarse texture — The texture exhibited by sands, loamy sands, and sandy
barns except very fine sandy loam. A soil containing large quantities
of these textural classes (United Hates usage). See sand, sandy, and
crystalline rock — A rock consisting of various minerals that have crystal-
lized in place from magma. See igneous rock and sedimentary rock.
moderately’ coarse texture.
cultivation — A tiliage operation used in preparing land for seeding or trans-
cobblestone — Rounded or partially rounded rock or mineral fragments be-
planting or later for weed control and for loosening the soil.
tween 3 and 10 inches in diameter. See coarse fragments.
cyclic salt — Salt deposited on the soil by wind blowing off the sea or off
cobbly — Containing appreciable quantities of cobblestones. (Said of soil
inland salt lakes.
and of land. The term angular cobbly” is used when the fragments
are less rounded.) See coarse fragments.
1)
colluvium — A deposit of rock fragments and soil material accumulated at
the base of steep slopes as a result of gravitational action. See creep.
D layer — (Obsolete) An unconsolidated geological stratum, beneath the
solum of some soils, that does not conform to the parent material from
which the overlying soil was formed even though it may influence the
color — See Munsell color system.
genesis or behavior of the soil. See soil horizon.
columnar soil structure — See soil structure types.
Darcy’s equation — See Darcy’s law.
complex, soil— See soil complex.
Darcy’s law — (i) A law describing the rate of flow of water throughporous
media. (Named for Henry Darcy of Paris who formulated it in 1856
compost — Organic residues, or a mixture of organic residues and soil, that
have been piled, moistened, and allowed to undergo biological decom-
from extensive work on the flow of water through sand filter beds.)
As formulated by Darcy the law is:
position. Mineral fertilizers are sometimes added. Often called “arti-
ficial manure” or “synthetic manure” if produced primarily from plant
+e
= He
residues,
where
Q is the volume of water passed in unit time,
concentrated flow — The flowing of a rather large accumulated body of water
over a relatively narrow course. It often causes serious erosion and
g ”Uymg.
S is the area of the bed,
e is the thickness of the bed,
H is the height of the water on top of the bed, and
‘k is a coefficient depending on the nature of the sand” and for
concretion — A local concentration of a chemical compound, such as calcium
cases where the pressure “under the filter is equal to the weight
carbonate or iron oxide, in the form of a grain or nodule of varying
size, shape, hardness, and color.
of the atmosphere.”
(ii) Generalization for three dimensions: The rate of viscous flow of
water in isotropic porous media is proportional to, and in the direction
conductivity, hydraulic — See soil water,
of, the hydraulic gradient. (iii) Generalization for other fluids: The
rate of viscous f tow of homogeneous fluids through isotropic porous
consistency — (i) The resistance of a material to deformation or rupture.
media is proportional to, and in the direction of the driving force.
(ii) The degree of cohesion or adhesion of the soil mass. Termsused
for describing consistency at various soil moisture contents are:
decalcification — The removal of calcium carbonate or calcium ions from
the soil by leaching.
WET SOfL - nonsticky, slightly sticky, sticky, very sticky, nooplastic,
aligbtly plastic, plastic, and very plastic.
decomposition — See mineralization.
MOIST SOIL -loose, very friable, friable, firm, very firm, and us-
tremely firm.
deflation— The removal of fine soil particles from soil by wind.
DRY SOIL — loose, soft, slightly hard, hard, very hard, and extremely
deflocculate — (i) To separate the individual components of compound par-
hard.
tides by chemical and/or physical means. (ii) To causethe particles
CEMENTATION — weakly cemented, strongly cemented, and thd ted.
of the disperse phase of a colloidal system to become suspended in the
dispersion medium.
consistency constants — See liquid limit, plastic limit, and plasticity number.
corrugation — See Irrigation
Degraded Chernocem — A zonal great soil group consisting of soils with a
very dark brown or black Al horizon underlain by a dark gray, weakly
cradle knoll — A small knoll formed by earth that is raised and left by an
expressed A2 horizon and a brown B (?) horizon; formed in the forest-
uprooted tree. (A microrelief term.)
prairie transition of cool climates.
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deriitrification — The biochemical reduction of nitrate or nitrite to gaseous
nitrogen either as molecular nitrogen or as an oxide of nitrogen.
deposit — Material left in a new position by a natural transporting agent
such as water, wind, ice, or gravity, or by the activity of man.
Depression Podzol — Poorly drained depressional soils of the grassland and
parkland regions of Canada with bleached A2 horizons and finer-textured
B horizons. Also referred to as Bluff Podzol, Meadow Podzol, or
Slough Podzol.
desert crust — A hard layer, containing calcium carbonate, gypsum, or
other binding material, exposed at the surface in desert regions.
desert pavement — The layer of gravel or stones left on the land surface in
desert regions after the removal of the fine material by wind erosion.
Desert soil — A zonal great soil group consisting of soils with a very thin,
light- colored surface horizon, which may be vesicular and is ordinarily
underlain by calcareous material:formed in arid regionsunder sparse
shrub vegetation.
diatoms — Algae having siliceous cell walls that persist as a skeleton after
death. Any of the microscopic unicellular or colonial algae constituting
the class Bacillariaceae. They occur abundantly in fresh and salt waters
and their remains are widely distributed in soils.
diatomaceous earth- A geologic deposit of fine,grayish siliceous material
composed chiefly or wholly of the remains of diatoms. It may occur
as a powder or as a porous, rigid material.
differential water capacity — See soil water.
direct counts — In soil microbiology any one of several methods of estimating
the total number of microorganisms in a given mass of soil by direct
microscopic examination.
disintegraf ion — See physical weaihering.
disperse - (i) To break up compound particles, Such as aggregates, into the
individualcomponent particles. (ii) Todistribute or suspend fine par-
ticles, such as Clay, in or throughout a dispersion medium, such as water.
dispersion medium — The portion of a colloidal system inwhichthe disperse
phase is distributed.
diversion dam — A structure or barrier built to divert part or all of the
water of a stream to a different course.
double layer — In colloid chemistry, the electric charges on the surface of
the disperse phase (usually negative), and the adjacent diffuse layer
(usually positive) of ions in solution.
drag — The force retarding the flow of water or wind over the Surface of the
ground.
drain, to— (i) To provide channels, such as open ditches or drain tile, so
that excess water can be removed by surface or by internal flow. (ii)
To lose water (from the soil) by p, rcolation.
drainage, excessive — Too much or too rapid loss of water from soils.
either by percolation or by surface flow. Lossgreaterthanthatneces—
sary to prevent the development of an anaerobic condition for any
appreciable length of time.
drainage terrace — See terrace.
drain tile — Concrete or ceramic pipe used to conduct water from the soil.
dry aggregate — A compound or secondary soil particle that is not destroyed
by dry sieving.
dryland farming— The practice of crop production in low-rainfall areas
Without irrigation.
dry-weight percentage — The ratio of the weight of any constituent (of a soil)
to the oven-dry weight of the soil. See oven-dry soil.
duff mull.- Atype of forest humus transitional between mull and mor: H and
F layers as well as the Al horizon.
dust mulch — A loose, finely granular, or powdery condition on the surface
of the soil, usually produced by shallow cultivation.
dv — Finely divided, partially decomposed organic material accumulated in
peal soils in the transition zone between the peal and the underlying
mineral material.
dynamometer — An instrument for measuring draft of tillage implements
and for measuring resistance of soil to penetration by tillage imple-
ments,
E
ecology — The science that deals with the interrelations of organisms and
their environment.
ectodynamomorphic soils — Soils with properties that have been produced
or influenced mainly by factors other than parent material. See endo-
dynamomorphic soils.
edaphic — (i) Of or pertaining to the soil. (Li) Resulting from or influenced
by factors inherent in the soil or other substrate, rather than by cli-
matic factors.
edaphology — The science that deals with the influence of soils on living
things, particularly plants, including man’s use of land for plant growth.
effective precipitation — That portion of the total precipitation which be-
comes available for plant growth.
electrokinetic potential — (i) In a colloidal system, the difference in po-
tential between the immovable layer attached to the surface of the dis-
persed phase and the dispersion medium. (ii) The work done in bringing
a unit charge from infinity to a reference point in the liquid layer
attached to the solid phase in a colloidal system.
eluvial horizon— A soil horizon that has been formed by the process of
eluviation. See illuvial horizon.
eluviation — The removal of soil material in suspension (or in solution) from
a layer or layers of a soil. (Usually, the loss of material in solution is
described by the term “leaching.” See illuviation and leaching.
endodynamoniorphic soils — Soils with properties that have been influenced
primarily by parent material.
endotrophic -. Nourished or receiving nourishment from within, as fungi or
their hyphae receiving nourishment from plant roots in a mycorrhizal
association.
enrichment culture — A technique in which environmental (including nutri-
tional) conditions are controlled to favor the development of a specific
organism Or group of organisms.
equivalent diameter — In sedimentation analysis the diameter assigned to a
nonspherical particle, it being numerically equal to the diameter of a
spherical particle of the same density and velocity of fall.
equivalent weight of a soil colloid— The weight of clay or organic colloid
that has a combining power equivalent to 1 gram atomic weight of hy-
drogen.
erode — To wear away or remove the land surface by wind, water, or other
agents.
erodible — Susceptible to erosion. (Expressed by terms such as highly erod-
ible. slightly erothbte. etc.)
erosion — (i) The wearing away of the land surface by running waler, wind,
ice, or other geological agents, including such processes as gravita-
tional creep. (n) ,Detachment and movement of soil or rock by water,
wind, ice, or gravity. The following terms are used to describe differ-
ent types of water erosion:
accelerated erosion — Erosion much more rapid than normal, natural,
geological erosion, primarily as a result of the influence of the
activities of man Or, in some cases, of animals.
geologicat erosion — The normal or natural erosion caused by geolog-
ical processes acting over long geologic periods and resulting in
the wearing away of mountains, the building up of flood plains,
coastal plains. etc. Synonymous with natural erosion.
gully erosion — The erosion process whereby water accumulates in
narrow channels and, over short periods, removes the soil from
this narrow area to considerable depths, ranging front 1 or 2 feet
to as much as 75 to 100 feet.
natural erosion — Wearing away of the earth’s surface by water, ice, or
other natural agents under natural environmental conditions of cli-
mate,vegetation.etc.,undisturbed by man. Synonymous with geo-
logical erosion.
normal erosion — The gradual erosion of land used by man which does
not greatly exceed natural erosion. See natural erosion.
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rUt erosion — An erosion process in which numerous small chatmels
of only several inches in depth are formed: occurs mainly on re -
cently cultivated soils. See nil.
sheet erosion. The removal of a fairly uniform layer of soil from the
land surface by runoff water.
splash erosion — The spattering of small soil particles caused by the
impactof raindrops on very wet soils. The loosened and spattered
particles may or may not be subsequently removed by surface
runoff.
erosion classes — A grouping of erosion conditions based on the degree of
erosion or on characteristic patterns. (Applied to accelerated erosion:
not to normal, natural, or geological erosion.) Four erosion classes
are recognized for water erosion and three for wind erosion. Specific
definitions for each vary somewhat from one climatic zone, or major
soil group, to another. (For details see Soil Survey Staff, SCS, USDA.
