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IV. Nature and Importance of Some Clay-Organic Complexes in Soils and Sediments

IV. Nature and Importance of Some Clay-Organic Complexes in Soils and Sediments

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clay fraction of the soil will exert its influence by interaction with undecomposed organic matter, intermediate products of decomposition, and

end products of this process. Not only that, the clay may exert direct

influence on the biological agents themselves, such as enzymes, microorganisms, plant roots. The work summarized by McLaren and Peterson

( 1965) is particularly interesting in this regard and should be consulted

by the reader.

In studying the relationship of soil clay mineralogy to the occurrence of

Fusarium wilt in bananas in Panama, Stotzky et al. (1 96 1 ) found a high

correlation between resistance to the disease and soils which contained

montmorillonite in the clay fraction. Soils which did not contain montmorillonite had short lifetimes with respect to the production of bananas

because of the buildup of the Fusarium wilt. It is not clear exactly what

the function of the montmorillonite was in reducing the disease. Several

possibilities exist; for example, the clay might act as an absorbent for

toxins from the Fusarium or perhaps interact with the Fusarium, or even

interfere in the interactions between the plant and the pathogen in a

beneficial way.

It was noted earlier that a number of workers utilizing infrared absorption techniques have found clay mineral surfaces to be more acidic than

would be expected from pH measurements of water suspensions of these

clays. In anticipation of these findings, biologists utilizing enzymes in

clay mineral systems have noted greater acidic properties at clay surfaces

than the pH measurement would indicate. Thus Peterson ( 1957),

McLaren and Seaman ( 1968) found that for some enzyme actions in clay

systems may be shifted by as much as two pH units toward the alkaline

region suggesting that the acidity at the clay surfaces is lower than the

ambient buffer solution. McLaren and Peterson (1965) suggest that apparently the enzymes respond to the concentration of hydrogen ions rather

than activity, which would be the same at all points in the system at equilibrium, but it is also likely that the true activity may not be reflected by

pH measurements with glass electrodes in colloidal systems. Some

enzyme systems maintain activity after adsorption at clay mineral surfaces while others are apparently deactivated or at least reduced in

activity (McLaren and Peterson, 1965; Galstyan et al., 1968).

Bacteria-clay interactions are of importance as suggested by the work

of Marshall (1968). He found that r. trifolii survived high temperatures

much better in the presence of montmorillonite or illite than without them.

He suggested an edge-to-face association between the clay platelets and

the bacteria. The extrapolation of these results were to suggest that the

ability of these bacteria to survive exposure to high temperatures in dry



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M. M. MORTLAND



soils was related to a protective effect of the soil colloids. It was suggested that a clay covering might protect the bacteria from exposure to

high temperatures by modifying the rate of water loss from the cells.

Jenny and Grossenbacher ( 1963) utilizing electron microscopy methods

observed very intimate contact between mucigel surrounding plant roots

and clay surfaces. They propose that the mucigel and clay particles are

actually bound together by chemical interaction, perhaps through carboxyl-polyvalent cation linkages, for example, A13+on the minerals. The

ramifications of the plant-mucigel-mineral matrix regarding diffusion,

transpiration, and metabolic processes are obvious.

The organic materials associated with clays in nature are often extremely complex materials. In this connection the work of Stevenson

(1969), Degens (1967), and Murphy er al. (1969) should be consulted.

The end products of the biological and diagenetic processes are such

materials as kerogen, coal, and petroleum. As diagenesis proceeds,

oxygen decreases, carboxyl and alcoholic groups are lost, there is a

lowered nitrogen content, and a concomitant increase in carbon occurs.

The relative distribution of various kinds of organic matter between the

clay mineral interfaces and the bulk of the soil or sediment has apparently

not received great attention. Some compounds, such 3s polysaccharides,

amino acids, peptides, proteins, which normally would be decomposed by

microorganisms, may be protected to some extent when they are adsorbed

within the interlamellar regions of clay minerals. However, they may

undergo chemical alteration at the clay surfaces as indicated in the section

on catalysis. This may account for the fact that ammonium ion often

occurs in abundance in illites which could have arisen from diagenetic

changes of a swelling clay, vermiculite, or montmorillonite that had

adsorbed considerable quantities of amino acids or peptides.

The adsorption of freshly formed humic substances from aqueous

extracts of humified clover by several types of clays was studied by

Wada and lnoue ( I 967) and lnoue and Wada (1968). They found much

greater adsorption by allophane than by such crystalline clay minerals

as montmorillonite and halloysite; this seems to concur with the observation of the high organic matter contents of soil clays high in allophane.

