956 ISSN 1064-2293, Eurasian Soil Science, 2007, Vol. 40, No. 9, pp. 956–961. © Pleiades Publishing, Ltd., 2007. Original Russian Text © G.N. Fedotov, E.I. Pakhomov, A.I. Pozdnyakov, A.I. Kuklin, A.Kh. Islamov, V.I. Putlyaev, 2007, published in Pochvovedenie, 2007, No. 9, pp. 1071–1077. INTRODUCTION Experiments on the study of the stickiness, electrical conductivity, enzymatic activity, thermal conductivity, and some other soil properties depending on the dura- tion of the soil–water interaction after addition of water to dry soils confirmed the existence of interactions between soil colloidal particles with the formation of gel structures incorporating the soil solution. Methods of scanning and transmission electron microscopy, as well as small-angle neutron scattering, showed that colloidal particles in soils are fixed apart in a gel matrix formed by organic molecules. The results obtained [12, 13, 16] suggest that organic–mineral soil gels present on the surface of soil particles and binding these particles are composed of soil humus occurring in the gel-like state and reinforced by organic and inorganic colloidal particles. In the interaction with water, the reinforced humus gel (RHG) behaves as many polymers: it swells and increases in volume when absorbing water, and it shrinks when dry. Different impacts on the soil affect the state of the reinforced humus polymer gel, which results in changes in soil properties. At the same time, studies of the gel composition [1, 5, 6, 11] indicate that soil gels are structurally non- homogeneous; therefore, the general concepts of the structure and functioning of soil gels [12, 13, 16] need to be specified and rectified. It may be supposed that information on the princi- ples of structural rearrangement of soil colloids could be derived from the analysis of changes in the structural organization of soil colloids under the effect of thermal treatment and changes in the soil water content. EXPERIMENTAL Small-angle neutron scattering (SANS) is a method that can be used to study the structural organization of the whole organic–mineral gel, including internal lay- ers of soil gels without their separation [8]. This method can provide information on the fractal organi- zation of soils, which is an integral characteristic of their colloidal structure [14, 15]. The objects of the study were the humus-accumula- tive horizons of contrasting soils: a leached chernozem, a soddy-podzolic soil, and a krasnozem. The properties of the soils were described earlier [12]. These soils dif- fered in many parameters, which could provide infor- mation on the effect of some soil properties on the structural organization of soil colloids. The specific sur- faces of the chernozem and krasnozem exceeded that of the soddy-podzolic soil by an order of magnitude. Hence, the contents of colloidal particles in them were significantly higher than in the soddy-podzolic soil. The krasnozem was enriched in positively charged col- loidal particles of iron and aluminum hydroxides com- SOIL PHYSICS Structure and Properties of Soil Organic–Mineral Gel G. N. Fedotov a , E. I. Pakhomov a , A. I. Pozdnyakov b , A. I. Kuklin c , A. Kh. Islamov c , and V. I. Putlyaev d 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, 119992 Russia c Joint Institute for Nuclear Research, Dubna, Moscow oblast, 141980 Russia d Faculty of Chemistry, Moscow State University, Leninskie gory, Moscow, 119992 Russia Received September 27, 2006; in final form, February 2, 2007 Abstract—Changes in the fractal dimension and scattering intensity of colloidal structures in a chernozem, soddy-podzolic soil, and a krasnozem were studied by small-angle neutron scattering at different temperatures and soil water contents. The character of the neutron scattering by soil colloids indicated that the latter were mass fractals in all of the soils studied; i.e., the colloidal particles were located apart from one another even in dry soils. The obtained results confirmed the supposition about the distribution of colloidal particles in the humus gel matrix. The changes in the fractal parameters of the soddy-podzolic soil and chernozem with increas- ing water contents were nonmonotonic in character, which indicated complex structural rearrangements of the colloidal component in these soils. From the results obtained, a conclusion was drawn that the destruction of the molecular network of reinforced humus gel occurred upon heating the soils to high temperatures: colloidal particles reinforcing the humus gel began to move and coagulate with the formation of dense aggregates. The electron-microscopic study of gel films released from the predried and then capillary wetted aggregates in water showed that the gel films were nonhomogeneous and included zones of humus gel reinforced by colloidal par- ticles and zones almost free from these particles. DOI: 10.1134/S1064229307090050