1951. Soil surveymanual. U. S.Dept. Agr.llandbookl8. U. S.Govern-
ment Printing Office, Washington.)
erosion pavement — A layer of coarse fragments, such as sand or gravel,
remaining onthe surface of the ground alter the removal of fine parti -
des by erosion.
eutrophic — Having concentrations of nutrients optimal (or nearly so) for
plant or animal growth. (Said of nutrient solutions or of soil solutions.)
evapotranspiration — The combined loss of water from a given area, and
duringaspecified period of time, by evaporation from the soil surface
and by transpiration from plants.
exchange acidity - The titratable hydrogen and aluminum that can be re-
placed from the adsorption complex by a neural salt Solution. Usually
expressed as mifliequivalents per 100 grams of soil.
exchange capacity — The total ionic charge of the adsorption complex active
in the adsorption of ions. See anion-exchange capacity and cation -
exchange capacity.
exchangeable -cation percentage — The extent to which the adsorption corn -
plex of a soil is occupied by a particular cation. It is expressed as
follows:
- Exchangeable cation (meq l00 g soil ) X 100
- Cation-exchange capacity (meq’lOO g soil)
exchangeable phosphate — The phosphate anion reversibly attached to the
surface of the solid phase of the soil in such form that it may go into
solution by anionic equilibrium reactions with isotopes of phosphorus
or with other anions of the liquid phase without solution of the colloid
phase to which it was attached.
exchangeable potassium — The potassium that is held by the adsorption
complex of the soil and is easily exchanged with the cation of neutral
nonpotassium salt Solutions.
exchangeable -sodium percentage — The extent to which the adsorption corn -
plex of a soil is occupied by sodium. It is expressed as follows:
ESP Exchangeable sodium (meq/l0{l g soil ) X 100
Cation-exchange capacity (meq 100 g soil)
F
F layer — A layer of partially decomposed litter with portions of plant
structures still recognizable. Occurs below the L layer (011 horizoui
on the forest floor in forest soils. It is the fermentation layer or the
012 layer. See L layer and soil horizon.
family, soil — In soil classification one of the categories intermediate be -
tween the great soil group and the soil series. See classification. soil.
fertility, soil — The status of a soil with respect to the amount and avail -
ability to plants of elements necessary for plant growth.
fertilizer — Any organic or inorganic material of natural or synthetic ori-
gin which is added to a soil to supply certain elements essential to
the growth of plants.
fertilizer grade - The guaranteed minimum analysis, in per cent, of the
maior plant nutrient elements contained in a fertilizer material or in
a mixed fertilizer. (Usually refers to the percentage of N-P 2 0 5 -K 2 0
but proposals are pending In change the designation to The percentage
of N-P-K.)
fertilizer requirement The quantity of certain plant nutrient elements
needed, in addition to the amount supplied by the soil, to increase
plant growth to a designated optimum.
field capacity (field moisture capacity) - (Obsolete in technical work.) The
percentage of water remaining in a soil 2 or 3 days alter having been
saturated and after free drainage has practically ceased. (The per-
centage may be expressed on the basis of weight or volume.) See
moisture tension.
film water — A layer of water surrounding soil particles and varying in
thickness from br 2 to perhaps 100 or more molecular layers. Usu-
ally considered as that water remaining after drainage has occurred,
because it is not distinguishable in saturated soils.
fine sand — (i) A soil separate. See soil separates. (ii) A soil textural
class. See soil texture.
fine sandy loam — See soil texture.
fine texture - Consisting of or containing large quantities of the fine frac-
tions, particularly of siltandclay. (Includes all clay barns and clays;
that is, clay loam, sandy clay loam, silty clay loam, sandy clay, silty
clay, and clay textural classes. Sometimes subdivided into clayey
texture and moderately fine texture.) See soil texture.
fire, ground — (Forestry) A fire that consumes all organic material of the
forest floor and also burns into the underlying soil itself, as, for ex-
ample. a peat fire. Differentiated from a surface fire on the basis of
vulnerability to wind: in a surface fire the flames are visible and
burning is accelerated by wind, whereas, in a ground fire, wind is
generally not a serious factor.
firm — A term describing the consistency of a moist soil that offers dis-
tincUy noticeable resistance to crushing but can be crushed with mod-
erate pressure between the thumb and forefinger. See consistency.
first bottom - The normal flood plain of a stream.
fixation — The process or processes in a soil by which Certain chemical
elements essential for plant growth are converted from a soluble or
exchangeable form to a much less soluble or to a nonexchangeable
form: for example, phosphate “fixation. Contrast with nitrogen
fixation.
fixed phosphorus (i) That phosphorus which has been changed to a less
soluble form as a result of reaction with the soil; moderately avail-
able phosphorus. More specifically, that quantity of soluble phos-
phorus compounds which, when added to soil, becomes chemically or
biologically attached to the solid phase of soil so as not to be re-
covered by extracting the soil with a ape cilied extractant under speci-
fied conditions. Such extractants include: water, carbonated water,
or dilute solutions of strong mineral acids with or without fluoride or
other exchangeable anion. (Li) Applied phosphorus that is not ab-
sorbed by plants during the first cropping year. (iii) Soluble phos-
phorosthathas become attached to the solid phase of the soil in forms
highly unavailable to crops: unavailable phosphorus; phosphorus in
other than readily or moderately available forms.
flaggy - See coarse fragments.
flagstone — A relatively thin fragment, 6 to 15 inches long, of sandstone,
limestone, slate, shale or, rarely, of settist. See coarse fragments.
flooding — See irrigation methods.
flood plain — The land bordering a stream, built up of sediments from
overflow of the stream and subject to inundation when the stream is
at flood stage. See first bottom.
flow velocity (of water in soil) — The volume of water transferredper unit
of time and per unit of area normal to the direction of the net flow.
fluviogtacial — See glaciofluvial deposits.
foliar diagnosis — An estimation of the extent to which plants are getting
certain necessary chemical elements from the soil by examination of
the color and growth habits of the foliage of the plants.
forest floor — All dead vegetable or organic matter, including litter and
unincorporated humus, on the mineral soil surface under forest
vegetation.
forest soils — (i) Soils developed under forest vegetation. (ii) Soils
formed in temperate climates under forest vegetation (European
usage).
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fragipan — A natural subsurface horizon with high buIlt density relative to
the solum above, seemingly cemented when dry, but when moist show-
ing a moderate to weak brittleness. The layer is low in organic mat-
ter, mottled, slowly or very slowly permeable to water, and usually
shows occasional or frequent bleached cracks forming polygons. It
may be found in profiles of either cultivated or virgin soils but not in
calcareous material.
friable — A consistency term pertaining to the ease of crumbling of soils.
See consistency.
frost, concrete — Ice in the soil in such quantity as to constitute a virtually
solid block.
frost, honeycomb — Ice in the soil in insufficient quantity to be continuous,
thus giving the soil an open, porous structure permitting the ready
entrance of water.
fulvic acid— A term of varied usage but usually referring to the mixture
of organic substances remaining in solution upon acidification of a
dilute alkali extract from the soil.
furrow irrigation — See irrigation methods.
G
genetic— Resulting from, or produced by, soil-forming processes; for ex-
ample, a genetic soil profile or a genetic horizon.
gi lgai — The microrelief of soils produced by expansion and contraction
with changes in moisture. Found in soils that contain large amounts
of clay which swells and shrinks considerably with wetting and drying.
Usually a succession of microbasins and microknolls in nearly level
areas or of microvalleys and microridges parallel to the threction of
the slope. See niicrorelief.
glacial drift — Rock debris that has been transported by glaciers and de-
posited, either directly from the ice or from the meltwater. The
debris may or may not be heterogeneous.
glaciofluvial deposits .- Material moved by glaciers and subsequently sorted
and deposited by streams flowing from the melting ice. The deposits
are stratified and may occur ir the form of outwash plains, deltas,
kaxnes, eskers, and kame terraces. See glacial drift and till, (i).
Gleyzation — A soil-forming process resulting in the development of gley
soils. See Itumic Gley soil and Dark Gray Gleysolic. soil.
Gley soil — (Obsolete in the USA) Soil developod under conditions of poor
drainage resulting in reduction of iron and other elements and in gray
colors and mottles.
glycophytes — Nonhalophytic plants or plants that do not grow well when
the osmostic pressure of the soil solution rises above two hers.
grain density — See particle density.
granular structure — See soil structure and soil structure types.
granule — A natural soil aggregate or ped which is relatively nonporous.
See soil structure and soil structure types.
gravefly — Containing appreciable or significant amounts of gravel. (Used
to describe soils or lands.) See coarse fragments.
gravitational potential — See soil water.
gravitational water — Water which moves into, through, or Out of the soil
under the influence of gravity.
Gray-Brown Podzolic soil — A zonal great soil group consisting of soils
with a thin, moderately dark Al horizon and with a grayish-brown A2
horizon underlain by a B horizon containing a high percentage of
bases and an appreciable quantity of itluviated silicate clay; formed
on relatively young land surfaces, mostly glacial deposits, from ma-
terial relatively rich in calcium, under deciduous forests in humid
temperate regions.
Gray Desert soil — A term used in Russia, and. frequently in the United
Bates, synonymously with Desert soil. See Desert soil.
Great soil group— One of the categories in the system of soil classifica-
tion that has been used in the United Rates for many years. See
classification, soil.
green manure— Plant material incorporated with the soil while green, or
soon alter maturity, for improving the soil.
green-manure crop— A crop grown for use as green manure, See green
manure.
groundwater — That portion of the total precipitation which at any particu-
lar time is either passing through or standing in the soil and the un-
derlying strata and is free to move under the influence of gravity. See
water table.
Ground-Water Laterite soil — A great soil group of the intrazonal order
and hydromorphic suborder,consisting of soils characterized by hard-
pans or concretional horizons rich in iron and aluminum (and some-
times manganese) that have formed immediately above the water
table.
Ground-Water Podzol soil — A great soil group of the intrazonal order
and hydromorphic suborder, consisting of soils with an organic mat
on the surface over a very thin layer of acid humus material under-
lain by a whitish-gray leached layer, which may be as much as 2 or 3
feet in thickness, and is underlain by a brown, or very dark-brown,
cemented hardpan layer; formed under various types of forest vege-
tation in cool to tropical, humid climates under conditions of poor
drainage.
gully — A channel resulting from erosion and caused by the concentrated
but intermittent flow of water usually during and immediately follow-
ing heavy rains. Deep enough to interfere with, and not to be obliter-
ated by, normal tillage operations.
gully erosion— See erosion, (ii).
gyttja — Sedimentary peat consisting mainly of plant and animal residues
precipitated from standing water.
H
H layer— A layer occurring in mor humus consisting of well-decomposed
organic matter of unrecognizable origin. The 02 horizon. See soil
horizon.
Half-Bog soil— A great soil group, of the intra.zonal order and hydromor-
phic suborder consisting of soil with dark-brown or black peaty ma-
terial over grayish and rust mottled mineral soil; formed under
conditions of poor drainage under forest, sedge, or grass vegetation
in cool to tropical humid climates.
halomorphic soil — A suborder of the intrazonal soil order, consisting of
saline and alkali soils formed under imperfect drainage in arid reg-
ions and including the great soil groups Solonchak or Saline soils,
solonetz soils, and soloth soils.
halophytic vegetation— Salt-loving or salt-tolerant vegetation, usually hav-
ing fleshy leaves or thorns and resembling desert vegetation.
hardpan— A hardened soil layer, in the lower A or in the B horizon, caused
by cementation of soil particles with organic matter or withmaterials
such as silica, sesquioxides, or calcium carbonate. The hardness
does not change appreciably with changes in moisture content and
pieces of the hard layer do not slake inwater. See caliche and claypan.
heavy soil — (Obsolete in scientific use.) A soil with a high content of the
fine separates, particularly clay, or one with a high drawbar pull and
hence difficult to cultivate. See fine texture.
heterotrophic — Capable of deriving energy for life processes only from
the decomposition of organic compounds and incapable of using in-
organic compounds as sole sources of energy or for organic synthesis.
Contrast with autotrophic.
horizon— See soil horizon.
hue — One of the three variables of color. It is caused by light of certain
wavelengths and changes with the wavelength. See Munsell color sys-
tem. chroma, and value, color.
humic acid — A mixture of variable or indefinite composition of dark-
colored organic substances, precipitatedopon acidification of a dilute-
alkali extract from soil. (Used by some workers to designate only the
alcohol-soluble portion of this precipitate.) (In chemical literature,
it is sometimes used to designate a preparation obtained by the treat-
ment of sugars with mineral acids.)
Humic Gley soil — Soil of the intrazonal order and hydromorphic suborder
that includes Wisenboden and related soils, such as Half-Bog soils.
which have a thin muck or peat 02 horizon and an Al horizon. Devel-
oped in wet meadow and in forested swamps.
