740 ISSN 1064-2293, Eurasian Soil Science, 2007, Vol. 40, No. 7, pp. 740–746. © Pleiades Publishing, Ltd., 2007. Original Russian Text © G.N. Fedotov, G.V. Dobrovol’skii, V.I. Putlyaev, E.I. Pakhomov, A.I. Kuklin, A.Kh. Islamov, 2007, published in Pochvovedenie, 2007, No. 7, pp. 823–830. INTRODUCTION Experiments on the study of the stickiness, electrical conductivity, enzymatic activity, structural-mechanic, and other soil properties depending on the duration of the soil–water interaction after addition of water to dry soils confirmed the interactions between soil colloidal particles with the formation of gel structures incorpo- rating the soil solution. Methods of scanning and transmission electron microscopy showed that colloidal particles in soils are fixed apart in a gel matrix formed by organic molecules, which is semitransparent for an electron beam. The results obtained [11, 12, 15] suggest that orga- nomineral soil gels present on the surface of soil parti- cles binding these particles compose soil humus occur- ring in a gel-like state reinforced by organic and inor- ganic colloidal particles. In the interaction with water, the reinforced humus gel behaves as many polymers: it swells and increases in volume when absorbing water and shrinks when dried. Different impacts on the soil affect the state of the reinforced polymer humus gel, which induces changes in the soil properties. However, these studies are complicated by problems of isolating the structures studied in undisturbed state. Experiments confirmed the existence of a reinforced humus gel in soils, but they provide no unambiguous proof of the absence of other types of gel structures in soils [3]. An electron-microscopic study of soil solutions showed that colloidal particles are arranged in space on a substrate to form structures very similar to fractal clusters [11]. This suggested that soil colloids have a fractal structure. Nonetheless, no unambiguous conclu- sion on the fractal organization of soil colloids can be drawn, because similar structures could be generated during the preparation of soil samples for electron- microscopic study. Methods should be found to directly observe the colloidal structures in soils rather than study separate fragments of colloidal formations isolated from soils subjected to external impacts. These methods include small-angle scattering of neutrons and X-rays [8]. Small-angle neutron scattering studies [13, 14] con- firmed the supposition about the fractal organization of colloids in soils. However, the physicochemical mech- anism of this distribution of colloidal particles in the humus gel matrix was not clear. The aim of this work was to elucidate the mecha- nism of fractal organization in soil colloids. SOIL PHYSICS Physicochemical Principles of the Fractal Organization of Soil Colloids G. N. Fedotov a , G. V. Dobrovol’skii b , V. I. Putlyaev c , E. I. Pakhomov a , A. I. Kuklin d , and A. Kh. Islamov 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 Faculty of Chemistry, Moscow State University, Leninskie gory, Moscow, 119992 Russia d Joint Institute for Nuclear Research, Dubna, Moscow oblast, 141980 Russia Received November 13, 2006; in final form, December 20, 2006 Abstract—An electron-microscopic study was conducted of gel films collected from aggregates from humus- accumulative horizons of chernozem and soddy-podzolic soils. The aggregates were dried and then capillary- wetted and immersed in water, Solutions obtained by pressing from these soils were also studied. Based on the results obtained, a hypothetical mechanism was proposed for the development of fractal organization of soil colloids, which involves the fixation of micron-size mineral particles in the humus gel and their transformation under the effect of aggressive substances with the formation of colloidal particles of reaction products diffusing in the humus gel. Humus macromolecules contain many polar groups; therefore, the colloidal particles pass some distance and are then fixed on these groups. The greater the distance from a coarse particle in the center of a cluster the smaller the number of colloidal particles capable of traversing it. Therefore, the concentration of colloidal particles decreases when going from the cluster center to its periphery according to an exponential law, which results in the development of the fractal organization in the colloidal soil component. Results of soil studies using the small-angle neutron scattering method were analyzed in terms of the hypothesis proposed. DOI: 10.1134/S1064229307070058