ISSN 0012-5008, Doklady Chemistry, 2006, Vol. 411, Part 2, pp. 240–242. © Pleiades Publishing, Inc., 2006.Original Russian Text © G.N. Fedotov, Yu.D. Tret’yakov, E.I. Pakhomov, 2006, published in Doklady Akademii Nauk, 2006, Vol. 411, No. 6, pp. 785–787.

240

It has been recently proposed to treat soils as soilcolloids organized into gel structures and containinglarge amounts of soil water. Colloidal particles areaccommodated in cells of a three-dimensional networkof organic humus molecules. The soil gel structure maybe treated as a humus gel reinforced with colloidal par-ticles, or reinforced humus gel (RHG). When exposedto water, the RHG behaves as many polymers: it swells(takes up water and enlarges); when dried, it shrinks.Various exposures change the state of the polymericRHG, which changes the soil properties [1–3].

Soils are known for their capacity for salt overreten-tion relative to ion exchange [4]. Soils behave in thisrespect as granulated ion exchangers: the latter can alsoretain salts and are organic gel structures [5].

It is noteworthy that gels can “remember” how theywere prepared. For example, if two gels (one preparedfrom a dilute and the other from a concentrated gelatinsolution) are dried at a low temperature to the samewater content and then again exposed to water, theformer swells far more strongly than the latter [6]. Thereason for this different behavior is that dried gels con-serve the internal structure that existed during their for-mation; the evolution of structure takes longer timesthan the duration of swelling experiments.

In our experiments intended to elucidate how thewater content affects the salt-retention ability of soils,we proceeded precisely from this feature of the gels: weassumed that the RHG would behave in the same man-ner.

The goal of this work was to elucidate how the evo-lution of soil gel structures induced by changing watercontent affects salt leaching.

Samples were taken from the high-humus horizonsof leached Kuban chernozem, an uncultured soddy-podzolic soil from the Oka Terrace National Park, and

a cultured soddy-podzolic soil from the YakhromaRiver floodplain.

Test samples were prepared as follows: air-dry soilswere moistened to a required level, and the moistenedsamples were exposed for at least two weeks. Toluenewas added to inhibit microbial activity [7].

Water extracts from soils were prepared as follows:a 20-g soil sample was placed in 50 ml of deionizedwater, shaken on a rotator for 1 h, and centrifuged [8].

The pH was measured with an Orion Research EA940 pH meter. Potassium, sodium, and calcium concen-trations were measured with an FPL-1 flame photome-ter. The measurement error was within 3%.

A soil is a very complex structure; when the soil isexposed to water, its ions and molecules yield to water.Salts of strong acids [9] also yield to the extract [9]. ThepH of the extract changes due to hydrolyzable salts,whose concentration may be about two orders of mag-nitude lower than the cation concentrations in theextracts (Figs. 1–3). Therefore, a correlation betweenthe pH of an aqueous extract and the cation concentra-tion cannot be found; it is pertinent to analyze generaltrends.

From the results displayed in Figs. 1–3, we see thatthe pH of extracts increases with increasing water con-tent in soddy-podzolic soils. From the fact that thedeionized water used had a pH of 5.5–5.6, we may inferthat, in chernozem, the yield of pH-increasing com-pounds decreases with increasing soil water content; insoddy-podzolic soils, the yield of these compoundsincreases.

It is noteworthy that there are flex points on the pHand concentration versus water content plots (Figs. 1–3), dividing the plots into three portions. The first por-tion extending to a water content of 5% is observed forboth chernozem and soddy-podzolic soils. The secondportion extends from 5 to 15–17% water for chernozemand from 5 to about 10% for soddy-podzolic soils. Thethird one lies above 15–17 and 10% water for cher-nozem and soddy-podzolic soils, respectively.

For chernozem, the pH increases until the soil watercontent reaches 5%, while the sodium concentration

Effect of the Soil Water Content on Soil Gels and Salt Extraction from Soil Gels

G. N. Fedotov

a

,

Academician

Yu. D. Tret’yakov

b

, and E. I. Pakhomov

a

Received August 14, 2006

DOI:

10.1134/S0012500806120056

a

Moscow State Forestry University, Mytishchi-5, Moscow oblast, 141005 Russia

b

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

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EFFECT OF THE SOIL WATER CONTENT ON SOIL GELS AND SALT EXTRACTION 241

remains unchanged and the potassium concentrationslightly decreases in this water content range. For watercontents from 5 to 15–17%, the pH and sodium concen-tration both decrease; when the water content is above15–17%, these quantities are unchanged.