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humification — The processes involved in the decompostion of organic
matter and leading to the formation of humus.
humin — The fraction of the soil organic matter that is not dissolved upon
extraction of the soil with dilute alkali.
humus — (i) That more or less stable fraction of the soil organic matter
remaining after the major portion of added plant and animal residues
have decomposed. Usually it is dark colored. (ii) Includes the F and
K Layers in undisturbed forest soils. See soil organic matter. mor,
mull, and soil horizon.
hydraulic conductivity — See soil water.
hydraulic gradient — See soil water.
hydraulic head — See soil water.
hydrogenic soil — Soil developed under the influence of water standing
Within the profile for considerable periods; formed mainly in cold,
humid regions.
hydrologic cycle — The fate of water from the time of precipitation until
the water has been returned to the atmosphere by evaporation and is
again ready to be precipitated.
hydromorphic soils — A suborder of mtrazonal soils, consisting of seven
great soil groups, all formed under conditions of poor drainage in
marshes, swamps, seepage areas, or flats. See classifiCation, soil.
hydrous mica —A silicate clay with 2:1 lattice structure, but of indefinite
chemical composition since usually part of the silicon in the silica
tetrahedral layer has been replaced by aluminum, and containing a
considerable amount of potassium which serves as an additional bond -
ing between the crystal units, resulting in particles larger than nor-
mal in montmoriflonite and, consequently, in a lower cation-exchange
capacity. Sometimes referred to as illite. See clay mineral.
hygroscopic water —Water adsorbed by a dry soil from an atmosphere of
high relative humidity, water remaining in the Sm’ after “air -drying,”
or water held by the soil when it Is in equilibrium with an atmosphere
of a specified relative humidity at a specified temperature, usually
98% relative humidity at 25C.
hymatomelanic acid — The fraction of humus that is soluble in alcohol,
after having been extracted with alkali and precipitated with acid, and
which, upon distillation of the alcohol and drying, forms a brittle mass
that is insoluble in alcohol. Contains certain tannins and the alcohol-
soluble part of humic and ulmic acids.
igneous rock — Rock formed from the cooling and solidification of magma,
and that has not been changed appreciably since its formation.
illite —A hydrous mica. See hydrous mica.
ifluvial horizon — A soil layer or horizon in which material carried from
an overlying layer has been precipitated from solution or deposited
from suspension. The layer of accumulation. See eluvial horizon.
illuviation —The process of deposition of soil material removed from one
horizon to another in the soil; usually from an upper to a lower hori-
ann in the soil profile. See eluviation.
immature soil -. A soil with indistinct os’ only slightly developed horizons
because of the relatively short time it has been subjected to the sari -
ous soil -forming processes. A soil that has not reached equilibrium
with its environment.
immobilization — The conversion of an element from the inorganic to the
organic form in microbial tissues or in plant tissues, thus rendering
the element not readily available to other organisms or to plants.
impeded drainage — A condition which hinders the movement of water
through soils under the influence of gravity.
indicator plants — Plants characteristic of specific soil or site conditions.
infiltration - The downward entry of water into the soil.
infiltration rate — A soil characteristic determining or describing the
maximum rate at which water can enter the soil under specified con-
ditions, including the presence of an excess of water. It has the
dimenSions of velocity (i.e., cm 5 cm- 2 secl cm sec- 1 ). (Formerly,
the infiltration capacity.) See infiltration velocity.
infiltration velocity — The actual rate at which water is entering the soil
at any given time. It may be less than the maximum (the infiltration
rate) because of a limited supply of water (rainfall or irrigation). It
has the same units as the infiltration rate. See infiltration rate.
infiltrometer — A device for measuring the rate of entry of fluid into a
porous body, e.g., water into Soil.
intergrade - A soil that possesses moderately well-developed distinguish-
ing characteristics of two or more genetically related great soil
groups.
intrazonal soils — (i) One of the three orders in soil classification. See
classification, soil. (ii) A soil with more or less well-developed soil
characteristics that reflect the dominating influence of some local
factor of relief, parent material, or age, over the normal effect of
climate and vegetation.
intrinsic permeability — The property of a porous material that relates to
the ease with which gases or liquids can pass through it. The Darcy
“k’ multiplied by q pg.
where
, is the viscosity of the fluid in poises,
p is the density of the fluid in g/cm 3 , and
g is the acceleration of gravity in cm/sec- 2 .
See permeability and soil water.
ions — Atoms, groups of atoms, or compounds, which are electrically
charged as a result of the loss of electrons (cations) or the gain of
electrons (anions).
ion activity — The effective concentration of a particular ion in a solution
or soil-water system. It is expressed analogously to pH, as “pCa,”
“pNa,” etc.
iron-pan — An indurated soil horizon in which iron oxide is the principal
cementing agent.
irrigation — The artificial application of water to the Coil for the benefit of
growing crops.
irrigation efficiency — The ratio of the water actually consumed by crops
on an irrigated area to the amount of water diverted from the source
onto the area.
irrigation methods — The manner in which water is artificially applied to
an area. The methods and the manner of applying the water axe as
follows:
border-strip — The water is applied at the upper end of a strip with
earth borders to confine the water to the strip.
check-basin — The water is applied rapidly to relatively level plots
surrounded by levees. The basin is a small check.
corrugation — The water is applied to small, closely-spaced furrows,
frequently in grain and forage crops, to confine the flow of irri-
gation water to one direction.
flooding — The water is released from field ditches and allowed to
flood over the land.
furrow — The water is applied to row crops in ditches made by tillage
implements.
sprinkler — The water is sprayed over the soil surface through nozzles
from a pressure system.
subirrigation — The water is applied in open ditches or tile lines until
the water table is raised sufficiently to wet the soil.
wild-flooding — The water is released at high points in the field and
distribution is uncontrolled.
isodyne — Points of equal dynamometer pull of a cultivating implement; a
line on a map of a cultivated field connecting points of equal dyna-
mometer pull.
isomorphous substitution — The replacement of one atom by another of
similar size in a crystal lattice without disrupting or changing the
crystal structure of the mineral.
K
kaine — An irregular ridge or hill of stratified glacial drift.
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kaolin — Ci) An aluminosilicate mineral of the 1:1 crystal lattice group;
that is, consisting of one silicon tetrahedral layer and one aluminum
oxide-hydroxide octahedral layer. (ii) The 1:1 group or family of
aluminosilicates.
L
L layer (litter) — The surface layer of the forest floor consisting of fresh-
ly fallen leaves, needles, twigs, stems, bark, and fruits. This layer
may be very thin or absent during the growing season. The 01 hori-
zon. See soil horizon.
lacustrine deposit — Material deposited in lake water and later exposed
either by lowering of the water level or by the elevation of the land.
lacostrine soil — (Obsolete) Soil formed on or from lacustrme deposits.
lagg — The depressed margin of a raised bog.
land classification — The arrangement of land units into various categories
based upon the properties of the land or its suitability for some par-
ticular purpose.
landscape — All the natural features such as fields, hills, forests, water,
etc., which distinguish one part of the earth’s surface from another
part. Usually that portion of land or territory which the eye can com-
prehend in a single view, including all its natural characteristics.
landslide — (i) A mass of material which has slipped down hill under the
influence of gravity, frequently assisted by water (that is, when the
material is saturated). (ii) Rapid movement down elope of a mass of
soil, rock, or debris.
land type — See soil association.
land, wild— Uncultivated land; it may or may not be maintained by the
owner for its productive vegetative cover or for wood, forage pro-
duction, recreation, or wildlife.
Lateritic soil — A suborder of zonal soils formed in warm, temperate, and
tropical regions and including the following great soil groups: Yellow
Podzolic, Red Podzolic, Yellowish—Brown Lateritic, and Lateritic.
See classification, soil and Latosol.
Latosol — A suborder of zonal soils including soils formed under forested,
tropical, humid conditions and characterized by low silica-sesquioxide
ratios of the clay fractions, low base-exchange capacity, low activity
of the clay, low content of most primary minerals, low content of sol-
uble constituents, a high degree of aggregate stability, and usually
having a red color. See classification, soil and Lateritic soil.
lattice energy — The energy required to separate the ions of a crystal to
an infinite distance from each other.
lattice structure — The orderly arrangement of atoms in a crystalline
material.
Leached Saline soils — (i) Soils from which the soluble salts have been
removing by leaching. (ii) Soils that havebeen saline and stillpossess
the major physical characteristics of saline soils but from which the
soluble salts have been leached, generally for reclamation.
leaching — The removal of materials in solution from the soil. See
eluviation.
lime, agricultural — A soil amendment consisting principally of calcium
carbonate but including magnesium carbonate and perhaps other ma-
terials, and used to furnish calcium and magnesium as essential ele-
ments for the growth of plants and to neutralize soil acidity.
lime concretion — An aggregate of precipitated calcium carbonate, or of
other material cemented by precipitated calcium carbonate.
lime-pan — A hardened layer cemented by calcium carbonate.
lime requirement— The mass of agricultural limestone, or the equivalent
of other specified liming material, required per acre to a soil depth
of 6 inches (or on 2 million pounds of soil) to raise the pH of the soil
to a desired value under field conditions.
liquid limit — The minimum percentage (by weight) of moisture at which a
small sample of soil will barely flow under a standard treatment.
Synonymous with ‘upper plastic Limit.’ See plastic limit and plasti-
city number.
lithosequence — A group of related soils that differ, one from the other, in
certain properties primarily as a result of differences in the parent
rock as a soil-forming factor.
Lithosols— A great soil group of azonal soils characterized by an incom-
plete solum or no clearly expressed soil morphology and consisting
of freshly and imperfectly weathered rock or rock fragments.
loam — A soil textural class. See soil texture,
loamy — Intermediate in texture and properties between fine-textured and
coarse-textured soils. Includes all textural classes with the words
“loam” or “loamy” as a part of the class name, such as clay loam or
loamy sand. See loam and soil texture.
bess — Material transported and deposited by wind and consisting of pre-
dominantly silt—sized particles.
lysimeter — (i} A device for measuring percolation and leaching losses
from a column of soil under controlled conditions. (ii) A device for
measuring gains (precipitation and condensation) and losses (evapo-
transpiration) by a column of soil.
M
macronutrient — A chemical element necessary in large amounts (usually
>1 ppm in the plant) for the growth of plants and usually applied
artificially in fertilizer or liming materials (“macro” refers to quan-
tity and not to the essentiality of the element). See micronutrient.
macroscopic velocity — See flow velocity.
made land — Areas filled with earth, or with earth and trash mixed, usually
by or under the control, of man. A miscellaneous land type.
manure — (i) The excreta of animals, with or without the admixture of
bedding or litter, in varying stages of decomposition. Also referred
to as “barnyard manure” or “stable manure.” (The usual meaning of
the term as used in the USA; in many other countries “manure” refers
to any fertilizer.)
marl — Soft and unconsolidated calcium carbonate, usually mixed with
varying amounts of clay or other impurities.
marsh — Periodically wet or continually flooded areas with the surface
not deeply submerged. Covered dominantly with sedgea, cattails,
rushes, or other hydrophytic plants. Subclasses include fresh-water
and salt-water marshes. See swamp.
mature soil — A soil with well-developed soil horizous produced by the
natural processes of soil formation and essentially in equilibrium
with its present environment.
maximum water-holding capacity — The average moisture content of a dis-
turbed sample of soil, 1 cm high, which is at equilibrium with a water
table at its lower surface.
meander land — Unsurveyed land along a lake shore or stream border that
has developed by the receding of the shore line or of the stream since
the last cadastral survey of the area. A miscellaneous land type.
mechanical analysis (Obsolete) See particle-size analysis and particle-
size distribution.
medium-texture — Intermediate between fine-textured and coarse-textured
(soils). (It includes the following textural classes: very fine sandy
loam, loam, silt loam, and silt.)
mellow soil — A very soft, very friable, porous soil without any tendency
toward hardness or harshness. See consistency.
mesophilic bacteria— Bacteria whose optimum temperature for growth
falls in an intermediate range of approximately 15 to 45C.
metamorphic rock— Rock derived from pre-existing rocks but thatdiffer
from them in physical, chemical, and mineralogical properties as a
result of natural geological processes, principally heat and pressure,
originating within the earth. The pre-existing rocks may have been
igneous, sedimentary, or another form of metamorphic rock.
microclimate — (i) The climatic condition of a small area resulting from
the modification of the general climatic conditions by local differ-
ences in elevation or exposure. (ii) The sequence of atmospheric
changes within a very small region.
79
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microfauna That part of the animal population which consists of indi-
viduals too small to be clearly distingtiished without the use of a
microscope. Includes protozoa and nematodes,
microflora — That part of the plant population which consists of individuals
too small to be clearly distinguished without the use of a microscope.
Includes actinomycetes, algae. bacteria, and fungi.
micronutrient— A chemical element necessary in Only extremely small
amounts ( < 1 ppm in the plant) for the growth of plants. Examples
are: B, Cl, Cu, Fe. Mn, and Zn. (“micro” refers to the amount used
rather than to its essentiality.) See macronutrient.
microrelief — Small-scale, local differences in topography, including
mounds, swales, or pits that are only a few feet in diameter and with
elevation differences of up to 6 feet. See coppice mound, cradle tcnoll,
and gilgal.
mine dumps— Areas covered with overburden and other waste materials
from ore and coal mines, quarries, and smelters, and usually with
little or no vegetative cover. A miscellaneous land type,
mine wash— Water-deposited accumulations of sandy, silty. or clavey ma-
terial recently eroded in mining operations. It may clog streams and
channels, and damage land on which it is deposited. A miscellaneous
land type.
mineralization—- The conversion of an element from an organic form to an
inorganic state as a result of microbial decomposition.
mineralogical analysis — The estimation or determination of the hinds or
amounts of minerals present in a rock or in a soil.
mineral soil — A soil consisting predominantly of, and having its proper-
ties determined predominantly by. mineral matter. Usually Contains
(20’t organic matter, but may contain art organic surface layer up to
30 cm thick.
minor element — (Obsolete) See micronutrient.
miscellaneous land type — A mapping utut for areas of land that have little
or no natural soil, that are too nearly inaccessible for orderly exami-
nation, or that for any reason it is not feasible to classify the soil.
See badland. made land, meander land, mine dumps, mine wash, oil
wasteland, river wash, rough broken land, rubble land, scoria land,
slickens, stony land. swamp, tidal flats, urban land, volcanic-ash
land, and waste land,
moderately- coarse texlure - - Consisting predominantly of coarse particles.