They also found that the humified clover extract was not affected by the

nature of the exchangeable cation on the clay and that it did not penetrate

the interlamellar spaces of montmorillonite to any great degree. This is

in contrast to results with soil humic or fulvic acids, where Kodama and

Schnitzer ( 1968) have shown a great effect of the exchangeable cation on

adsorption of a soil humic compound by montmorillonite. In addition, it



CLAY-ORGANIC COMPLEXES A N D INTERACTIONS



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was shown by Schnitzer and Kodama ( 1 966) in the laboratory that a

fulvic acid from soil organic matter would penetrate the interlamellar

regions of montmorillonite. Adsorption and penetration of the fulvic acid

was very much pH dependent, being greatest at the low pH of 3 and decreasing as pH increased to 7. Thermal studies on fulvic acid-montmorillonite complexes by Kodama and Schnitzer (1970) suggest that it is

possible to differentiate between externally adsorbed and internally

adsorbed fulvic acid on montmorillonite. The externally adsorbed

material decomposed before the combustion of that retained in the

interlamellar spaces. Isothermal experiments also showed that the

fulvic acid complexed with the montmorillonite delayed the thermal

decomposition as compared with uncomplexed fulvic acid, which relates

to the observed stability of clay-organic matter complexes in nature. In

contrast to this, McLaren and Peterson ( I 965) pointed out that they were

unable to find proof that organic matter has penetrated the internal surfaces of montmorillonites isolated from soil.

Changes in the nature of organic matter when clay (Wyoming bentonite) was applied to sand soils was studied by Colom and Wolcott (1967).

In these experiments clay application rates up to 50 tons per acre had

been applied to Plainfield sand, mixed in the top 6 inches, then cropped

for several years. Colom and Wolcott found positive correlations between

clay rates and various categories of acid- and alkali-soluble organic

fractions. Total nitrogen increased and the carbon-nitrogenratio decreased

with increasing clay rates. While it was obvious that the clay had affected

the development of organic matter in the soil, it was not clear what the

relationship was between the organic matter and clay, that is, whether the

resulting material was in fact an organic-clay complex, discrete organic

material, or a combination of these two categories.

The role of clay-organic complexes in soil structure has long been appreciated. The adsorption of organic materials by clays modifies the relationship of the clay to the surrounding environment in a fundamental

way. These modifications include the interaction with water and salts as

well as effects on swelling properties. Inter- as well as intraparticle bonds

result, and clay-organic complexes form aggregates of varying size and

stability which are of major importance in determining the physical nature

of soil and, therefore, the environment it provides for plant growth.

Greenland (1965b) has well documented the work in this area.

The recent work of Edwards and Bremner (1967) is of particular

interest because it attempts to relate bonding mechanisms described

earlier in this work that have been established on a fundamental basis,



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M. M. MORTLAND



with actual aggregate formation and stability. The proposal is made that

organic materials and clay particles are linked via polyvalent metal

cations and that microaggregates consist of groupings of these complexes

resulting from bridging through polyvalent metals in .various combinations with clay and organic matter. These workers point out that a Nasaturated resin shaken in water with the soil aggregates has a dispersing

effect which probably results from the adsorption of the polyvalent ions

such as Ca, Mg, Fe, and A1 by the resin, and a substitution of sodium in

the aggregate weakens the interparticle bonds which promotes bond rupture required for dispersion. Dispersive effects of alkali metal cations

has usually been explained in the past on the basis of zeta-potential

effects. But Edwards and Bremner ( 1 967) pointed out that the aggregates

described above could easily be dispersed by sonic or ultrasonic vibration without previous saturation with an alkali metal ion and showed no

tendency to flocculate on standing after vibration treatment. The bonds

formed in the aggregate formation are suggested to be weak enough to be

broken by the vibrations imposed upon them and that stable microaggregates are formed by a mechanism which is a reversal of the process

by which they may be dispersed.

The concepts of Edwards and Bremner (1967) on microaggregates of

soils are extremely intriguing because they coincide to a degree with the

basic mechanisms of adsorption of polar organic molecules by clays

established through infrared and other studies. That is, that the nature of

the exchangeable cation is of paramount importance in determining the

nature and energy of the adsorption process. Where protonation is not

involved, ion-dipole or coordination-type interactions, either direct or

through a water bridge, are decisive, as pointed out in earlier sections.

