ISSN 0012-5008, Doklady Chemistry, 2007, Vol. 415, Part 2, pp. 200–204. © Pleiades Publishing, Ltd., 2007.Original Russian Text © G.N. Fedotov, G.V. Dobrovol’skii, 2007, published in Doklady Akademii Nauk, 2007, Vol. 415, No. 6, pp. 767–771.

200

Soils are frequently treated as systems with a multi-level architecture [1]. In particular, molecular–ionic,texture, soil structure, and horizon levels are distin-guished. Experimental soil science, however, fre-quently employs a simplified approach, treating soils asan ensemble of particles with various sizes, includingcolloidal particles, not as a system, and ignoring parti-cle interactions.

The simplified approach based on the polydispersemixture model is employed to determine the particle-size composition of soils, measure their specific surfacearea, study ion exchange and the state and movement ofwater in soils, and so on. Fundamental soil propertiesare frequently determined in this way; they are used toadvance models and hypotheses, and the constraintsunderlying their determination are, as a rule, ignored.As a consequence, the interpretation of experimentalresults is based on the hypothesis that a polydispersemixture is generated in the evolution of living and non-living matter, this mixture having a very limited abilityfor development and for data storage and transmissionand having virtually no structural or functional resem-blance to living systems.

Soil science employs several approaches to studysoil organics. These approaches are based on the solu-bility [2], ability of soils for hydrophilic and hydropho-bic interaction [3], particle-size separation undermechanical action and density fractionation [4], andother factors. All these approaches are, however,destructive: the organomineral assemblage is degradedto a mechanical mixture, and debris in this mixture arestudied and identified. As a consequence, the conclu-sions are perfectly adequate for the analyzed mixturebut are hardly adaptable to the test soil.

In our soil studies, we applied a systemic approach.This approach involves studies of the system’s integrity,namely, relations in the system [5]. It was shown in [6]

that soil colloids linking soil particles are a hybridmaterial [7]; the base of this hybrid material is thehumus, a jelly reinforced with inorganic colloids.

For improving our approach, it was necessary torefine some aspects of the behavior of the soil humus.Humus chemistry has been studied by soil scientistssince long ago and comprehensively, and the accumu-lated results are fairly sufficient for understanding therole of the humus in the existence of soil colloids.

Some of these previous results are listed below.(1) Soil organics are 70–90% bound to soil miner-

als; unlike detritus, they do not buoy in liquids withdensities of 1.9–2.0 [8].

(2) Fragments are eliminated from humus polymers,and the polymer structure is recovered through theincorporation of similar fragments into the humus [9].

(3) Elimination of side chains occurs nonselectivelyfor humic acid (HA) and fulvic acid (FA) macromole-cules [9].

(4) The sizes of HA and FA macromolecules arebelieved to differ by many tens of times, all beingformed of tens and hundreds of structurally similarfragments [2].

(5) Destruction of HAs produces FAs, among otherproducts [2].

(6) When HAs of a certain molecular weight areadded to a soil, the humus subsequently extracted fromthe soil shows the whole molecular-weight spectrum ofpolymers and is not enriched in HAs with molecularweight corresponding to the added HAs [8].

(7) Soil freezing strongly increases the concentra-tion of HAs and humins (humic substances stronglybound to inorganic soil components) [10].

(8) Seasonal variations are observed in the soilhumus. The

c

HA

/

c

FA

ratio in soils can vary by severaltimes during one month, with the carbon content beingvirtually unchanged [10].

(9) Studies of the in-year dynamics of the humuscomposition of loamy soddy-podzolic soils showedthat the dynamics of the overall FA content is oppositeto that of the HA content [10].

The above results contradict the behavior of a soil asa polydisperse system. It is likely for this reason that

Humus as the Base of Soil Colloids

G. N. Fedotov

a

and

Academician

G. V. Dobrovol’skii

b

Received April 27, 2007

DOI:

10.1134/S001250080708006X

a

Moscow State Forestry University, ul. Pervaya Institutskaya 1, Mytishchi-5, Moscow oblast, 141005 Russia

b

Moscow State University, Vorob’evy gory, Moscow,119992 Russia

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HUMUS AS THE BASE OF SOIL COLLOIDS 201

their interpretation meets some difficulties. For exam-ple, the statistically identical elimination of side chainsfrom HAs and FAs and their subsequent inclusion intothese macromolecules cannot be explained if HAs existin a globular conformation (as single molecules ormolecular associations) and FAs exist in an extendedconformation on mineral surfaces or in soil solutions.To explain this, one should suggest the identical acces-sibility of the terminal chains of HAs and FAs and,therefore, their unified spatial-structural organizationand their indistinguishability in the structure of thehumus gel. The change in the ratio between FA and HAmacromolecules in favor of the latter upon freezing andother results also cannot be explained in terms of apolydisperse mixture.