(In soil textural ctasslttcation, it includes all the sandy barns except
the very fine sandy loam.) See coarse texture.
moderately-fine texture— Consisting predominantly of intermediate-size
(soil) particles or with relatively small amounts of fine or coarse
particles. (In soil textural classification, it includes clay loam. sandy
clay loam, and Silty clay loam.) See fine texture.
moisture equivalent — The weight percentage of water retained by a pre-
viously saturated sample of soiL 1 cm in thickness aSter it has been
subjected to a centrifugal force of one thousand timeS gravity for 30
mi i i.
moisture-release curve — See moisture-retention curve,
moisture-retention curve— A graph showing the soil moisture percentage
(by weight or by volume) versus applied tension (or pressure). Points
on the graph are usually obtained by increasing (or decreasing) the
applied tension or pressure over a specified range.
moisture tension (or pressure ( — The equivalent negative pressure in the
soil waler, It is equal to the equivalent pressure that must be applied
to the soil water to bring it to hydraulic equitibrium.through a porous
permeable wall or membrane, with a pool of water of the same com-
position. See soil water. The pressures used and the corresponding
percentages most commonly determined are:
80
fifteen-atmosphere percentage — The percentage of water contained in
a soil that has been saturated, subjected to. and is in equtlibrium
with, an applied pressure of 15 atm. (Pressure applied in a pres-
sure membrane or ceramic pressure plate apparatus. Usually
expressed as a weight percentage but may be expressed as a vol-
ume percentage. Approximately the same as fifteen-bar percent-
age.t
fifteen- bar percentage — The percentage of water contained in a soil
that has been saturated, subjected to. and is in equilibrium with,
an applied pressure of 15 bars. (Pressure applied in a pressure
membrane or ceramic pressure plate apparatus. Usually cx-
pressed as a weight percentage but may be expressed as a volume
percentage. Approximately the same as the fifteen-atmosphere
percentage.)
one-third- atmosphere percentage — The percentage of water contained
in a soil that has been saturated, subjected to. and is itt equilib-
rium with. an applied pressure of 1-3 atm. (Pressure applied in
a ceramic plate apparatus, Usually expressed as a weight per-
centage. but may be expressed as a volume percentage. Approx-
imately the same as One-third bar percentage. Also,for medium-
to coarse-textured soils approximately numerically equal to
moisture equivalent.)
one-third bar percentage — The percentage of water contained in a
soil that has been saturated, subjected to. and is in equilibrium
with, an applied pressure of 1 3 bar. (Pressure applied in a
ceramic plate apparatus. Usually expressed as a weight per-
centage. Approximately the same as one-third atmosphere per-
centage. Also, for medium- to coarse-textured toils approxi-
mately numerically equal to moisture equivalent.)
six’tv—centimeter percentage — The percentage of water contained in a
soil that has been saturated. subjected to. and is in equilibrium
with, an applied pressure (Or tension) equivalent 10 a column of
water 60 cm high. (The pressure may be applied in a pressure
plate apparatus or. as a tension, on a tension table. May be ex-
pressed on a weight or volume basis. Considered by many to
approximate the”field-moisture capacity.” especially in medium-
to coarse-textured soils.)
moisture volume percentage— The ratio of the volume of water in a soil
to the total bulk volume of the soil,
moisture weight percentage — The moisture content expressed as a per-
centage of the oven-dry weight of soil, See dry-weight percentage.
muntmorillonite — An aluminosilicate (‘lay mineral with a 2:1 expanding
crystal lattice: that is, wtth two silicott tetrahedral layers enclostng
an aluminum octahedral layer. Considerable expansion max’ be caused
along the C axis by water moving between silica layers of contiguous
units, See montmorillonite group.
montmorillonite group— Clay minerals with 2:1 crystal lattice structure:
that is, two silicon tetrahedral layers enclosing an aluminum octahed-
ral layer, Consists of montinorillonite, beidellite. nontrunite, sapon-
ite, and others.
inor - A type of forest humus in which the H layer is present and in which
there is practically no mixing of surface organic matter with mineral
soil: that is. the transition front the H layer to the Al horizon is
abrupt, (Sonietinies differentiated into thick mor, thin mor, granular
mar, greasy mor. or felty mor,)
mottied zone ‘- A layer that is marked with spots or blotches of different
color or shades of color, The pattern of mottling and the size. abun-
dance, and color contrast of the mottles may var’,’ considerably and
should be specified in soil description.
mottling — Spots or blotches of different color or shades of color inter-
spersed with the dominant color,
muck — Highly decomposed organic material in which the original plant
parts are not recognizable. Contains more mineral matter and is
usually darker in color than peat. See muck soil, peat, and peat soil,
muck soil — (i) A soil containing between 20 and 50T of organic matter.
(ii) An organic soil in which the organic matter is well decomposed
(USA usage),
mulch — (ii Any material such as straw, sawdust. leaves, plastic film.
loose soil. etc.. that is spread upon the surface of the soil to protect
the soil and plant roots from the effects of raindrops, soil crusting.
freezing. evaporation. etc. ii) To apply mulch to the soil surface.
mulch farming — A system of farming in which the organic residues are
not plowed into or otherwise mixed with the soil but are left on the
surface as a mulch,
mull — A type of forest humus in which the F la ’er may or may not be
present and in which there 15 no H layer. The Al horizon consists of
an intinrate mixture of organic matter and mineral soil with gradual
transition between the Al and the horizon beneath. (Sometimes dif-
ferentiated into firm mull. sand mull. coarse niull. nedium mull. and
fine mull.)
Munsell color system — Acolurdesignation system that specifies the rela-
tive degrees of the three simple variables of color: hue, value, and
chroma. For example: 1OYR 6 4 is a color (of soil) with a hue =
1OYR. value = 6, and chroma = 4, These notations can be translated
into severaldiiferent systems of color names as desired, See chroma.
hue, and value, color.
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mycorrhiza — The association, usually symbiotic, of fungi with the roots
of seedplants. See ectotrophic mycorrhiza and endotrophic myc orrhiza.
N
natural erosion — See erosion, (ii).
neutral soil — A soil in which ihe surface layer, si least to normal plow
depth, is neither acid nor alkaline in reaciion. See scid soil, alkaline
soil, pH, and reaction, soil.
nitrate reduction — The biochemical reduction of nitrate.
nit rification — The biochemical oxidation of animonium to nil rate.
nitrogen assimilation — The incorporation of nitrogen into organic cell
substances by living organisms.
nitrogen cycle — The sequence of biochemical changes undergone by nitro-
gen wherein it is used by a living organism, liberated upon the death
and decomposition of the organism, and converted to its original slate
of oxidation.
nitrogen fixation — The conversion of elemental nitrogen (N 2 ) to organic
combinafions or to forms readily utilizable in biological processes.
nodule bacteria — See rtiizobia.
0
O horizon — See soil horizon.
oil wasteland — Land on which oily wastes have accumulated. Includes
slush pits and adjacent areas affected by oil waste. A miscellaneous
land type.
one-third atmosphere percentage — See moisture tension )or pressure).
one-third bar percentage — See moisture tension (or pressure).
organic phosphorus — Phosphorus present as a constituent of an organic
compound, oragroup of organic compounds such as glycerophosphoric
acid, inositol phosphoric acid, cylidylic acid, etc.
organic soil —A soil which contains a high percenfage (>15 or 20° .) of
urganic matter throughout the solum.
ortatein — An indurated layer in the B horizon of Podzols in which the
cementing material consists of illuviated sesquioxides (mostly iron)
and organic matter.
osmotic potential — See soil water.
osmotic pressure — See Soil water.
oven-dry soil — Soil which has been dried at lOSC until if reaches constant
weight.
P
pans — Horizons or layers, in soils, that are strongly compacted. indur-
ated, or very high in clay content. See caliche. claypan, fragipan,
and har an.
pan, genetic — A natural subsurface soil layer of low or very low perme-
ability, with a high concentration of small particles, and differing in
certain physical and chemical properties, from the soil immediately
above or below the pan. See claypan. fragipan, and hardpan, all of
which are genetic pans.
pan, pressure or induced — A subsurface horizon or soil layer having a
higher bulk density and a lower total porosity than the soil directly
above or below it. as a result of pressure that has been applied by
normal tillage operations or by other artificial means. Frequently
referred to as plowpan, plowsole, or traffic pan.
parent material — The unconsolidated and more or less chemically wea-
thered mineral or organic matter from which the solum of soils is
developed by pedogenic processes.
partial sterilization — The elimination of a portion of a population of mi-
croorganisms usually by treatment with heat or chemicals. The pro-
cess is selective, with certain organisms or groups of organisms
being destroyed to a greater extent than others.
particle density — The mass per unit volume of the soil particles. to
technical work, usually expressed as grams per cubic centimeter.
See bulk density, soil.
particle size — The effective diameter of a particle measured by sodimen-
tation, sieving, or micrometric methods.
particle-size analysis — Determination of the various amounts of the dif-
ferent separates in a soil sample, usually by sedimentation, sie iog,
micrometry, or combinations of these methods.
particle-size dfslrtbutioo — The amounts of the various soil separates in a
soil sample, usually expressed as weight percentages.
parts per million (ppm) —Weight units of any given substance per one mil-
lion equivalent weight units of oven-dry soil; or, in the case of soil
solution or other solution, the weight units of solute per million weight
units of solution.
peat — Unconsolidated soil material consisting largely of undecomposed,
or only slightly decomposed, organic matter accumulated under con-
ditions of excessive moisture.
peat soil — An organic soil containingmore than 50’ organic matter. Used
in the United States to refer to the stage of decomposition of the or-
ganic matter, ‘peat” referring to the slightly decomposed or ondecom -
posed deposits and “muck” to the highly decomposed materials, See
peat, muck, and muck soil,
ped — A unit of soil slrocture such as an aggregate. cmmb, prism, block,
or granule, formed by natural processes )in contrast with a clod.
which is formed artificially).
pedalfer — )Obsolete) A subdivision of a soil order comprising a large
group of soils in which sesquioxides increased relative to silica dur-
ing soil formation.
pedocal — )Obsotete) A subdivision of a soil order comprising a large
group of soils in which calcium accumulated during soil formation.
peneplain — A once high. mgged area which has been reduced by erosion
to a low, gently rolling surface resembling a plain.
penetrability — The ease with which a probe can be pushed into the soil.
(May be expressed in units of distance, speed, force, or work de-
pending on the type of penefrometer used,)
percolation, soil water — The downward movement of water through soil.
Especially ; the downward flow of waler in saturated or nearly satu-
rated soil at hydraulic gradients of the order of 1.0 or less.
permafrost — (i) Permanently frozen material underlying the solum. )ii)
A perennially frozen soil horizon.
permafrost table — The upper boundary of the permafrost [ see permafrost.
(i) ] coincident with fhe tower limit of seas-nat fhaw.
permanent charge — The net negative (or positive) charge of clay particles
inherent in the crystal lattice of the particle; not affected by changes
in pH or by ion-exchange reactions,
permeability, soil — )i) The ease with which gases. liquids, or plant roots
penetrate or pi.ss through a bulk mass of soil or a layer of soil.
Since different soil horizons vary in permeability, the particular hori-
zon under question should be designated. (ii) The property of a por-
ous medium itself that relates to the ease with which gases, liquids.
or other substances can pass lhrough it, Previously, frequently con-
sidered the “k” in Darcy’s law. The’K” in intrinsic permeability.
See intrinsic permeability, Darcy’s law, and soil water,
pF (Obsolete) The logarithm of the soil moisture tension expressed in
centimeters height of a column of wafer.
pH. hydrolytic — The arithmetical difference bet ween the pH value of a soft
as measured on the soil paste and the value obtained on a 1:10 soil-
water suspension.
pH, isohydric — The pH vaiue of a soil identical with that of a buffer solu-
tion that remains unchanged when mixed with the s&i.
pH, soil — The negative logarithm of the hydrogen-ion activity of a soil.
The degree of acidity (or alkalinity) of a soil as determined by means
of a glass, quinhydrone, or other suitable electrode or indicator at a
specified moisture content or soil-water ratio, and expressed in
terms of the pH scale.
81
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p 11-dependent charge — That portion of the total charge of the Soil (clayt
particles which is affected by. and varies with, changes in ph.
phase. Soil —A subdivision of a soil type or other unit of classification
having characteristics that affect the use and management of the soil
but which do not vary sufficiently to differentiate it as a separate type.
A variation in a property or characteristic such as degree of slope.
degree of erosion. content of stones. etc.
photomap — A mosaic map made from aerial photographs with physical
and cultural features shown as on a planimetric map. See planisaic
and toposaic-.
physical properties (of soils) — Those characteristics, processes, or re-
actions of a soil which are caused by physical forces and which can
be described by, or expressed in. physical terms or equations. Some-
times confused with and difficult to separate from chemical proper-
ties: hence, the terms “physical-chemical” or “physicochemical.”
