738 ISSN 1064-2293, Eurasian Soil Science, 2006, Vol. 39, No. 7, pp. 738–747. © Pleiades Publishing, Inc., 2006. Original Russian Text © G.N. Fedotov, G.V. Dobrovol’skii, V.I. Putlyaev, A.V. Garshev, V.K. Ivanov, E.I. Pakhomov, 2006, published in Pochvovedenie, 2006, No. 7, pp. 824–835. INTRODUCTION Much attention was paid to the study of soil colloids until the mid-1950s [1, 6, 18]. Most studies in this field dealt with the ion-exchange capacity of soils [2, 4, 13]. However, the studies of soil colloids were reduced to determining their composition and examining the absorbing capacity of soils. No reliable data are presently available on the col- loidal structure of soils. From general considerations, it is clear that soil should have a structural organization, but it is still not unambiguously characterized. Unfortu- nately, it is even not safe to say that a colloidal struc- tures exist in soils. In fact, the occurrence of structure- less conglomerates of colloidal particles may also be stated in the absence of proof. The lack of experimental data on the structural orga- nization of the colloidal component of soils prompted us to perform a number of studies in this area [19–27]. The results obtained indicated that the soil solutions are structured colloidal formations, in which organic and mineral colloidal particles interact presumably via long-range aggregation 1 [8] and form stable periodic colloidal structures [26], 2 which determine many prop- erties of soils. 1 Long-range aggregation is the fixation of colloidal particles at a distance from one another. 2 Periodic colloidal structures are systems in which colloidal parti- cles are fixed at a distance from one another in energetic minima of the system. It is notable that the energetic periodicity does not always entail a geometric periodicity. However, this interpretation of the experimental results reduces the role of soil humus to its occurrence as colloidal particles in colloidal soil structures. The aim of this work was to specify the role of the organic soil component in the formation of colloidal structures and to examine the effect of colloidal struc- tures on some physical properties of soils. EXPERIMENTAL Experiments were conducted on samples taken from the humus-accumulative horizons of a soddy-podzolic soil (near the Yakhroma River, Moscow region), a leached chernozem (Kuban region), and a krasnozem (Chakva, Georgia). The electron-microscopic studies were performed on a Zeiss SUPRA 50 VP scanning electron microscope equipped with an autoemission source at an accelerat- ing voltages of 3–10 kV and with an InLens secondary electron detector. Original soil samples and those pretreated with 30% H 2 O 2 to remove organic matter [31] were used. The clay fractions of the soils, which were prepared according to the conventional procedure [28], were also studied. The air-dry soils were thoroughly triturated with a 4% sodium pyrophosphate solution in a mortar at room temperature, sieved through a sieve of 250 μm, placed in a cylinder of 1 l with distilled water, and thor- oughly mixed. After settling for 24 h, samples with par- ticles <1μm in size were taken from the suspensions and applied onto a copper substrate. Gel Structures in Soils G. N. Fedotov a, b , G. V. Dobrovol’skii b , V. I. Putlyaev c , A. V. Garshev d , V. K. Ivanov c , and E. I. Pakhomov a a Moscow State Forestry University, ul. Pervaya Institutskaya 1, Mytishchi-5, Moscow oblast, 141005 Russia b Faculty of Soil Science, Moscow State University, Leninskie gory, Moscow, 119899 Russia c Department of Chemistry, Moscow State University, Leninskie gory, Moscow, 119899 Russia d Department of Materials Science, Moscow State University, Leninskie gory, Moscow, 119899 Russia Received July 6, 2005 Abstract—The colloidal structures in soils were studied by scanning and transmission electron microscopy. Small-angle neutron scattering was used in the pioneering study of the colloidal soil structures and their rear- rangements under the effect of different factors. It was found that colloidal particles are fixed apart in a gel matrix formed by organic molecules. The results obtained suggest that the organomineral gel is composed of soil humus occurring, at least in part, in a gel-like status and reinforced by organic and inorganic colloidal par- ticles. In the interaction with water, the reinforced humus gel behaves as many polymers: it swells, absorbing water and increasing in volume; it shrinks under drying conditions. Different impacts on the soil affect the status of the reinforced humus gel, which results in the observed changes of the soil properties. DOI: 10.1134/S1064229306070076 SOIL PHYSICS