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II. Formation and Stabilization of Aggregates

II. Formation and Stabilization of Aggregates

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4



JAMES P. MARTIN



et al.



on the first. This may be justified in some soils having exceptionally

stable and characteristic aggregates which are highly resistant to destruction either by tillage or natural processes and which thus impart

to the soils physical properties which do not change readily as a result

of management. Some of the prairie and chernozem soils are striking

examples of this type. In most soils, however, the aggregates are not

resistant. The determination of the aggregate size distribution in such

soils is probably meaningless (except as a measure of relative stability),

since the size distribution obtained is, to a large extent, dependent on

the treatment given during the determination.

The size and shape of aggregates as they exist in the soil would certainly be expected to have considerable influence on the pore spaces. It

can be readily seen, however, that the same aggregates arranged differently will impart quite different size and continuity of soil pores

within the root zone. The fact that fairly good agreement is usually

obtained between degree of aggregation and crop yields indicates that

physical characteristics do in general tend to be better where the soil

is more highly aggregated. It is possible, although unusual, to have a

highly aggregated soil which still has poor physical properties. This

would result if the aggregates were themselves rather dense and packed

closely together.

The second of the above aspects of aggregation is certainly the most

important as far as plant growth is concerned; yet strangely it has received the least attention. Present methods of dissecting the soil to

determine the size of individual units eliminate the possibility of determining the function of the aggregates in place except by inference.

The third factor, stability, is one which is of obvious importance and

which has received considerable study, but in spite of this it is still not

possible to make an accurate measurement of the stability of soil aggregates except on an empirical, relative basis. The ordinary wet sieve

analysis measures relative stability more nearly than any other characteristic. It is difficult to interpret results of the analysis in terms of the

stability one might expect under field conditions. However, by standardizing the conditions of the determination and repeating it on composite samples at different times, it has been possible to show general trends in the levels of aggregation through the growing season as a

result of tillage or cropping. Much more certainly needs to be done

in this area, but until we can arrive at a fuller understanding of the

forces affecting aggregation and of the nature of the processes which

cause aggregates to remain in the soil, it is doubtful that we can

make much progress toward finding a better method for characterizing

stability.



SOIL AGGREGATION



5



2. Mechanisms Involved in Aggregation



Three kinds of mechanisms have been proposed to explain the formation of aggregates in the soil: ( I ) living bacteria and fungi (and

possibly actinomycetes) bind soil particles together; (2) gelatinous

organic materials such as gums, resins, or waxes are thought to surround the soil particles and thus hold them together through a cementing or encapsulating action; and ( 3 ) the clay particles themselves

cohere and thus entrap or bridge between larger sand and soil grains.

All of these types of binding are undoubtedly important and they may

operate singly or in combination to different degrees in different soils

(Hubbel and Chapman, 1946; Kroth and Page, 1947; Martin, 1946;

Martin and Waksman, 1940; Peterson, 1946; Russell and Russell,

1950).

The evidence supporting the first view is partly direct and partly

circumstantial. Where the mycelia of fungi extend quite thoroughly

through the soil the particles are entrapped and held together. This is

apparent even with the naked eye, and under magnification small

clumps of particles can be seen clinging to the mycelia. Since most

colonies of bacteria growing on artificial media appear somewhat slimy

and gelatinous, it has been deduced that bacteria in the soil may serve

to bind particles together; this appears to be a greatly oversimplified

explanation. In any case the binding action of the living microorganisms would disappear when the food supply is exhausted and the numbers of microorganisms decline. It is comparatively easy to demonstrate

this action during the course of simple experiments in the laboratory,

but it is difficult to determine how important it becomes under actual

field conditions, where keener competition exists between the different

microorganisms, and food sources are usually not as readily available.

It is quite probable that aggregates which are formed as a result of the

presence of liuing microorganisms are ephemeral and quite possibly of

little importance in most agricultural soils. Certainly other explanations

will have to be found to explain the long-lived stable structural aggregates commonly found in many soils even where readily decomposable

organic matter is low and decomposition is not occurring rapidly.

As will be discussed in a later section, it can be argued quite justifiably that some organic compound or compounds which are synthesized during the process of decomposition or which are by-products of

the decomposition process are actually the important factors in producing stable soil aggregates. The available evidence supports this view

quite strongly, and several workers have directed their attention to determining the composition and characteristics of these compounds. Chief



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JAMES P. MARTIN



et aL.



attention has been directed toward the polysaccharides, of which the

derivatives of uronic acid have been most intensely studied. These do

appear to occur in rather large proportions during the process of active

microbial decomposition and may play quite an important role in the

formation of a kind of aggregate.