Examples of physical properties are bulls density, ‘water-holding Ca-
pacity. hydraulic conductivity. porosity, pore-size distribution, etc.
physical weathering — The breakdown of cork and mineral particles Into
smaller particles by physical forces such as frost action. See wea-
thering.
phvsiosorption — The process of attachment of nonionic substances such
as polar water molecules, acetic acid molecules, or nucleic acids to
clays or to other solid-phase surfaces. The attachment of large
molecules to clay particles by ionic processes is not physiosorption.
phytogenic soils — (Obsoletel Soils developed under the dominant influ-
ence of the natural vegetation, mainly in temperate regions.
phytonieter A plant or plants used to measure the physical iactors of
the habitat in terms of physiological actit’ities.
phvtomorphlc soils - - tCanadal Well -drained soils of an association which
have developed under the dominant intl Uclici’ of the natural vegetatior
characteristic of a region. The zonal soils of an area.
pitchv - Dense and hard when dry, breaking with sii;uoth. somewhat Ian -
trous i’onchoidal fracture into sharp-angled fragments. Wet pitchv
peat is very plastic and f squeezed in the hand. oozes between the
fingers.
planisaic A photomap in which iheplaninietric deiait is shown by over-
prints in colors. See photomap and toposaic.
Planos Ol - A great soil group of the intrazonal order and hvdron:orphic
suborder consisting of soils with eluviated surface horizons under-
lain by B horizons more strongly eluvialed. cemented, or compacted
than associated normal Soil.
plastic soil — A soil capable of being molded or deformed continuously and
permanenU . by relatively moderate pressure, into various shapes.
See consistency.
plastic linuil — The minimum moisture percentage by weight al which a
small sample of soil material can be deformed without rupture. Syn-
onymous with ‘lower plastic limit. See liquid limit and plasticity
number.
podzolization - A process of soil formation resulting in the genesis of
Podzols and Podzolic soils.
pore -size distribution .- The volume of the various sizes of pores in a soil.
Expressed as percentages of the bulk volume (soil plus pore space).
pore space — Total space not occupied by Soil particles in a bulk volume
of soil.
porosite — The volume percentage of the total bulk not occupied by solid
particles.
potassium fixation- The process of converting exchangeable or water -
soluble potassium to moderately soluble potassium: i.e.. to a form
not easily exchanged from the adsorption complex with a cation of a
neutral salt solution.
potassium -supply power of soils — The capacity of the soil to supply po-
tassium to growing plants from both the exchangeable and the mod-
erately available forms.
Prairie soils — A zonai great soil group consisting of soils formed under
temperate to cool-temperate, humid regions under tall grass vege-
tation. See classification, soil.
precipitation interception — The stopping, interrupting and temporary hold-
ing of precipitation in any form by a vegetative canopy or vegetation
residue.
pressure membrane — A membrane. permeable to water and only very
slightly permeable to gas when wet. through which water can escape
from a soil sample in response to a pressure gradient.
primary mineral A mineral that has not been altered chemically since
deposition and crystallization from molten lava. See secondary
mineral.
prismatic soil Structure - - A soil structure type with prisnilike aggregates
that have a vertical axis much I anger than the horizontal axes. See
soil structure types.
productivity, soil The capacit Y of a soil, in its normal environnient. for
producing a specified plant or sequence of ptants under a speciiied
system of management. The specified’ limilations are necessary
since no soil can produce all crops with equal success nor can a
single system of inanagenient produce the same effect on all soils.
Productivity emphasizes the capacity of soil to produce crops and
should be expressed in terms of yields.
productive soil - A soil in which the chemical, physical, and biological
conditions are favorable for the economic production of crops suited
to a particular area.
profile, soil — A vertical section of the soil through all its horizons and
extending into the parent material,
psamrnophvtes -- Plants which prefer or tolerate sand, particularly fine
to medium sand, as a habitat.
pure culture — The growth of a single strain of an organism in the ab-
sence of any other living Species or strain.
plasticity constants — See liquid fimit. plastic limit, and plasticity number.
plasticity number — The numerical difference between the liquid and the
plastic limil or. synonymously, between the lower plastic limit ansi
the upper plastic limit. Sometimes called “plasticity index.”
plasticitr range — The range of moisture weight percentage within which a
susall sample of sofl exhibits plastic properties.
plate count - - A roust of the number of colonies formed on a culture mcdi -
urn which has been inoculated with a small amount of soil for the par -
pose of estimating the number of certain organisms present in the
soil sample.
platy — Consisting of sail aggregates that are developed predominately
along the horizontal axes: laminated: flaks’. See soil structure types.
pocosin - A swamp, usually containing organic soil, and partly or com-
pletely enclosed by a sandy rim. The Carolina Bays of the South-
eastern United States.
Podzal A great soil group of the zonal order consisting of soils formed
in cool -temperate to temperate, humid climates, under coniferous or
mixed coniferous and deciduous forest, and characterized particularly
by a hi hir-leached. whitish-gray (Podzol) A2 horizon.
It horizon See soil horizon.
It
rainfall interception — See precipitation interception
reaction, soil — The degree of acidity or alkalinity of a soil, usually ex-
pressed as a pIt value. Descriptive terms conunionfu’ associated with
certain ranges in ph are: exireniely acid. ‘:4.5: very strongly acid.
4.5-5.0: strongly acid. 5.1-5.5: moderately acid. 5.6-tO: slightly
acid. 6.1-6.5; neutral. 6.6-7.3: slightly alkaline. 7.4-7.8: moderately
alkaline. 7.9 -8.4:stronglv alkaline. 8.5-9.0: and very stroi il’ alkaline.
“9.1
Red Desert soil — A zonal great soil group consisting of soils formed un-
der war in -temperate to hot, dry regions under desert-type vegetation.
mostly shrubs.
red earth — Highly leached, red clayev soits of the humid tropics, usually
with very deep protiles that are low in silica and high in sesquioxides.
Red-Yellow Podzolic soils —A combination of the zonal great soil groups.
Red Podzolic and Yellow Podzolic. consisting of soils formed under
warm-temperate to tropical. humid climates, under deciduous or
coniferous forest vegetation and usually, except for a few members
of the Yellow Podzolic Group, under conditions of good drainage.
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regolitli — The unconsolidated mantle of weathered rock and soil material
on the earth’s surface, loose earth materials above solid rock. (Ap-
proximately equivalent to the term ‘soil” as used by many engineers
Regosol — Any soil of the azonal order without definite genetic horizons
and developing from or on deep, unconsolidated, soft mineral deposits
such as sands, bess, or glacial drift.
Regur — An intrazonal group of dark calcareous soils high in clay, which
is mainly montmorillonitic, and formed mainly from rocks low in
quartz; occurring extensively on the Deccan Plateau of India.
Rendzina — A great soil group of the intrazonal order and calcimorphic
suborder consisting of soils with brown or black friable surface hori-
zons underlain by light gray to pale yellow calcareous material: de-
veloped from soft, highly calcareous parent material under grass
vegetation or mixed grasses and forest in humid and semiarid cli-
mates.
residual material — Unconsolidated and partly weathered mineral materi-
als accumulated by disintegration of consolidated rock in place.
residual shrinkage — The decrease in the bulk volume of soil in addition
to that caused by the loss of water.
residual soil — (Obsolete) A soil formed from, or resting on, consolidated
rock of the same kind as that from which it was formed, and in the
same location, See residual material.
retentivity profile, soil — A graph showing the retaining capacity of a soil
as a function of depth. The retaining capacity may be for water, for
water at any given tension, for cations, or for any other substances
held by Soils.
reticulate mottling — A network of streaks of different color; most com-
monly found in the deeper profiles of Lateritic soils.
reversion — The changing of essential plant nutrient elements from solu-
ble to less soluble forms as a result of interaction with, or reactions
in, the soil. Usually restricted to the conversion of monocalciurn
phosphate to the less soluble dicalcium phosphate.
rhizobia — Bacteria capable of living symbiotically with higher plants,
usually legumes, from which they receive their energy, and capable
of using atmospheric nitrogen; hence, the term symbiotic nitrogen-
fixing bacteria. (Derived from the generic name Rhizobium.)
rhizoplane — The external surface of plant roots, (As used by some, it
includes the surface of the adhering soil particles.)
rhizosphere — That portion of the soil directly affected by plant roots.
nil — A small, intermittent water course with steep sides; usually only a
few inches deep and, hence, no obstacle to tillage operations.
nil erosion — See erosion, (ii).
river wash— Barren alluvial land, usually coarse-textured, exposed along
streams at low water and subject to shifting during normal high water.
A miscellaneous land type.
rock land— Areas containing frequent rock outcrops and shallow soils.
Rock outcrops usually occupy from 25 to 90% of the area. A miscel-
laneous land type.
rough broken land — Land with very steep topography and numerous inter-
mittent drainage channels but usually covered with vegetation. See
miscellaneous land type and badlands.
rubble land— Land areas with 90% or more of the surface covered with
stones and boulders. A miscellaneous land type.
runoff — That portion of the precipitation on an area which is discharged
from the area through stream channels. That which is lost without
entering the soil is called surface runoff and that which enters the
soil before reaching the stream is called ground water runoff or seep -
age flow from ground water. (In soil science “runoff” usually refers
to the water lost by surface flow; in geology and hydraulics “runoff”
usually includes both surface and subsurface flow.)
saline-alkali soil — (i) A soil containing sufficient exchangeable sodium
to interfere with the growth of most crop plants and containing appre-
ciable quantities of soluble salts, The exchangeable-sodium percent-
age is >15, the conductivity of the saturation extract >4 miilimhos
per Centimeter (at 25C), and the pH is usually 8.5 or less in the satu-
rated soil, (ii) A saline-alkali soil has a combination of harmful
quantities of salts and either a high alkalinity or high content of ex-
changeable sodium, or both, so distributed inthe profile that the growth
of moat crop plants is reduced, (Often called saline- sonic soil.)
saline soil — A nonalkali soil containing sufficient soluble salts to impair
its productivity. (This name was formerly applied to any soil con-
taining sufficient soluble salts to interfere with plant growth.)
salinizalion — The process of accumulation of sails in soil.
salt-affected soil — Soil that has been adversely modified for growth of
most crop plants by the presence of certain types of exchangeable ions
or of soluble salts. (includes soil having an excess of salts, or an ex-
cess of exchangeable sodium, or both.) See saline-alkali soil, saline
soil, and sodic soil.
sand— (i) A soil particle between 0.05 and 2.0mm in diameter. (ii) Any
one of five soil separates, namely: very coarse sand, coarse sand,
medium sand, fine sand, and very fine sand. See soil separates. (iii)
A soil textural class. See soil texture.
sandy — Containing a large amount of sand. (Applied to any one of the soil
classes that contains a large percentage of sand.) See class, soil and
soil texture.
sandy clay loam — A soil textural class. See class, soil and soil texture.
sandy clay — A soil textural class. See class, soil and soil texture.
sandy loam — A soil textural class. See class, soil and soil texture.
saturate — (i( To fill all the voidsbetween soil particles with a liquid. (ii)
To form the most concentrated solution possible under a given set of
physical conditions in the presence of an excess of the solute. (iii)
To fill to capacity, as the adsorption complex with a cation species;
e.g., H-saturated, etc.
scoria land— Areas of slaglike clinkers, burned shale, and fine-grained
sandstone; characteristic of burned-out coal beds. Such areas com-
monly support a sparse cover of grasses, but are of low agricultural
value. A miscellaneous land type.
second bottom — The first terrace above the normal flood plain of a stream.
secondary mineral — A mineral resulting from the decomposition of a pri-
mary mineral or from the reprecipitation of the products of decom-
position of a primary mineral. See primary mineral.
sedimentary rock — A rock formed from materials deposited from sus-
pension or precipitated from solution and usually being more or less
consolidated. The principal sedimentary rocks are sandstones, shales,
lime stones, and conglomerates.
self-mulching soil — A soil in which the surface layer becomes So well
aggregated that it does not crust and seat under the impact of rain
but instead serves as a surface mulch upon drying.
separate, soil — See soil separate.
series, soil— See soil series.
shaly — (i) Containing a large amount of shale fragments, as a soil. (ii)
A soil phase as, for example, shaly phase. See coarse fragments.
shear — Force, as of a tillage implement, acting at right angles to the di-
rection of movement.
sheet erosion — See erosion, (ii).
sterozein — A zonai great soil group consisting of soils withpale grayish A
horizons grading into calcareous material at a depth of 1 foot Or less,
and formed in temperate to cool, arid climates under a vegetation of
desert plants, short grass, and scattered brush.
silica-alumina ratio—The molecules of silicon dioxide (S10 2 )per molecule
of aluminum oxide (A1 2 0 3 ) in clay minerals or in soils.
siiica-sesqiiioxide ratio — The molecules of silicon dioxide (Si0 2 ) per
molecule of aluminum oxide (A1 2 0 3 ) plus ferrix oxide (Fe 2 O 3 ) in clay
minerals or iii soils,
S
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silt (i) A soil separate consisting of particles between 0.05 and 0.002 mm
in equivalent diameter. See sot1 separates. (iii A soil textural class.