The above experimental data leave no doubt that thehumus matrix exists, but the question arises of the roleof HAs and FAs in its formation. Are HAs and FAs con-stituents of the matrix like amino acids in proteins, orare they only relatively stable fragments that arise upondegradation? Fulvic acids and, especially, humic acidsare not autonomous elements, at least in terms of thesystemic approach [11], because they are only elimi-nated by a sufficiently severe impact on soils.

We should note that it is sufficient to suggest that thehumus matrix is degraded by chemicals at its weakestlinks and that its debris are classified as HAs or FAsdepending on their solubility in acid solutions, and theinterpretation of the experimental facts as arising fromthe existence of a unified humus matrix in soilsbecomes consistent. In this case, a change in the densityof crosslinks of the organic matrix in which polargroups participate should lead to a change in thestrength of the matrix and the density of free polargroups. Therefore, the solubility of debris shouldchange and, as a result, the ratio between HAs and FAsshould change, which is observed in the above results.

This approach easily explains the above experimen-tal data. It is hardly possible that one-third of FA mac-romolecules are removed from the soil during onemonth, being substituted by an equal amount of HAmacromolecules. This can only be explained by anincrease in the crosslink density of the humus polymermatrix and the degradation of the matrix during the rou-tine determination of the fractional composition of thehumus due to chemical action on larger fragments thatwe perceive as HA macromolecules. The same expla-nation fits the effect of freezing on the decrease in theFA content and the increase in the HA and humin con-tent.

We showed in [6] that the humus gel in soils is rein-forced with inorganic colloidal particles. In materialsscience such structures are called hybrid materials [7].Organics in such nanosystems stabilize the existence ofinorganic nanoparticles, keeping them from coarsen-ing. The effect of the humus on the specific surface areaof the soil is perceived quite differently. The existenceof a high specific surface in soils is impossible without

some stabilizing factor. The humus matrix is apparentlysuch a factor. The humus was thought to have a veryhigh specific surface area. It is sufficient to remove thehumus by oxidation in order for the specific surfacearea of the soil to decrease dramatically. However,given that reinforced humus gels exist, when the humusis removed, the specific surface area decreases not onlydue to its disappearance, but also due to the coagulationof colloids whose separation was provided by thehumus matrix.

In order to understand the behavior of the humus geland soils themselves in response to changing watercontent, one should not treat the humus matrix as a uni-form organic substance. The existence of a great num-ber of hydrophobic regions and the participation of lip-ids in the humus organization should be taken intoaccount. In such systems, a change in water contentinduces the conformational rearrangement of themolecular network [12]. When the water content is low,the humus gel should be a water-in-oil emulsion. Thismeans that, when water is removed from the humus gel,the gel structure changes so that the nonpolar parts ofmacromolecules turn to increase their areas of contactwith one another, nonpolar lipid parts, and air. Thepolar parts are in contact with remnant water in the gelstructure and with minerals, coating them with protec-tive films (figure). As the water content increases, firstthe water regions coated with films are united into aframework [12]. Under these conditions, the soil gel isa system of two interpenetrating frameworks (onehydrophobic and the other hydrophilic; see figure). Thevalue of the hydrophobic framework and its bondingstrength are, however, greater, and the hydrophobicframework controls the behavior of soils with thesewater contents. Higher water contents induce the rever-sal of the structure: now, the hydrophilic frameworkcontrols the behavior of soil gels and soils. With stillhigher water contents, the hydrophobic frameworkstarts to degrade to autonomous regions; i.e., the humusorganization changes to an oil-in-water emulsion.

However, the above model of the structural evolu-tion of the humus gel as a function of water content isonly a simplified scenario. Micelles in water are knownto change their structure depending on the ratio of theirareas occupied by the hydrophilic and hydrophobicparts of biphilic molecules; this ratio is characterizedby the parameter

α

[12]. When

α

is less than 1/3 andwhen the surfactant concentration is low, sphericalmicelles are formed. As

α

progressively increases toreach

α

= 1, cylindrical micelles change to lamellarones. These rules were determined for systems of sim-ple surfactants. For surfactant mixtures, intricate (syn-ergetic or antagonistic) and virtually unpredictablerelationships are observed. For soils, the situation isadditionally complicated by the existence of high-molecular-weight surfactants (proteins and, to a certaindegree, FAs and HAs) in addition to various low-molecular-weight lipids. Moreover, these compounds

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are not free; rather, they are linked with one another inthe humus matrix.

