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Doklady Chemistry, Vol. 393, Nos. 4–6, 2003, pp. 279–282. Translated from Doklady Akademii Nauk, Vol. 393, No. 4, 2003, pp. 497–500.Original Russian Text Copyright © 2003 by Fedotov, Tret’yakov, Pozdnyakov, Zhukov, Pakhomov.

Factors responsible for the origin of natural electri-cal fields (NEFs) in soils have been analyzed, and theorganomineral gel (OMG) of soils has been shown toaffect these fields.

Experiments in which the electrical conductivity ofsoils and the potentials between soils changed as therecovery of the OMG network progressed have proventhat cation diffusion through the OMG is the key factorin the appearance of NEFs.

A nonequilibrium cation distribution over soil lay-ers resulting from colloid peptization in the upper soillayer during water infiltration is suspected to be respon-sible for the appearance of NEFs.

NEFs, discovered in rocks and soils more than50 years ago [1], were first related to differentiated ionmobilities in heterogeneous systems [2]. A previouslyadvanced geophysical model [3] was applied, withinsignificant modifications, to interpret the origin ofNEFs in soils.

Now it is known [4] that many inorganic solids arecovered with a gel layer. Gelling is especially pro-nounced in soils that are formed through weathering [5].

Our goal in this work was to refine the extant ideasof the mechanism of NEF formation in soils taking intoaccount the existence of OMG on the surface of solidparticles.

We chose soils easily available to us for use in thestudy: peat and soddy-podzolic soils from the flood-plain of the Yakhroma River and its surroundings, agreenhouse substrate, and leached Kuban chernozem.This choice was dictated by our desire to study soilsdiffering significantly in their properties. The soil prop-erties were determined using routine procedures.Determination errors fell within the errors of the meth-ods employed. The results displayed in Table 1 showthat the soils differed strongly in their water content,density, organic and free salt contents, overall specific

surface area, acidity, cationic composition, and otherproperties.

The ac electrical resistivity was measured in plasticcells using the four-point probe method. Test soil sam-ples were stored at

105°ë

until they achieved a constantweight (the absolutely dry (AbD) state). Then, distilledwater was added until the soils achieved their naturalwater content level (NW), which was 0.8–0.9 of theleast field water capacity. Watered samples were placedin a measurement cell, and measurements were madeperiodically. A GFG-82117 A device was used as thevoltage source. Mastech M890 digital multimeters withan internal resistance of 10 M

Ω

were used as current andvoltage gages. The error in resistivity was within 2%.

Air-dry (AD) soils were prepared by removingwater from soils at

40°ë

.

The resistivity of solutions was measured at 1000 Hzusing a standard procedure. The measurement errorwas within 1%.

CHEMISTRY

Role of an Organomineral Gel in the Formation of Natural Electrical Fields in Soils

G. N. Fedotov*,

Academician

Yu. D. Tret’yakov**, A. I. Pozdnyakov**, D. V. Zhukov*, and E. I. Pakhomov*

Received August 21, 2003

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

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

100

80

60

40

20

0 30 60 90 120 150 180 210

Resistivity

×

100,

Ω

cm

Time, h

Soddy-podzolic soil

Greenhouse substrate

Peat soil

Chernozem

Fig. 1.

Electrical resistivity vs. time for (AD + H

2

O) soils.

280

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2003

FEDOTOV

et al

.

The potential difference was measured between twosoil samples brought in contact. Standard Ag/AgClelectrodes were used; they were connected to soil sam-ples through agar-thickened salt bridges filled with asaturated KCl solution. The potential difference at thecontact surface between soil layers or soil samples wascalled the diffusion–adsorption potential (DAP). Thepotential difference between electrodes (DAP) wasmeasured on the specified multimeter. The error waswithin 10% of the measured value.

Since electrical properties are ordinarily the moststructure-sensitive, we started with measuring the soilresistivity as a function of time. The time zero was themoment when water was added to AbD soils (Fig. 1).The results show that processes occurring in soils uponaddition of water decrease ion mobilities.

To understand the results, let us imagine two identi-cal soils with the only difference that an OMG structureincluding a soil solution exists in one soil and does notexist in the other. The transfer of the soil solution fromthe free state to the OMG matrix must decrease ionmobilities. Therefore, given the same water content, asoil binds ions of the soil solution with a higher energyif the soil contains an OMG matrix. Removing waterfrom the soil, we partially destroy the OMG through

coagulation of colloidal particles; the observed resistiv-ity rise proves that the OMG structure has been recov-ered, its force field has increased, and its ion mobilityhas decreased in proportion. In chernozem (which isknown to be the richest in colloidal particles), the resis-tivity rise is the slowest; in the soddy-podzolic soil(which is the poorest in these particles), the resistivityceases rising almost immediately.