Most of the emphasis has been placed on the nature of the organic

fraction involved in aggregation. This can be readily understood since

most of the investigators have been soil microbiologists, but in terms of

the over-all problem it now appears that the role of the clay particles

has been unnecessarily minimized and that the nature of the combination between clay particles and the polar organic compounds needs to

be investigated intensively. The importance of the clay in soil aggregate

formation has been stressed by several investigators. Baver (1935) , in

a study of 77 different soils in the United States, correlated aggregation,

clay content, organic matter, and exchangeable calcium. A very high

correlation was found between the <0.005 mm. clay and the >0.05 mm.

aggregates. The correlation was greater as the organic matter content

decreased. At t.he higher organic matter contents the effect of the clay

became insignificant. It was concluded that clay was important in stable

aggregate formation in the soil but that organic matter was probably

more important. Mazurak (1950) studied the aggregation of the inorganic fraction of Hesperia sandy loam. It was found that the 0.03 p

particles were associated with water stability of synthetic aggregates.

The important factor in aggregation is probably the presence of some

chemical compound or group of compounds which appears in one way

or another during the process of decomposition and which then combines with the clay to help make aggregates.

The third proposed mechanism of aggregate formation involves the

belief that clay is the chief binding agent, and that organic materials

do not act primarily to hold the clay and sand and silt grains together.

Rather their chief role may be to modify the forces by which the clay

particles themselves are attracted to one another. According to this

view, the cohesive force between clay particles rather than the cementing action of organic molecules is thought to be the binding force in aggregation. The magnitude of these forces between clay particles may be

very great, leading even to solidification in some cases. This last condition would obviously be unfavorable for agriculture, but the same

types of forces between clay particles appear to be involved in producing desirable structure in agricultural soils as are active in solidification

of puddled soils.

The cohesive forces which may operate between clay particles to



SOIL AGGREGATION



7



hold them together may act in lhree ways: (1) by linkage due to chains

of water dipoles; (2) by bridging or tying together with certain polar,

long-chain, organic molecules; ( 3 ) by cross-bridging and sharing of

intercrystalline ionic forces and interactions of exchangeable cations

between oriented clay plates.

It is quite likely that the first of these (linkage due to water dipoles)

is of importance under moist conditions and probably accounts for

some of the resistance to dispersion observed in some soils. It is difficult

to see, however, how such a mechanism may be active in causing o r a t

least affecting orientation of adjacent clay particles as they are dried

out. The second mechanism in which polar, probably long-chain, organic compounds hold clays together, may prove to be of great significance and certainly needs to be investigated more intensively. There

is evidence that many such compounds can be strongly adsorbed by

clays (Gieseking, 1949). It appears logical that they could serve as binding agents to hold soil particles together either by hydrogen bonding or

direct bridging. It is known that different compounds vary tremendously in the degree to which they are held by clays and likewise that

the clays differ in the force with which different polar compounds are

adsorbed. Many such compounds are held tightly, and it has been reported that certain clay-organic complexes are resistant to redispersion

or crushing after drying. The synthetic long-chain polymers which

have been introduced for use in stabilizing soil structure have produced

striking results with certain types of clay soils, and part of the action

may be due to bridging of the type postulated above. The exact mechanisms by which these compounds are adsorbed to clay surfaces need

to be investigated further, and it should not be concluded that they

simply hold the soil particles together because of their apparent stickiness.

It is difficult to assess the importance of molecular binding forces

in the soil at our present stage of knowledge. There is no question but

that they are important. They may be the predominating forces under

certain conditions. However, under a great many other conditions, it is

believed that the intercrystalline ionic forces between clay particles

may themselves account for all of the binding necessary to explain

aggregation in the soil. It can readily be seen that under certain conditions cohesion between clay particles can give rise to extremely strong

forces, which could account for a11 the binding observed in soils containing clays. These forces are at a maximum when the clay particles are

in closest contact and in preferred orientation, so that the number of

points of contact and areas of contact are large. Puddling of soils or clays



a



JAMES P. MARTIN



et al.