See soil texture.
silting — The deposition of water-borne sediments in stream channels, lakes,
reservoirs. or onflood plains, usually resulting from a decrease in the
velocity of the water.
silt loam A soil textural class containing a large amount of silt and small
quantities of sand and clay. See soil texture and class. soil.
silty clay — A soil textural class containing a relatively large amount of silt
and clay and a small amount of sand. See soil texture and class, soil.
silt clay loam — A soil textural class containing a relatively large amount
of silt, a lesser quantity of clay, and a still smaller quantity of sand.
See clans, soil and soil texture.
site — Ii) In ecology, an area described or defined by its biotic. climatic.
and soil conditions as related to its capacity io produce vegetation. (ii)
An area sufficiently uniform in biotic, climatic, and soil conditions to
produce a particular climax vegetation.
site index — (it A quantitative evaluation of the productivity of a soil for
forest growth under the existing or specified environment. (ii) The
height to feet of the dominant forest vegetation taken at or calculated
to an index age, usually 50 or 100 years.
sixty-centimeter tension — See moisture tension (or pressurel.
slat — Containing a considerable quantity of slate fragments. (Used to mod-
ify soil texture class names as, “slat clay loam. etc.t See coarse
fragments.
slickens Fine-textured materials separated in placer mining and in ore-
mill operations: maybedetrimentalto plant growth unless confined in
specially constructed bastns. A miscellaneous land type.
slick spots Smallaroasinafieldthatareslickwhenwel.dueto a high con-
tent of alkali or of exchangeable sodium.
Slough Podzol See Depression Podzol.
sodic soil - ( l A soil that contains sufficient sodium to interfere with the
growth of most crop plants. (iii A soil in which the exchangeable-
sodium percentage is 15 or more.
soil -. (it Thewiconsolidatedmineralmaterial on the immediate surface of
the earth that serves as a natural medium for the growth of land plants.
(ii) The unconsolidated mineral niatteron the surface of the earth that
has been subjected to and influenced by genetic and environmental factors
of: parent material, climate (including moisture and temperature
effectst. macro- and microorganisms, and topography. all acting over
a period of time and producing a product—soil—that differs from the
material from which it is derived in many physical, chemical, biological
and morphological properties, and characteristics.
soil air — The soil atmosphere: the gaseous phase of the soil, being that
volume not occupied by solid or liquid.
soil alka.ljnitv — The degree or intensity of alkalinity of a soil, expressed by
a value >7.0 on the pH scale.
soil association — (i) A group of defined and named taxonomic soil units
occurring together in an individual and characteristic pattern over a
geographic region, comparable to plant associations in many vays.
(Sometimes called “natural land wpe.”) liii A mapping unit used on
general soil maps, in which two or more defined tax000mic units occur-
ring together in a characteristic pattern are combined because the scale
of the map or the purpose for which it is being made does not require
delineation of the individual soils.
soil auger — A tool for boring into the soil and withdrawing a small sample
for field or laboratory observation. Soil augers may be classified mb
several types as follows: (ii those with worm-type bits, uninctosed;
(ut those with worm-type bits inclosed in a hollow cylinder: and (iii
those with a hollow cylinder with a cutting edge at the lower end.
soil chemistry — A divisionofsoilscienceconcernedwiththe chemical con-
Siltation, the chemical properties, and the chemical reactions of
soils.
soil classification — See classification, soil.
scM complex — A mapping unit used in detailed soil surveys where two or
more defined taxonomic units are so intimately intermixed geographi-
cally that it is undesirable or impractical, because of the scale being
used, to separate them. A more intimate mixing of smaller areas of
individual taxonomic units than that described under soil association.
soil conservation (i) Protection of the soil against physical loss by erosion
or against chemical deterioration; ihat is, excessive loss of fertility
by either natural or artificial means. (ii) A combination of all manage-
ment and land use methods which safeguard the soil against depletion
ordeteriorationby natural or by man-induced factors. (iii) A division
of soil science concerned with soil conservation (i) and (ii ).
soil creep — See creep.
soil extract The solution separated from a soil suspension or from a soil
by filtration, centrifugation, suction, or pressure. (May or may not be
heated prior to separation.)
soil-formation factors — The variable, usually interrelated natural agencies
that are active in and responsible for the formation of soil. The factors
are usually grouped into five major categories as follows: parent rock,
climate, organisms, topography, and time.
soil genesis — (i) The mode of origin of the soil wtth special reference to
the processes or soil-forming factors responsible for the development
of the soltim. or true soil, from the unconsolidated parent material. (ii)
A division of soil science concerned with soil genesis (i).
soil geography A suhspecialization of physical geography concerned with
the areal distributions of soil types.
soil horizon A layer of soil or soil material approximately parallel to the
land surface and differing from adjacent genetically related layers in
physical, chemical, and biological properties or characteristics such
as color, structure, texture, consistency. kinds and numbers of or-
ganisms present. degree of acidity or alkalinity. etc. The following
table lists the designations and properties of the major soil horizons.
Very few if any soils have all of these horizons well developed but every
soil has some of them.
Horizon
designation Description
Organic horizons of mineral soils. Horizons:(i) formed
orformingintheupper part of mineral soils above the
mineral part: (ii) dominated by fresh or partly decom-
posedorganic material; and (iii) containing>30 or-
ganic matter if the mineral fraction is)50 clay, or
>20 organic matter if the mineral fraction has no
clay. Intermediate clay content requires proportional
organic matter content.
Organic horizons in which essentially the original form
of most vegetative matter is visible to the naked eye.
The 01 corresponds to the L (litter) and some F (fer-
mentation) layers in forest soils designations, and to
the horizon formerly called Aoo.
Organic horizons in which the original form of most
plant or animal matter cannot be recognized with the
naked eye. The 02 corresponds to the IT (humus) and
some F (fermentation) layers in forest soils designa-
tions, and to the horizon formerly called Ao.
Mineral horizons consisting of: (t) horizons of organic-
matter accumulation formed or forming at or adjacent
to the surface: (ii) horizons that have lost clay, iron,
or aluminum with resultant concentration of quartz or
other resistant minerals of sand or silt size; or (iii)
horizons dominatedby (i) or (it) above but transitional
to an underlying B or C.
Mineral horizons, formed or forming at or adjacent to
the surface, in which the feature emphasized is an
accumulation of humifted organic matter intimately
associated with the mineral fraction.
Mineral horizons in which the feature emphasized is
loss of clay, iron, or aluminum, with resultant concen-
tration of quartz or other resistant minerals in sand
and silt sizes,
A transitional horizon between A and B, and dominated
by properties characteristic of an overlying Al or A2
but having some subordinate properties of an under-
lying B.
A horizon transitional between A and B, having an upper
part dominated by properties of A and a lower part
dominatedbvpropertiesof B, and the two parts cannot
be conveniently separated into Al and Bl.
Horizons that would qualify for A2 except for included
parrsconslttuting <50 of the volume that would qual-
ify as B.
A horizon transitional between A and C, having subor-
dinate properties of both A and C, but not dominated by
properties characteristic of either A or C.
Any horizon qualifying as B iii >50% of its volume
including parts that qualify as A2.
0
01
02
A
Al
A2
A3
AB
A&B
AC
B& ’A
84
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Horizons in which the dominant feature or features is
oneor more of thefollowing: (i) an illuvialconcentra-
tion of silicate clay, iron, aluminum, or humus, alone
or in combination; (ii) a residual concentration of
sesqutoxides or silicate clays, alone or mixed, that
has formed by means other than solution and removal
of carbonates or more soluble salts: (iii) coatings of
sesquioxides adequate to give conspicuously darker.
stronger, or redder colors than overlying and under-
lying horizons in the same sequum but without apparent
illuviation of iron and not genetically related tolihori-
cons that meet requirements of (i) or (ii) in the same
sequum; or (iv) an alteration of material from us
original conditon in sequums lacking conditions defined
in(i),(ii),and(iii ) that obliterates original rock struc-
ture, that forms silicate clays, liberates oxides, or
both, and that forms granular, blocky, or prismatic
structure if textures are such that volume changes
accompany changes in moisture.
A transitional horizon between B and Al or between B
andA2 in whichthe horizon is dominated by properties
of anunderlying B2 but has some subordinate properties
of an overlying Al or A2.
That part of the B horizon where the properties on
which the B is based are without clearly expressed
subordinate characteristics indicating that the horizon
istransitional to an adiacentoverlyingAoranadjacent
underlying C or It.
A transitional horizon between B and C or It in which
the propertiesdiagnostic ofanoverlying B2areclearly
expressed but are associated with clearly expressed
properties characteristic of C or R.
A mineral horizon or layer, excluding bedrock, that
is either like or unlike the material from which the
solum is presumed to have formed. relatively little
affected by pedogenic processes, and lacking properties
diagnostic of A or H but including materials modified
by: (i) weathering outside the zone of major biological
activity; (ii) reversible ttemenlation, development of
brittleness, development of high bulk density, and other
properties characteristic of fragipans: (iii) gleving:
( iv) accumulation of calcium or magnesium carbonate
or more soluble salts: (v) cementation by accumula-
tions suchas calcium or magnesium carbonate or more
soluble salts: or (vi) cementation by alkali-soluble
siliceous material or by iron and silica.
Underlying consolidated bedrock, such as granite,
sandstone, or limestone. If presumed to be like the
parent rock from which the adjacent overlying layer
or horizon was formed, the symbol It is used alone.
If presumed to be unlike the overlying material, the
R is preceded by a Roman numeral denoting luthologic
discontinuity.
soil improvement - The process for, or the results of, making the soil more
productive for growing plants, by drainage, irrigation, addition of fertil-
izers and soil amendments, and the like.
soil management — (i) The sum total of all tillage operations, cropping prac-
tices, fertilizer, lime, and other treatments conducted on or applied to
a soil for the production of plants. (ii) A division of soil sciencecon-
eerned with the items listed under (i).
soilnianagement groups— Groups of taxonomic soilonits with similar adap-
tations or management requirements for one or more specific purposes,
such as: adapted crops or crop rotations, drainage practices, fertil-
ization, forestry, highway engineering, etc.
soil map— A map showing thedistributionofsoiltypesorothersoilmapping
units in relation to the prominent physical and cultural features of the
earth’s surface. The following kinds of soil maps are recognized in
the United Rates:
soil map,detailed — A soil map on which the boundaries are shown be-
tween all soil types that are significant to potential use as field-
management systems. The scale of the map will depend upon the
purposetobe served, the intensity of land use,the pattern of soilS.
and the scale of the other cartographic materials available. Tra-
verses are usually made at 1, 4-mile, or more frequent, intervals.
Commonly a scale of 4 inches=l mile (1:15,840) is now used for
field mapping in the U. S.
soil map, detailed reconnaissance — A reconnaissance map nfl which
some areas or features are shown in greater detail than usual, or
than others.
soil map, generalized — A small- scale soil map which shows the general
distributionofsoilswithin a largearea and thus in less detail than
on a detailed soil map. Generalized soil maps may vary from soil
association maps of a county, on a scale of 1 inchal mile (1:63,360).
to maps of larger regions showing associations dominated by one
Or more great soil groups.
soil map, reconnaissance — A map showing the distribution of soils over
a large area as determined by traversing the area at intervals vary-
ing from about 12 mile to several miles. The units shown are
soil associations. Such a map is usually made only for exploratory
purposes to outline areas of soil suitable for more intensive
development. The scale is usually much smaller than for detailed
soil maps.
soil map. Schematic — A soil map compiled from scant knowledge of the
soils of new andundeveloped regions by the application of available
information about the soil-formation factors of the area. Usually
on a small scale (1:1,000,000 or smaller).
See soil-formation factors.
soil mechanics and engineering — A subspecialization of soil science con-
cerned with the effect of forces on the soil and the application of
engineering principles to problems involving the soil.
soil microbiology — A subspecialization ol soil science concernedwith soil-
inhabiting microorganisms and with their relation to agriculture, in-
cluding both plant and animal growth.
soil mineral — (i) Any mineralthat occurs as a part ofor in the soil. (ii) A
natural inorganic compoundwith definite physiral,chemical.and crystal-
line properties (within the limits of isomorphism). that occurs in the
soil. See clay mineral.
soil mineralogy — A subspecializalion of soil science concerned with the
homogeneous inorganic materialsfound in the earth’s crust tothe depth
of weathering or of sedimentation.
soil moisture— Water contained in the soil.