Polyvalent cations are generally much more electrophilic than monovalent cations like the alkali metals and so form much firmer bonds with

functional groups of organic compounds which are able to furnish electrons (i.e., carboxyl and amino groups). Also, electrostatic adsorption

through neutralization of positive charges on polyvalent cations by

anionic groups on organic matter is a possibility. It would seem to the

author that the most important bonding mechanism which could be easily

broken by vibration might be organic matter-water bridge-polyvalent

cation-clay, since direct coordination between functional groups and

polyvalent cations would be quite energetic and less likely to be broken

by sonic or ultrasonic vibrations. Also, under natural conditions many

such polyvalent cations would be likely to retain their primary hydration

shell. The water bridge bond would be a much more likely candidate for

disruption than would direct coordinate bonds. It would seem that the



CLAY-ORGANIC COMPLEXES A N D INTERACTIONS



113



above concepts may provide a better explanation for microaggregate

formation and stability than those based on electrostatic considerations.

V.



Conclusions



The dominant factors determining the nature of clay-organic interactions are the properties of the organic molecule, the water content of

the system, the nature of the exchangeable cation on the clay surface, and

the unique properties of the clay mineral structures. The exchangeable

cations determine the surface acidity and therefore the possibilities of

protonation of the organic compound. However, even in a homoionic clay

the surface acidity will vary with hydration level, becoming more acidic

with decreasing water content. Where protonation of the organic molecule

is not involved, the exchangeable metal cations act as electron acceptors

by which they interact with electron-donatingfunctional groups of organic

compounds. Such ion-dipole or coordination type of bonding will vary

greatly in energy depending upon the nature of the exchangeable cation.

Here again the hydration level of the system may be a factor because the

exchangeable metal cation may retain its primary hydration shell, in

which case functional groups of organic materials may be absorbed to the

cation by hydrogen bonding via a water bridge. Where organic cations

occupy the exchange sites on the clay mineral, other organic compounds

may interact with them by hydrogen bonding to form an organic-organic

complex on the mineral surface. It should be remembered that for most

nonionic interactions with clay minerals, organic compounds are in competition with water for adsorption sites. It must be recognized that the

surface chemistry of clays, with regard to interaction with organics or

other molecules, is different in water suspensions compared with air-dry

environments, that results obtained in one situation may or may not apply

to the other, and that both extremes may exist in nature. The clay mineral

surface makes its contribution to adsorption of organic molecules through

hydrogen bonding between its oxygens or hydroxyls and appropriate

functional groups of the organic material. The contribution of the clay

surface itself to the total adsorption energy is maximal when ions of low

solvation energy occupy the cation exchange sites and is minimal when

they are occupied by cations of high solvation energy. In addition, the

charge density of the clay minerals will sterically affect the position and

orientation of organic molecules associated with exchange sites either as

cations or indirectly through ion-dipole interactions.

Much remains to be learned regarding the clay-organic systems and the

reactions taking place at this interface. In particular, it is quite likely that



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many reactions, which have not yet been recognized, are catalyzed by the

clay minerals. In the past, almost all alterations of organic materials in

soils and sediments were attributed to a biological agency, but the author

is confident that the future will record important effects of clay surfaces

on some processes.

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BI RDSFOOT TREFOIL

Robert R. Seaney a n d Paul R. Henson

Cornell University, Ithaca, New York

and U. S. Department of Agriculture, Beltsville, Maryland



Page

1.



Introduction ............................. ...... . ..... ...........................................

Origin and Distribution ...............................................................

B. Agricultural History ,.................................. .............................. ...

C . Economic Importance ................................ ... ... ...........................

Morphology . . .. .. ........... . ..................................................................

A. Root ......................

.....................

B. Stem and Leaf ........

A.



II.



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....................

...................................

C. Seedling Growth ........................................................................

D. Vegetative Propagation .............................. ....................... ..........

IV.

Adaptation..............................................

B. Soils and Soil Fertility

C . Inoculation

A.



..................................



...................................................................

H . Insects ......................................................................................

Utilization ............................................................. ..........................

A. Hay and Silage ..........................................................................

B. Pasture ....................................................................................

C. Feeding Value ........

..................................

VI. Genetics and Cytology ....

V.



...................................................................



B.



VII.



Inheritance of Characters ........................... .. . . ........ .....................



Breeding ....................,



..................................



B. Variability and Methods ..............................................................

C . Pod Dehiscence ........................................................................

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