favors such orientation, and the pieces resulting after puddled clays

are dried are strong and coherent. Crumbs resulting from drying of dispersed soils are usually much stronger than those from flocculated

clays, since in flocs the tendency is for random orientation. In most

agricultural soils which have not been mismanaged, clay particles will

not yet have been strongly oriented. Natural structure may still be

favorable and total cohesive force may not be high. With more nearly

random orientation the number and area of points of contact should be

at a minimum. Further, if, as is usually the case, water dipoles as well

as active organic molecules are adsorbed on the free clay surfaces, the

magnitude of any further cohesive forces which could become effective between clay particles will be even further reduced. Apparently

the same types of bonds would be involved a t existing points of contact

of clay particles as in puddled soils. However, with part of the surface

energy directed toward adsorption and orientation of water and organic

molecules and with these molecules serving as a protective layer over

free surfaces of the particles, any further expression of the normal cohesive forces would be markedly reduced. Thus these materials would

act to stabilize the existing structure, partly through cementation and

partly through modification of surface properties of the clay particles.

Swelling has been shown to cause the breakdown of aggregates

under certain conditions. Many polar organic compounds when adsorbed greatly reduce the swelling tendency of clays. Presumably this

is brought about because these compounds are preferentially adsorbed

by the same forces on the clays which attract water dipoles. They are,

however, much more tightly held. It should be emphasized that relatively small amounts of active organic material, even a monomolecular

layer, may exert a tremendous influence on swelling, cohesion, and

other physical characteristics of clays.

Clays differ in the surface activity and the ability to adsorb or orient

water and organic molecules, and this is reflected in soil properties.

They differ also in the magnitude of cohesive forces which would be

exhibited even under complete orientation and contact. Adsorbed

cations play an important role as well, presumably dependent upon the

degree of hydration of the adsorbed cations and whether they cause

dispersion or flocculation of the colloidal clay. It appears that soils

which are predominantly kaolinitic may not exhibit as strong cohesive

forces upon drying as are exhibited by soils which are predominantly

montmorillonitic. It would be unsafe to generalize, however, since too

little is known at the present time of the characteristics of the clay minerals in large numbers of soils. With either mineral type, granules

formed by drying from highly hydrated monovalent systems are less



S O I L AGGREGATION



(3



resistant to rehydration and dispersion than are those from soils

saturated with slightly hydrated cations.

3. Formation of Aggregates

I n the light of these considerations the following seems to best explain how aggregates are formed and stabilized in agricultural soils:

aggregates result primarily from the action of natural agencies or any

process by which parts of the soil are caused to clump together and

separate from adjacent masses of soil. If soils are initially dispersed (as

in alkali soils), flocculation is essential for aggregate formation; if they

are partially puddled or solid, fragmentation into smaller units is the

first essentiaI. Thus, there are two kinds of processes involved. The first

is concerned with the building up of aggregates from dispersed materials; the second involves the breaking down of larger coherent masses

into favorably sized aggregates. Since most soils become more dense and

compact with continued farming, the second case is of greater interest.

Separation of parts of the soil mass may result because of: (1) the action of small animals, particularly earthworms; (2) tillage processes;

( 3 ) pressures and differential drying caused by freezing; (4) compression due to roots; ( 5 ) localized shrinkage caused by removal of water

by roots or evaporation. Roots are undoubtedly tremendously important, acting to separate and compress small clumps of soil, to cause

shrinkage and cracking due to desiccation near the root, and to make

conditions favorable for the activity of microorganisms at the surfaces

of these units. Alternate wetting and drying causes cracks or cleavage

planes to develop owing to differential swelling and shrinking. Freezing

causes extreme localized pressures, again tending to cause the soil to

break up into rather small fragments or crumbs. When this occurs,

forces within the crumb which cause clay particles to cohere are

stronger than those between clay particles of adjacent crumbs. These

units tend to exist separately in the soil until forced back into intimate contact with neighboring groups. The size and shape of the

masses which are thus caused to form in the soil are extremely important but little is known of the factors governing the characteristics of

the aggregates resulting or of the specific role of the different clay

minerals.

The characteristics of the pore spaces in the soil obviously depend

upon the shape, size, and arrangement of aggregates. It has been suggested that kaolinitic clays tend to produce platy aggregates in contrast

to the blocky aggregates produced by montrnorillonitic clays, but it is

not felt that enough is known about the specific effects of these min'erals

to generalize a t this time.