It is certainly impossible to understand all detailsof the structural evolution of such a complex system.

Our model interprets the trends of fundamentalhydrophysical characteristics (figure) [1], the swell-ing behavior of soils, and some other physical prop-erties of soils.

7

6

5

4

3

2

1

P

F

Adsorbate water

Film waterMWA

MHWW

MMW + CBW

MCSW + LW

Capillary-film,

low-mobility waterLW

Capillary

CW

Gravitation water

W

Fundamental hydrophysical properties [1] and a suggested scenario of the structural transformation of the humus in response tovarying soil water content. Notations:

p

F

, soil water potential;

W

, soil water content. MWA, MH, WW, MMW, CBW, MCSW, LW,and CW are hydrophysical constants of soils, where MWA is maximum water adsorption capacity; MH, maximum hygroscopicity;WW, wilting water content; MMW, maximum molecular water capacity; CBW, capillary break water content; MCSW, maximumcapillary sorption water capacity; LW, least water capacity; and CW, capillary water capacity.

water

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HUMUS AS THE BASE OF SOIL COLLOIDS 203

Let us consider, as an example, soil swelling interms of the model advanced.

First, the model explains the absence of soil swell-ing for low water contents [1]. This observation was notclear: soil swelling was interpreted as the swelling ofsoil smectics. The existence of two interpenetrating(hydrophobic and hydrophilic) frameworks in the rein-forced humus gel and the decisive influence of thehydrophobic framework on the behavior of soils withlow water contents explain the experimentally observedabsence of swelling at low water contents.

Second, this model can explain the increase in soilswelling upon partial oxidation of organics with hydro-gen peroxide discovered in the middle of the 20th cen-tury [13]. In some soils, the increase in swelling is morethan 1.5-fold. There has been no adequate interpreta-tion of this fact. The interpretation of soil swelling ashumus gel swelling makes this fact quite natural: adecrease in the crosslink density of a polymer is knownto enhance swelling. Apparently, partially oxidizingorganics with hydrogen peroxide, Malquori destroyedorganic crosslinks between humus macromolecules,which enhanced swelling.

Third, the swelling pressure versus water contentdependence discovered by Mattson (cited from [14]),

where

W

is soil water content and

P

is the swellingpressure, acquires a clear physical meaning.

Poznyak’s relationship [15] known in polymer sci-ence relates the swelling pressure of polymers to thepolymer concentration in a gel:

P

=

AC

K

.

Here,

A

is a constant depending on the type of polymer,

C

is the weight concentration of the polymer in the gel,and

K

is a constant approaching three for most poly-mers.

The concentration

C

is the ratio of the polymerweight in the gel to the sum of the water and polymerweights in the gel. Applying this relationship to the soilhumus gel, we can take that the water weight at whichswelling starts is noticeably higher than the weight ofthe polymer in the gel. In this case,

C

is defined as theratio of the polymer weight in the gel to the waterweight in the gel.

In the same soil, the weight of the polymer in the gelis independent of the soil water content.

Therefore,

C

=

B

,

where

B

is a constant,

M

soil

is the weight of the abso-lutely dry soil, and

M

water

is the soil water weight.The ratio of the soil water weight to the weight of

the absolutely dry soil is taken as the water content ofthe soil.

W P3 const,=

Msoil

Mwater-------------

Substituting the expression for the polymer concen-tration

into Poznyak’s relationship and setting

K

= 3, we obtain

where

A

·

B

3

is a constant.As a result, we arrive at an expression identical to

the Mattson relationship.Thus, soil and polymer swelling are described by

identical mathematical relationships, and this verifiesour hypothesis that the soil humus is at least partially agel matrix that incorporates colloidal particles and con-trols the behavior of the soil.

The above analysis verifies the existence of ahumus gels, which controls the existence and behaviorof soil colloids. However, the current understanding ofthe general tendencies of the behavior of soil gels doesnot allow us to design an algorithm for predicting thebehavior of soils in various situations. Each particularproperty and the improvement of each particular soilparameter require special investigation. The under-standing of the principles of the architecture of soilcolloids and the mechanism of structural rearrange-ments in soils will only enhance the finding of ade-quate solutions.

ACKNOWLEDGMENTS

This work was supported by the Russian Foundationfor Basic Research (project no. 07–04–00144a).

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CBW-----=

W3P AB

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