Salt retention by soils is documented [6], but thereis no explanation for this phenomenon. It is hardlyexplicable in terms of the double electrical layer of aseparate colloidal particle, but we may suggest thatthese salts are retained by the OMG network as a wholeand must be released when the network is destroyed.We determined the electrical conductivity and the con-tents of selected cations in aqueous extracts of ther-mally processed soils (Table 2). Our results can beunderstood as follows: when water is removed, the net-work strength first increases (aqueous extracts from ADsoils have lower conductivities than NW soils); then,the network starts to disintegrate (the conductivity ofaqueous extracts increases).

The aforesaid means that OMG controls the ionmobility in all of the soils we studied. The occurrenceof OMG on the surface of solid soil particles suggests

Table 1.

Characteristics of soils

Property Greenhousesubstrate

Soddy-podzolic soil Peat Chernozem

Water content, % 79.7 23.5 140 23.5

Density, g/cm

3

0.42 1.20 0.27 0.80

Kutilek total specific surface area, m

2

/g 98 13 346 100

Solid-phase density, g/cm

3

1.96 2.02 1.46 2.10

Porosity, % 78 41 81 62

Aqueous extract pH 6.9 6.3 6.6 7.0

Salt extract pH 6.5 6.3 6.5 6.8

Nitrates, mg/kg 116 211 371 201

Phosphorus (P

2

O

5

), mg/100 g 25 27 17 12

Ammonium, mg/kg 41 27 41 35

Organics (loss on ignition), % 28.3 3.6 54.6 8.2

Potassium, mg/100 g 160 130 70 80

Sodium, mg/100 g 70 50 140 30

Calcium, mg/100 g 1200 160 1750 820

Total exchangeable bases, mg-equiv/100 g 82 15 104 42

Hydrolytic acidity, mg-equiv/100 g 1.8 <1 2.2 <1

Cation exchange capacity, mg-equiv/100 g 84 16 106 42

Base saturation, % 98 94 98 100

Salts, mg-equiv/100 g 0.9 2.4 3.5 2.5

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ROLE OF AN ORGANOMINERAL GEL 281

that soils are brought in contact through their OMGstructures (which exist in each soil) and the DAP is aresult of the interaction of the two OMG structures.

Let us consider the simplest case, namely, the inter-action of soils having identical compositions of theirOMGs (with their water contents ignored). The poten-tial difference between electrodes immersed into a pairof soils of the same type but differing in their water con-tents brought in contact with each other follows ans-shaped curve (Fig. 2). These results suggest that twotypes of limiting OMG structures exist; they correspondto the highest and lowest water contents of the soil in theexistence range of the OMG network. Therefore, theDAP value is controlled not only by the chemical com-position of the OMG but also by its structure.

The cation mobility (activity) will increase once theOMG network is destroyed. Therefore, when a soil witha destroyed OMG network is in contact with a soilwhose OMG network is not destroyed, the latter willacquire a positive charge due to cation transfer.

AbD soils were saturated with water until theyachieved their natural water content level (AbD +

ç

2

é

)and were brought in contact with NW soils.

As we expected, the NW soils were in all cases posi-tively charged: peat, 1.9 mV; soddy-podzolic soil, 1.5 mV;greenhouse substrate, 2.1 mV; chernozem, 2 mV.

The effect of the OMG network on the DAP is alsowell defined if one compares the potentials arisingbetween pairs of different NW soils and pairs of thesame soils with partially destroyed OMG networks thathave been first dried to the absolutely dry state and thenbrought to their natural water content level(AbD +

ç

2

é

) (Table 3).

From the results above, it is clear that the destroyednetwork in all cases decreases the DAP, but to variousextents. An explanation lies, first, in the decreasedstrength of OMG networks and the associated change(decrease) in the ratio of diffusion rates of singlycharged and doubly charged cations through the OMGand, second, in the existence of a free solution in whichthe diffusion rates are far less differentiated than in theOMG network.

Another argument in favor of the dominant effect ofthe OMG structure on the DAP is the change in theDAP between two soil samples of which one has itsOMG network structure changing over time. The great-est potential difference, 4 mV, was observed (providedthat free water existed in the system) between NWchernozem and (AD +

ç

2

é

) chernozem. When theOMG network was restored, the difference disappearedin 15–20 h, precisely as expected.

All of our results indicate that the OMG network isresponsible for the appearance of NEFs; once the net-work is destroyed, the DAP decreases.