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JAMES P. MARTIN



et al.



4 , Stabilization

The structural units once formed in the soil would readily disappear

and recombine with others in the soil if not stabilized. This is probably

the chief role of the active organic compounds. As pointed out above

and later, certain types of compounds and active groupings on organic

compounds have been shown to be strongly adsorbed on clay colloids.

The forces involved differ with the different compounds and different

kinds of clay, as well as the adsorbed inorganic cations which are already present. Some compounds may be adsorbed as cations, others as

anions, and others as molecules, the binding capacities in this latter case

not appearing to be related to either anion or cation adsorptive

capacities.

Strong adsorption of active organic molecules on clay surfaces

would have a profound effect in modifying the forces between clay

particles which cause the particles to cohere. Those particles within the

aggregate where a degree of orientation and close contact had already

occurred would be less affected than those on the outer surfaces, where

clay surfaces would be exposed and available for adsorption. With outer

surfaces essentially saturated or occupied with active organic compounds, but little residual force would be left which could act to cause

coherence between clay particles of adjacent aggregates. I n this situation the requirements for aggregates would have been met, namely,

stronger cohesive forces between particles within the aggregate than

between aggregates, and the unit could exist in the soil as a separate

entity. Such a unit would tend to be stable, even when wet, if organic

molecules were so strongly adsorbed that further hydration or swelling

and consequent weakening of bonds between clay particles did not occur, or if the compounds themselves tended to hold adjacent clay

particles together through cross linkage or mutual adsorption. Apparently both mechanisms are of importance, and it is probable that they

operate concurrently.

Polar organic compounds may be thought of as playing two important roles in soil structure tending to stabilize naturally formed

aggregates: (1) weakening the otherwise strong cohesive bonds between clay particles, thus permitting formation into aggregates instead

oE a solid mass; and (2) linking clay particles together through mutual

adsorption of such compounds by two or more clay particles. There is

insufficient evidence available to indicate which of these two functions

is the more important. It is almost certain, however, that both are important and that both actions may occur concurrently in stabilizing

soil structure.



SOIL AGGREGATION



11



Recent work with synthetic soil additives which will be presented

iii detail in a later section has shown that these highly polymerized

straight-chain compounds are extremely tightly held by clays. They do

not appear to be replaced by ordinary exchange and are quite resistant

to microbial attack. It is significant that these materials will not create

good structure but act instead to stabilize whatever structure is found

when the material is applied. If the soil can be prepared into favorably

sized aggregates or fragments, the materials do a n effective job of

stabilizing the aggregates so they do not tend to run back together upon

further wetting.

Earlier literature stressed the importance of flocculation in soil structure, but it has been found that colloids in almost all nonalkali soils tend

to be flocculated. Both Ca++and H+ions produce flocculation, and further, adsorption of most polar organic molecules causes complete flocculation. It is considered that most nonalkali soil clays are already

flocculated and that changes occurring in soil structure are not primarily changes in degree of flocculation but rather in degree of expression of cohesive forces between already flocculated clay particles.

It should be re-emphasized that clays are essential in structure formation and that the primary role of organic matter is in modifying the

physical properties of the clay. Since the mechanism involves an adsorption process, only very small amounts of the active compounds may

be involved at any one time, but the effect on clay and hence on soil

properties is tremendous. The amount and composition of the organic

materials in the soil at any one time are dependent upon the activity

of microorganisms, with the result that physical properties of the clay

organic matter system may change rather rapidly. During decomposition the microorganisms themselves exert a direct and usually favorable

effect on structure, but the effects produced through adsorption of the

compounds produced are thought to be much the most significant. The

specific organic compounds which combine with and modify the characteristics of the clay are not yet known, but their importance is tremendous, and studies of the nature of these compounds and the clayorganic matter combination should prove to be fruitful in helping us to

arrive a t an understanding of how aggregates are formed and stabilized.

Following sections will discuss certain organic fractions found in soil

and present evidence of the action of microorganisms in producing and

stabilizing soil aggregates.