For the uncultured soddy-podzolic soil from theOka Terrace National Park, the calcium concentrationdecreases until the soil water content is 5%; at a watercontent of about 10%, the pH and sodium concentrationcurves change their trends.

For the cultured soddy-podzolic soil from theYakhroma River floodplain, the pH decreases, thesodium concentration increases, and a flex appears onthe potassium concentration curve while its water con-tent is less than 5%. For water contents from 5 to 10%,the pH increases and the sodium concentrationdecreases. At about 10% water, the pH and sodium con-centration curves change their trends.

The above-described features may be interpreted asarising from structural rearrangements occurring in theRHG at the specified water contents.

Let us consider the composition of the humus gel.Humic acids and fulvic acids involved in the formationof the humus gel have both hydrophilic and hydropho-bic portions [10, 11]. It also should be taken intoaccount that lipids are contained in the humus gel; theircontent in mineral soils is 2–15% and can reach 25% inorganogenic horizons [12]. These low-molecular-weight compounds, likely, should have a greater free-dom to move than macromolecules. Our initial assump-tion was that the RHG swells upon taking up water andshrinks when dried. The high concentrations of film-forming molecules [13] in the humus gel help us tounderstand gel behavior in response to changing watercontent. The humus gel with low water contents shouldhave a W/O (water-in-oil emulsion) type structure [13].This means that removal of water from the humus gelcauses it to reorganize so that the apolar parts of mac-romolecules rotate to increase the contact area betweenthem, apolar parts of lipids, and air. The polar parts arein contact with the remnant water and minerals in thegel structure, forming protective films around them.The increasing water content has the following effect:first, the water regions coated with protective filmsincrease; at higher water contents, the inversion of thegel structure occurs: the W/O organization changes tothe O/W (oil-in-water emulsion) organization [13].

Presumably, the internal RHG organization insoddy-podzolic soils and chernozem starts to changefrom the W/O to O/W type when the soil water contentis about 5%; the reorganization ends at 15–17% waterin chernozem and at about 10% water in soddy-pod-zolic soils.

It is noteworthy that the data we obtained in thiswork coincide with the water contents in chernozemand cultured soddy-podzolic soils at which flex pointsappear on fractal dimension and scattered neutronintensity curves [14]. It was suggested in [14] that,

7.0

20

7.2

7.3

7.4

7.5

4 6 8 10 12 14 16 18 20 22 24

7.1

0.5

1.5

2.0

2.5

1.0

6.9

Water content, %

mg-

equi

v

pH

1

2

3

4

5.5

20 4 6 8 10 12 14 16 18 20

7.0

7.5

7.0

6.5

6.0

0.05

0.55

0.45

0.35

0.15

0.25

0.65

5.0

pH

Water content, %

mg-

equi

v

12

4

3

6.20

20 24222018161412108646.15

6.50

6.45

6.40

6.35

6.30

6.25

0.1

0.6

0.5

0.4

0.3

0.2

Water content, %

mg-

equi

v

pH

1

2

3

4

Fig. 1.

Effect of the water content of leached Kuban cher-nozem samples on (

1

) the pH of aqueous extracts and(

2

) sodium, (

3

) potassium, and (

4

) calcium concentrationsin the extracts.

Fig. 2.

Effect of the water content of soddy-podzolic soilsamples from the Oka-Terrace National Park on (

1

) thepH of aqueous extracts and (

2

) sodium, (

3

) potassium, and(

4

) calcium concentrations in the extracts.

Fig. 3.

Effect of the water content of soddy-podzolic soilsamples from the Yakhroma River floodplain on (

1

) thepH of aqueous extracts and (

2

) sodium, (

3

) potassium, and(

4

) calcium concentrations in the extracts.

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when a soddy-podzolic soil reaches a certain water con-tent, its RHG expands with the separation between rein-forcing colloidal particles and colloidal-sized aggre-gates being increased. Quite likely, such an expansionoccurs after the inversion of the internal RHG organiza-tion from the W/O to O/W type.

In summary, we obtained consistent data using fun-damentally different methods, which highlights theevolution of the RHG upon structural reorganizationsinduced by the changing soil water content.

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