Processes in soils occur on another time scale thanthose occurring in chemical systems; therefore, even

though concentrations are leveled very slowly, DAPs mustdisappear unless some process replenishes this difference.

In natural settings, upper soil layers exposed tomoving water lose their colloidal particles, which trans-

Table 2.

Properties of aqueous extracts from soils (soil-to-water ratio, 1 : 10)

Soil Electrical resis-tivity,

Ω

cmK

+

,mg/l

Na

+

,mg/l

Ca

+

,mg/l

Soddy-pod-zolic soil

NW 5400 16 17 40AD 7000 14 8 46AbD 5500 16 8 53

Greenhouse substrate

NW 2700 25 23 120AD 3500 32 28 92AbD 2000 35 28 200

PeatNW 2250 11 24 240AD 5200 7 12 160AbD 1800 12 15 400

ChernozemNW 5150 3 8 92AD 7200 4 7 92AbD 5050 5 7 132

Note: NW, soils with a natural water content (0.8–0.9 of the leastfield water capacity); AD, air-dry soils; AbD, absolutely drysoils.

20

18

16

14

12

10

8

6

4

2

15 30 45 60 75 90 105 1350

DAP, mV

W

, %

Greenhouse substrate

Chernozem

Peat

Soddy-podzolic soil

22

24

Fig. 2.

DAP vs. the water content of soil samples in contact.

282

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et al

.

fer to lower soil layers. Colloids whose double electri-cal layer is enriched in singly charged cations show thehighest tendency toward peptization. The activity (con-centration) of singly charged cations in lower soil lay-ers increases compared to upper soil layers as a resultof the transfer of colloidal particles, and this gives riseto soil NEFs.

The existence of soil NEFs apparently explains rootelectrotropism (root growth in the direction of a nega-tive electrode) [8] and increased plant growth rates inthe presence of a potential difference in a soil [9],although at present it is difficult to answer the questionof whether a soil electrical field is merely a source thatinforms plants on the state of the soil [10] or has somephysiological function.

From the above, we may state that DAPs appear insoils due to the existence of a structured OMG thatinvolves the soil solution.

REFERENCES

1. Dakhnov, V.N. and Kobranova, V.N.,

Trudy Mosk-ovskogo neftyanogo instituta

(Proceedings of MoscowPetroleum Institute), Moscow, 1947, no. 5, pp. 117–126.

2. Vendel’shtein, B.Yu.,

Prikladnaya geofizika

(AppliedGeophysics), Moscow: GNTI neftyanoi i gorno-toplivnoi literatury, 1960, no. 26, pp. 186–217.

3. Berezin, P.N. and Kipnis, V.M.,

Vestn. Mosk. Gos. Univ.,Ser. 17: Pochvovedenie

, 1978, no. 2, pp. 15–19.

4. Lipson, G.A. and Kolodieva, T.S.,

Kolloidn. Zh.

, 1972,vol. 34, no. 2, p. 235.

5. Zolotareva, B.N.,

Gidrofil’nye kolloidy i pochvoobrazo-vanie

(Hydrophilic Colloids in Soil Formation), Mos-cow: Nauka, 1982.

6. Pinskii, D.L.,

Ionoobmennye protsessy v pochvakh

(IonExchange Processes in Soils), Pushchino, 1997.

7.

Elektrokineticheskie svoistva kapillyarnykh sistem

(Electrokinetic Properties of Capillary Systems),Moscow: Akad. Nauk SSSR, 1956.

8. Gordeev, A.M., Prutinskaya, N.I., and Solomatin, K.V.,

Primenenie elektromagnitnykh polei v sel’skokhozyaist-vennykh issledovaniyakh i proizvodstve

(Use of Electro-magnetic Fields in Agricultural Studies and Industry),Leningrad: AFI, 1988, pp. 132–138.

9. Savich, V.I., Gordeev, A.M., and Solomatin, K.V.,

Vestn.S.–kh. Nauki

(Moscow), 1990, no. 4, pp. 13–19.

10. Savich, V.I. and Dokuchaeva, T.I.,

Izv. Timiryazevsk.S

−

kh. Akad.

, 1993, no. 1, pp. 56–69.

Table 3.

DAP (mV) appearing between different soils

Pair no. Soil pair NW soils AbD + H

2

O soils

1 Greenhouse substrate–sod-dy-podzolic soil

–

8.4

–

2.2

2 Greenhouse substrate–peat

–

6.5

–

2.8

3 Greenhouse substrate–cher-nozem

–

4.3

–

4.1

4 Soddy-podzolic soil–cher-nozem

1.8 0