5. Iron and Aluminum Oxides

I n addition to the mechanisms discussed in the preceding sections,

oxides or hydrated oxides of iron and aluminum may serve as cement-



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JAMES P. MARTIN



et al.



ing or binding agents in many soils. In lateritic or semilateritic soils,

for example, iron, or iron and aluminum oxides are important binding

substances. Lutz ( 1936) found a high positive correlation between the

free iron oxide in lateritic type soils and aggregation. He suggested that

the free iron serves a dual purpose, namely, that the iron in solution

acts as a flocculating agent for the clays and the precipitated iron acts

as a cementing agent. At the pH of the soils studied, the iron would be

precipitated as a hydrated gel, which would become a good cementing

agent upon dehydration. Studies by Weldon and Hide (1942) demonstrated that the amount of sesquioxides extracted from well-aggregated

fractions of several prairie soils was considerably greater than that extracted from the poorly aggregated fractions. These investigators

stressed the probability that sesquioxides act as cementing agents in the

formation of aggregates in prairie soils as well as in lateritic soils.

Kroth and Page (1947) concluded from studies with the electron

microscope that iron and aluminum oxides provide a continuous matrix

which binds soil particles into secondary units by physical forces alone.



111. EFFECTOF ORGANICRESIDUESON AGGREGATION



1. Microbial Decomposition

The preceding section presented a generalized discussion of the

possible mechanisms involved in the process of forming and stabilizing

soil aggregates. With these considerations in mind a review of the literature and a discussion of the role of organic substances, microorganisms, and products of microbial activities in soil aggregation will be

presented.

Numerous investigators have demonstrated an improvement in soil

aggregation following organic matter applications. Although some complex organic materials may contain soil-binding substances, the increased aggregation has been shown to be largely contingent upon the

decomposition of the residues by soil organisms. Under sterile conditions only slight to moderate benefit or none will ensue. For example,

studies by Martin and Waksman (1940) and Peele (1940) demonstrated that when a microbial energy source such as sucrose or cellulose

is added to a soil, and the soil is sterilized, no improvement in soil

aggregation will take place. If the mixture becomes contaminated or is

inoculated with a soil suspension or with certain soil microbes, however,

a marked aggregating effect will follow upon incubation. Studies with

complex residues such as alfalfa, grass, and cereal straws, on the other

hand, demonstrated the presence of water-soluble soil-binding sub-



SOIL AGGREGATION



13



stances (Martin, 1942). The concentration or quality of the watersoluble materials increased during the early and intermediate stages

of composting, and decreased during the rater stages. When the materials were mixed with the soil and allowed to decompose, however,

much greater aggregation occurred than that produced by the water

extracts of the fresh and composted materials.

The importance to soil aggregation of organic substances which are

apparently produced largely through microbial activity has been

stressed by numerous investigators. Robinson and Page ( 1951) tested

artificial aggregates of Brookston clay loam for resistance to slaking by

the wet sieving procedure before and after oxidation of the organic

fraction with hydrogen peroxide. The stability of the aggregates of the

oxidized soil was very poor in comparison with the unoxidized soil. It

was concluded that the organic matter associated with the clay was

largely responsible for aggregate stability. Studies by Metzger and Hide

(1938) and Weldon and Hide (1942) indicated that the organic matter

content of severaI soils was much higher in the well-aggregated fractions than in the poorly aggregated fractions. Baver (1935) suggested

that certain organic materials bind soil particles together through

physicochemical processes. Several investigators have demonstrated the

existence of organo-clay complexes (Ensminger and Gieseking, 1942;

Springer, 1940; Myers, 1937). Ensminger and Gieseking (1939, 1942)

found that certain proteins were adsorbed within the crystal lattice

structure of montmorillonite type clays and that adsorption made them

more resistant to enzymatic action. Bartholomew and Goring (1948)

and Goring and Bartholomew (1950, 1951) worked with certain organic phosphorus compounds and found that decomposition was retarded by clay which adsorbed the phosphorus compounds. Fixation by

the clays varied greatly depending on the nature of the organic compound, the type of clay, and the pH of the systems.

Kroth and Page (1947) made a study of natural and synthetic soil

aggregates in which the electron microscope was utilized as one approach to the problem. All parts of the investigation indicated that the

aggregating substances were uniformly distributed throughout the

aggregates and in contact with each soil particle. No evidence of aggregate capsules or coatings was found. In a later study, Robinson and

Page (1951) stated that the basis of aggregate stabilization by organic

matter is a modification of the properties of clay. It was concluded from

their work and that of others that the organic matter promotes aggregate stability by reducing swelling of montmorillonite-type clays and

by reducing the destructive forces of entrapped air during wetting of



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