Soil-moisture tension — See moisture tension (or pressure).
soil monolith — A vertical section of a soil profile removed from the soil
and mounted for display or study.
soil morphology — (i) The physical constitution, particularly the structural
properties, of a soil profile as exhibited by the kinds, thickness, and
arrangement of the horizons in the profile, and by the texture, struc-
ture, consistency, and porosity of each horizon. (ii) The structural
characteristics of the soil or any of its parts.
soil organic matter The organic fraction of the soil; includes plant and
animal residues at various stages of decomposition, cells and tissues
of soil organisms, and substances synthesized by the soil population.
Usually determined on soils which have been sieved through a 2.0-mm
sieve.
soil piping or tunneling— Acceleraiederosionwhich results in subterranean
voids and tunnels.
soil population — All the organisms living in the soil, including plants and
animals.
soil pores — That part of the bulk volume of soil not occupied by soil parti-
cles: interstices: voids.
soil porosity— See porosity.
soil province — (Obsolete) Areas similar in mode of origin of the soil parent
materials or in geological or geographic features.
soil reaction — See reaction, soil and pH, soil.
soil salinity — The amount of soluble salts in a soil, expressed in terms of
percentage, parts per million, or other convenient ratios.
soil science — That science dealing with soils as a natural resource on the
surface of the earth including Soil formation, classification and mapping,
and the physical, chemical, biological, and fertility properties of soils
per Se: and these properties in relation to their managemeni for crop
production.
Horizon
designation Description
Bi
B2
B3
85
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soil separates —Mineralparticles, <2.0 mminequivalentdiameter,ranging
between specified size limits. The names and size limits of separate
recognized intheUnitedStatesare: very coarse sand, 12.0 to 1.0 mm;
coarse sand, 1.0 to 0.5 mm; medium sand, 0.5 to 0.25 mm: fine sand,
0.2StoO.10 mm; veryfinesand, 0.lOtoO.05mm; silt,0.05 to 0.002mm;
and clay, 2 <0.002 mm.
The separates recognized by the International Society of Soil Science
are: I) coarse sand, 2.0 to 0.2 mm; U) fine sand, 0.2 to 0.02 mm;
III) silt, 0.02 to 0.002 mm: andW) clay, <0.002 mm.
soil series .— The basic unit of soil classification being a subdivision of a
family and consisting of soils which are essentially alike in all major
profile characteristics except the texture of the A horizon.
soil solution — The aqueous liquid iase of the soil and its solutes consisting
of ions dissociated from the surfaces of the soil particles and of other
soluble materials.
soil structure — The combination or arrangement of primary soil particles
into secondary particles, units, or peds. These secondary units may
be, but usually are not, arranged in the profile in such a manner as to
give a distinctive characteristic pattern. The secondary units are
characterized and classified on the basis of size, shape, and degree of
distinctness into classes, types, and grades, respectively. See soil
structure classes, soil structure grades, and soil structure types.
Also see Table 1.
‘Prior to 1947 this separate was called ‘fine gravel; now fine gravel
includes particles between 2.0mm and about 12.5 mm in diameter.
2 Prior to 1937, ‘clay included particles 0.005 mm in diameter, and
“silt,” those particles from 0.05 to 0.005 mm.
soil structure classes — A grouping of soil structural units or peds on the
basis of size. See soil structure, soil structure types, and Table 1.
soil structure grades — A grouping or classification of soil structure on the
basis of inter- and intra-aggregate adhesion, cohesion, or stability
within the profile. Four grades of structure designated from 0 to 3
are recognized as follows:
0) Structureless — no observable aggregation or no definite and orderly
arrangement of natural lines of weakness. Massive, if coherent;
single-grain, if noncoherent.
I) Weak — poorly formed indistinct pads, barely observable in place.
2) Moderate — well-formed distinct peda, moderately durable and evi-
dent, but not distinct in undisturbed soil.
3) Strong — dural)le pads that are quite evident in undisturbed soil,
adhere weakly to one another, withstand displacement, and become
separated when the soil is disturbed.
soil structure types — A classification of soil structure based on the shape
of theaggregatesorpeds andtheir arrangementin theprofile. See soil
structure, soil structure classes, soil structure grades, and Table 1.
soil survey — The systematic examination, description, classification, and
mapping of soils in an area. Soil surveys are classified according to
the kind and intensity of field examination.
soil texture — The relative proportions of the various soil separates in a soil
asdescribedby theclasses ofsoiltextureshownin Fig. I. Thetextural
classes may be modified by the addition of suitable adjectives when
Table I — Types and classes of soil structure
Fine Fine
prismatic; columnar;
10-20mm 10-20mm
Fine angular
blocky;
5 - 10 mm
Medium angular
blocky;
10 - 20 mm
Subangutar
blocky g
Fine Fine
granular; crumb;
1-2mm 1-2mm
Methum Medium
granular; crumb;
2-5mm 2-5mm
Coarse Thick platy; Coarse
or thick <5 - 10mm prismatic;
50-100mm
Class
Type (shape and
arrangement of peds)
Platelike with
one dimension
Prismlike with two dimensions
(the horizontal) limited and
Blocklike; polyhedronlike, or spheroidal, with
three dimensions of the same order of magnitude,
(the vertical)
limited and
greatly less
than the other
two; arranged
around a hori-
zontal plane;
faces mostly
considerably less than the
vertical; arranged around a
vertical line; vertical faces
well defined: vertices angular
arranged arouncla point
Blocklike; blocks or polyhedrons Spheroids of polyhedrons
having plane or curved surfaces having plane or curved sur-
that are casts of the molds formed faces which have slight or
by the faces of the surrounding peds no accommodation to the
faces of surrounding peds
— --- -—--——
Faces flattened: Mixed rounded Relatively Porous
Without Wtth
horizontal
rounded rounded
caps caps
most vertices and flattened nonporous pads
sharply angular faces with pads
many rounded
vertices
Platy Prismatic Columnar
Very fine Very thin
or very platy;
thin <1mm
Fine or Thin platy;
thin <1—2mm
Very fine
prismatic;
<10 mm
(Angular)
Blockyt
Very fine
columnar;
<10 mm
Very fine
angular
blocky:
<5 mm
Granular Crumb
Medium Medium platy; Medium Medium
<2 - 5 mm prismatic; columnar;
20-5Omm 20-50mm
Very fine Very fine
granular; crumb;
<1 mm <1 mm
Very fine
subangular
blocky;
<5 mm
Fine sub-
angular
blocky;
5 — 10mm
Medium sub-
angular
blocky:
10 - 20 mm
Very
coarse
or very
thick
Very thick
platy;
>10 mm
Coarse columnar;
50- 100mm
Very coarse
columnar;
>100 mm
Very coarse
prismatic;
>100 mm
Coarse angular Coarse sub-
blocky; angular
20 - 50 mm blocky;
20 - 50 mm
Very coarse
angular blocky;
>50 mm
Coarse
granular;
S - 10 mm
Very coarse
granular;
>10mm
Very coarse
subangular
blocky;
>50mm
From: Soil Survey Staff, SCS, USDA, 1951, Soil survey manual. U. S. Dept. Agr. Handbook 18, p. 228, U. S. Government
Printing Office, Washington.
+ (a) Sometimes called nut. (b) The word “angular” in the name ordinarily can be omitted.
• Sometimes called nuciforin, nut, or subangular nut. Since the size connotation of these terms is a source of great confusion to
many, they are not recommended.
86
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soil water 3 — (Cont)
C. SYMBOL, DIMENSION, AND UNIT FOR THE ABOVE GIVEN TERMS
WHEN ONE GRAM MASS IS USED AS UNIT QUANTITY OF WATER
total pressure —The pressure(positiveornegative),retative to the ex-
ternalgaspressureonthesojiwater, to which a pool of pure water
must be subjected in order to be in equilibrium through a semi-
permeable membrane with the soil water. Total pressure is thus
equal to the sum of soil water pressure and osmotic pressure.
Total pressure may also be derived from the measurement of the
partial pressure of the waler vapor in equilibrium with the soil
water. May be identified with the total potential defined above when
gravitational and external gas pressure potentials can be neglected.
hydraulic head The elevation with respect to a specified reference
level at which water stands in a piezometer connected to the point
in question in the soil. Its definition can be extended to soil above
the water table if the piezometer is replaced by a lensiometer.
The hydraulic head In systems under atmospheric pressure may
be identified with a potentialexpressed in terms of the height of a
water column. More specifically it can be identified with the sum
of gravitational and capillary potentials, and may be termed the
hydraulic potential.
water content — The amount of water lost from the soil upon drying to
constant weight at 105C; expressed either as the weight of water
per unit weight of dry soil or as the volume of water per unit bulk
volume of soil. The relationships between water content and soil
water pressure can be ref erred to as the soil moisture character-
istic curve. Dependingupon whether the curve is determined with
decreasing or increasing water content one may designate it as a
desorption and adsorption curve, respectively.
differential water capacity - The absolute value of the rate of change
of water content with soil water pressure. The water capacity at
a given water content will depend on the particular desorption or
adsorption curve employed. Distinction should be made between
volumetric and specific water capacity.
B. TERMS RELATING TO THE MOVEMENT OF WATER IN SOIL
Experimentally it has been established that generally theflow of a fluid
in a porous medium can be described by Darcy’s law which states that
the flux of fluid is proportional to the drivingforce. In viscous flow of
water in soils, the driving force equals the negative gradient of the
hydraulic potential.
hydraulic conductivity — The pr ortionality factor in Darcys law as
applied to the visccsis flow of water in soil. i.e., the flux of water
per unit gradient of hydraulic potential. If conditions require that
the viscosity of the fluid be divorced from the conductivity of the
medium, it is convenient to define the permeability (intrinsic per-
meability has beenused insome publications) of the soil as the con-
duct ivity, expressed in g- 1 cm 3 sec multiplied by the viscosity in
poise. For the purpose of solving the partial differential equation
of the non-steady-state flow in unsaturated soil it is often convenient
to introduce a variable termed the soil water diffusivity.
soil water diltusivity — The hydraulic conductivity divided by the differential
watercapacity (care beingtaken to beconsistent withunits), ortheflux
of water per unit gradient of moisture content in the absence of other
force fields.
3 me abovetermsandtheirdefinitionsrelatingto soil water are essen-
tially those recommended by a committee of the International Society of Soil
Science and poblished in Bulletin 23 (1963, p. 7-lOt of that society. Some
editorialchangeshavebeenmadefollowingcomments received from a num-
ber of soil iysicists in response to an inquiry by the SSSA Committee on
Terminology. Because of the strong divergence of opinion expressed by
the respondents, the Terminology Committee is listing the terms without
approval or disapproval, but with the recommendation that society members
express their opinion to the Editor- in- Chief or to the next Committee chair -
man.
Term Symbol
Dimension
Unit
Total potential
Osmotic potential
Gravitational potential
Capillary potential
Soil water pressure
‘I’
0
Z
M
2 -2
L T
L 2 T 2
L 2 T 2
L 2 T 2
ML 1 T 2
-1 -1
erg g , joule kg
erg g , joule kg
erg g , joule kg
erg g 1 , joule kg
dyne cm 2 , bar. cm
Osmotic pressure
ML 1 T 2
water, cm Hg
dyne cm 2 , bar, cm
Total pressure
ML T2
water, cm Hg
dyne cm 2 , bar, cm
water, cm Hg
Hydraulic head
Hydraulic potential
Water content
Differential water capacity
H
w
C
L
L T 2
M 1 LT 2
cm, m
erg g 1 ,j outs kg 1
cm 2 cm ,
cm 2 dyne , bar 1
Hydraulic conductivity
Permeability
Soil water ditfusivity
K
k
D
L 2
L 2 T 1
cm 2 , Darcy
cm 2 secl
Symbols as C, K, and D may have w or h as a subscript if they in the
same paper are used for water as well as for heat.f The dimension
would depend on the units used to describe the driving force as shown
in the table below:
Driving force
Dimension
Hydraulic
conductivity
Dimension
Unit
Hydraulic potential
gradient
Hydraulic head gradient
Pressure gradient
LT 2
LL 1
ML 2 T2
T
LT 1
M 1 L T
sec
cm sec
g 1 cm 3 sec
soil water pressure — See soil water.
solclime — The temperature and moisture conditions of the soil; the soil
climate.
solodized soil — A soil that has been subjected to the processes responsible
for the development of a Soloth and having at least some of the charac-
teristics of a Soloth.
Solonchak — A great soil group of the intrazonal order and halomorphic sub-
order, consisting of soils with gray, thin, salty Crust 00 the surface,
and with fine granular mulch immediately below being underlain with
grayish,friable,salty soil; formed under subhumidtoarid, hot or cool
climate, under conditions of poor drainage and under a sparse growth
of halophytic grasses, shrubs, and some trees.
Solonetz — A great soil group of the intrazonal order and halomorphic sub-
order, consisting of soils with a very thin, friable, surface soil under-
lain by a dark, hard columnar layer usually highly alkaline; formed
under subhumid to arid, hot to cool climates, under better drainage
than Solonchaks, and under a native vegetation of halophytic plants.
soluble-sodium percentage (SSP) — The proportion of sodium ions in solution
in relation to the total cation concentration, defined as follows:
Soluble-sodium concentration (meg/liter ) 100
Total cation concentration (meq/liter)
solum (plural: sola) — The upper and most weathered part of the soil pro-
file; the A and B horizons.
splash erosion — See erosion, (ii).
spoil bank — Rock waste, banks, anddumps, from the excavation of ditches.
sprinkler irrigation — See irrigation methods.
sticky point — (i) A condition of consistency at which the soil barely fails
to stick to a foreign object. (ii) Specifically and numerically, the
weight moisture percentage of a welt-mixed, kneaded soil that barely
fails to adhere to a polished nickel or stainless steel surface when the
shearing speed is 5 cm/sec.
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Stokes’ law — An equation relating the terminal settling velocity of a smooth,
rigid sphereinaviscous fluid of known density and viscosity to the dia-
meter of the sphere when subjected to a known force field. Used in
particle_sizeanaiystsofsoilsbythe pipette, hydrometer, or centrifuge
methods. The equation is
= 2gr 2 (d 1 - d 2 )
where
V velocity of fafl (cm sec 1)
g acceleration of gravity (cm sec - 3 )
r ‘equivalent’ radius of particle (cm).
d 1 density of particle (g cm 3 ),
d 2 density of medium (g cm” 3 ), and
viscosity of medium (dyne sec crn ’ 2 ).
stones —Rock fragments >10 inches indiaxneter if rounded, and >15 inches
along the greater axis if flat. See coarse fragments.
stoniness — The relative proportion of stones in or on the soil. Used in
classification of soils. See coarse fragments.
stony — Containing sufficient stones to interfere with or to prevent tillage.
To be classified as stony, >O.O1r of the surface of the soil must be
covered with stones. Used to modify soil class, as stony clay loam or
clay loan, stony phase. See coarse fragments.
stony land — Areas containing sufficient Stones to make the use of machinery
impractical usually 15 to 90 of the surface is covered with stores.
A miscellaneous land type. See stoniness and rubble land.
stratified — Arranged in or composed of strata or layers.
strip cropping —The practice of growing crops which require different types
of tillage, such as row and sod, in alternate strips along contours or
across the prevailing direction nf wind.
structure, soil — See soil structure.
structure index — Any measurement of a soil physical properlY, such as
aggregation, porosity, permeability to air or water, or bulk density,
that denotes or indicates the structural condition of a soil.
stubble mulch — The stubble of crops or crop residues left essentially in
place onthe land as a surface cover before and during the preparation
of the seedbed and at least partly during the growing of a succeeding
Crop.
Sebarctic Brown Forest soils — Soils similar to Brown Forest soils except
having moore shallow sola and averagetemperaturesof
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coarse fragments are present in substantial amounts; for example,
“stony silt loam,” or “silt loam, stony phase.” (For other modifica-
tions see coarse fragments.) The sand, loamy sand, and sandy loam
are further subdivided on the basis of the proportions of the various
sand separates present. The limits of the various classes and sub-
classes are a.s follows:
sand — Soil material that contains 85% or more of sand; percentage of
silt, plus 1.5 times the percentage of clay, shall not exceed 15.
coarse sand — 25% or more very coarse and coarse sand, and <50%
any other one grade of sand.
sand — 25% or more very coarse, coarse, and medium sand, and
<50% fine or very fine sand.
fine sand — 50% or more fine sand (or) <25% very coarse, coarse,
and medium sand and <50% very fine sand.
very fine sand — 50% or more very fine sand.
loamy sand — Soil material that contains at the upper limit 85 to 90% sand,.
and the percentage of silt pius 1.5 times the percentage of clay is
not less than 15: at the lower limit it contains not less than 70 to
85% sand, and the percentage of silt plus twice the percentage of
clay does not exceed 30.
loamy coarse sand — 25% or more very coarse and coarse sand, and
40% line and
very fine sand, atleasthaif olwhichisvery line sand and <15%
very coarse, coarse, and medium sand.
loam -‘ Soil material that contains 71027% clay, 28 to so% silt, and <52%
sand.
silt loam — Soil material that contains 50% or more silt and 12 to 27%
clay (or) 50 to 80% silt and 12% clay.
silt — Soil material that contains 80% or more silt and <12% clay.
sandy clay loam — Soil material thatcontains 20 to 35% clay, <28% silt,
and 45% or more sand,
clay loam — Soil material that contains 27 to 40% clay and20 to 45% sand.
silty clay loam —Soil material that contains 27 to 40%ciay and <20%sand.
sandy clay — Soil material that contains 35% or more clay and 45% or
more sand.
silty clay — Soil material that contains 40% or more clay and 40% or
more silt.
clay —Soil material that contains 40% or more clay, <45% sand, and <40%
silt.
soil type — (i) The lowest unit in the natural system of soil classification;
a subdivision of a soil series and consisting of or describing soils that
are alike in all characteristics including the texture of the A horizon.
(ii) In Europe, roughly equivalent to a great soil group.
soil variant — A soil whose properties are believed to be sufficienily different
from other known soils to justify a new series name but comprising such
a limited geographic area that creation of a new series is not justified.
soil water 3 —
A. TERMS RELATING TO THE STATE OF WATER IN SOIL
Water in soil is subject to several force fields originating from: the
presence of the soil solid phase: the dissolved salts: the action of ex-
ternal gas pressure; and, the gravitational field. These effects may
be quantitatively expressed by assigning an individual component poten -
tial to each. Thesumofthesepotentialsisdesignatedthetotal potential
of soil water and may be identified with the partial specific Gibb’sfree
energy of the soil water relative to free pure water at the same tem-
perature. It should be noted that Soil water is understood to be the equi -
librium solution in the soil; pure water refers to the chemically pure
compound 1120.
total potential (of soil water) — The amount of work that must be done
per unit quantity of pure water in order to transport reversibly
and isothermally an infinitesimal quantity of water from a pool of
pure water, at a specified elevation and at atmospheric pressure,
to the soil water (at the point under consideration). The total poten -
tial (of soil water) consists of the following:
osmotic potential — The amount of work that must be done per unit
quantity of pure water in order to transport reversibly and
isothermally an infinitesimal quantity of water from a pool of
pure water, at a specified elevation and at atmospheric pres -
sure, to a pool of water identical in composition to the soil
water (at the point under consideration), but in all other re -
spects being identical to the reference pool.
gravitational potential — The amount of work that must be done per
unit quantity of pure water in order to transport reversibly and
isothermally an infinitesimal quantity of water, identical in
composition to the soil water, from a pool at a specified eleva-
tionandatatmosphericpressure, to a similar pooi at the ele-
vation of the point under consideration.
capillary potential — The amount of work that must be done per unit
quantity of pure water in order to transport reversibly and
isothermally an infinitesimal quantity of water, identical in
composition to the soil water, from a pool at the elevation and
the external gas pressure of the point under consideration, to
the soil water.
gas pressure potential — This potential component is to be con-
sidered only when external gas pressure differs from atmos-
pheric pressure as, e.g., iii a pressure membrane apparatus.
A specific term and definition is not given.
soil water pressure — The pressure (positive or negative), relative to
the external gas pressure on the soil water, to which a solution
identical in composition to the soil water mustbe subjected in order
to be in equilibrium through a porous permeable wall with the soil
water. Maybe identifiedwith the capillary potential defined above.
osmotic pressure — The pressure to which a pool of water, identical in
composition to the soil water, must be subjected in order to be in
equilibrium, through a semipermeable membrane, with a pool of
pure water (semipermeable means permeable only to water). May
be identified with the osmotic potential defined above.
3 The above terms andtheir definitions relatingto soil water are essen-
tially those recommended by a committee of the International Society of Soil
Science and published in Bulletin 23 (1963, p. 7-10) of that society. Some
editorial changes have been made following comments received from a num -
ber of soil physicists in response to an inquiry by the SSSA Committee on
Terminology. Because of the strong divergence of opinion expressed by
the respondents, the Terminology Committee is listing the terms without
approval or disapproval, butwith the recommendation that society members
express their opinion to the Editor -in-Chief or to the next committee chair -
man.
89
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toposaic — A photomap on which topographic or terrain-form lines are shown,
as on standard topographic quadrangles. See photomap and planisaic.
toposequence — A sequence of related soils that differ, one from the other,
primarilvbecause of topography as a soil-formation factor. See dm0-
sequence.
topsoil — (I) The layer of soil moved in cultivation. See surface soil. ii)
The A horizon. (iii) The Al horizon. (iv ) Presumably fertile soil
material used to topdress roadianks, gardens, and lawns.
total potential (of soil water) — See soil water.
total pressure — See soil water.
trace elements - (Obsolete) See micr000trient.
traffic pan — See pan, pressure or induced.
transitional soil — A soil with properties intermediate between those of two
different soils and genetically related to them
transported soil — (Obsolete) Any soil which was formedon unconsolidated
sedimentary rocks.
truncated— Raving lost all or part of the upper soil horizon or horizons.
tuff— Volcanic ash usually more or less stratified and in various states of
consolidat ion -
tundra — A level or undulating treeless plain characteristic of arctic
regions.
Tundra soils — (I) Soils characteristic of tundra regions. lii) A zoual
great soil group consisting of soils withdark-brown peaty laver over
grayish horizons mottled with rust and having continually frozen sub-
strata; formed under frigid, humid climates, with poor draina .e, and
native vegetation of lichens, moss, flowering plants, and Shrubs.
type, soil — See soil type.
volcanic-ash land— Areas of volcanic ash so recentl deposited llt. i the
is essentially unmodified and shows little or noevidence of soil develop-
nient. The areas have almost no vegetation on them A miscellaneous
land type
w
wasteland— Land not suitable for. or capable of, producing materials or
services of value. A miscellaneous land type.
water conductivity— See soil water.
water content — See soil water.
waterlogged— Saturated with water.
water- retention curve — See moisture- release curve and moisture- retention
curve.
water-stable aggregate A soil aggregate whtch is stable to the action of
water snch as falling drops, or agitation as in wet-sieving analysis.
water table — The upper surface of grouttd water or that level below which
the soil is saturated with water; locus of points in soil water at which
the hydraulic pressure is equal to atmospheric pressure -
water table, perched— The water table ul a saturated layer of soil which is
separated from an underlrittg saturated laser by an unsaturated layer.
weatheritw — All phYsical andchentacal changes produced in rocks, at or near
the earth’s surface, by atmospheric agents.
wild tloodt i. - See irr igatiot methods.
wilt ig coefficient -- tObsolete ) A calculated value of the approximate wilting
po ut t or permanent witting percentage. Calculated as follows;
- hvgruscopic coefficient
wilting coefficient - o 8
ulttmate particles— Individual sotl particles after a standard dispersing
treatment.
underground runoff (seepage) — Water flowing toward stream channels after
infiltration into the ground.
undifferentiated soil groups — Soil mapping units in which two or more
similar tax000rnic soil units occur, but not in a regular geographic
association. For example, the steep phases of two or more similar
soils might be shown as a unit on a mapbecause topography dominates
the properties. See soil association stud soil complex.
unsaturated flow — The movement of water in a soil which is not filled to
capacity with water.
upper plastic limit— See liquid limit.
urban land — Areas so altered or obstructed by urban works or structures
that identification 0 f soils is not feasible. A miscellaneous land tYpe.
- - moisture equivalent
wilting coefficient 8-4
wilting point— Same as ‘permanent wilting percentage as defined in stan-
dard plant physiology texts See fifteen-atmosphere percentage and
fifteen-bar percentage
windbreak — A planting of trees, shrubs, or other vegetation, usually per-
pendicular or nearly soto the principal wind direction, to protect soil,
crops, homesteads, roads, etc., against the effects of winds, such as
wind erosion and the drifting of soil and snow.
Y
yield, sustained — A continual annual, or periodic, yield of plants or plant
material from an area; implies management practices which will
maintain the productive capacitY of the land.
z
zeta potential — See electrokinetic potential.
zoival soil - (i) A soil characteristic of a large area or zone. (ii) One of
the three primary subdivisions (orders) in soil classification as used
in the United States. See classification, soil.
zu’mogenous flora — Organismsfoundin soils in large numbers immediately
following the addition of readily decomposable organic materials.
U
or,
x
xerophytes — Plants thatgrow in or on extremely dry soils or soil materials.
V
value, color — The relative lightness or intensity of color and approximately
a function of the square root of the total amount of light. One of the
three variables of color. See Munsetl color system, hue, and chroma.
variant - See soil variant.
varnish— See desert varnish.
verve — A distinct band representing the annual deposit in sedimentary
materials regardless of origin and usually consistmg cut two layers,
one a thick, light-colored layer of silt and fine sand atud the other a
thin, thrk-colored layer of Clay.
very coarse sand — See soil separates and soil texture.
very fine sand — See soil separates and soil texture.
very fine sandy loam — See soil texture,
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