ISSN 10642293, Eurasian Soil Science, 2015, Vol. 48, No. 7, pp. 735–741. © Pleiades Publishing, Ltd., 2015.

Original Russian Text © I.I. Sudnitsyn, 2015, published in Pochvovedenie, 2015, No. 7, pp. 843–850. 735

INTRODUCTION

The soil water regime can be optimized only on the basis of sufficiently exact prognostic calculation of water fluxes in soils, which requires knowledge of the geophysical properties of different particlesize frac tions of these soils, primarily the relationships of the soil water potential (or pressure) and hydraulic con ductivity with the soil water content, which are fre quently referred to (at the suggestion of Globus) as soil moisture characteristic curves (SMCC) [2, 8, 10]. The determination and analysis of SMCC are essential and urgent problems of soil hydrophysics, which have been elaborated over the last half of a century. Interest in these problems does not decrease so far [11–13, 21– 23, 25, 26, 28, 29, 31–33, 35–43, 45, 46, 48, 49]. The aim of this work was to reveal an essential component of the SMCC: the relationship between the soil water potential (pressure) and the water contents of different particlesize fractions.

OBJECTS AND METHODS

The solution of this problem requires adequate input information on the water content of soil parti clesize fractions at different levels of water pressure (potential). Such information is reported in the funda mental monograph by Rode Theory of Soil Moisture [9]. These are results of the precise studies of Kuron [34], one of the founders of soil physics, who is known for the high precision of his experiments. The data are presented as the content of water adsorbed by different particlesize fractions of light clayey brown soil from

Wegnersau (Lower Silesia, Poland) separated by sedi mentation at different relative water vapor pressures.

For the determination of relationship between the

SMCC and the size of elementary particles in different fractions, these data were subjected to mathematical analysis according to the following algorithm: (1) the values of water potential (J/g water) were first calcu lated from the data on the equilibrium relative water vapor pressure; (2) the natural logarithms of water potential modules and the pF values were then calcu lated; (3) the water contents of different soil particle size fractions at different levels of equilibrium relative water vapor pressure were calculated from the data of

Kuron ([34], cited from [9]).

The next essential stage of study was to find a rela tively compact (elementary) analytical mathematical expression (function) for the adequate description of

SMCC in the entire range of hygroscopic moisture, because such an elementary function is necessary to present the effect of the size of elementary soil parti cles on the SMCC in the generalized form. Different analytical expressions were earlier presented for the description of SMCC in the hygroscopic moisture range; however, each of them gives satisfactory results only within a certain part of this range [9, 22].

The search for an adequate analytical function describing the SMCC is usually performed by means of pedotransfer methods. There are several main groups of these methods [21]: (1) methods of physi cally based models; (2) pointregression methods; (3) functional parametric regression methods. The methods of physically based models are preferable because they derive the most representative functions (the SMCCs obtained by these methods for one soil

SOIL PHYSICS

Effect of the Size of Elementary Soil Particles on the Soil Moisture Characteristic Curve

I. I. Sudnitsyn

Faculty of Soil Science, Moscow State University, Moscow, 119991 Russia, email: iisud@mail.ru

Received May 30, 2013

Abstract—Statistical analysis of water vapor sorption by light clayey brown forest soil and its elementary par ticles of different diameters has revealed extremely close correlations and linear relationships between the logarithm of total soil water potential (pressure) and the water contents in the separated particlesize fractions (due to the hydration of exchangeable cations in the diffuse layer near the surface of soil solid phase), as well as between the water content of particlesize fractions and the logarithm of their diameter (due to the differ ences in the specific surface area and mineralogy of these particles).

Keywords: soil water content, soil water potential, soil water pressure, specific soil surface area, exchangeable bases, correlation, regression

DOI: 10.1134/S1064229315050117 736

EURASIAN SOIL SCIENCE Vol. 48 No. 7 2015

SUDNITSYN can be used for determining the SMCCs of other anal ogous soils). The complexity of the structural and functional properties of soils makes the creation of physically based models difficult; however, a happy example of such model is available for the SMCC within the hygroscopic moisture range [14–20]. This model is based on the formation of a diffuse layer of exchangeably adsorbed cations in the soil solution contacting with the surface of soil solid phase; these cations are retained at this surface (usually negatively charged) by the Coulomb forces of electrostatic attraction. According to the theory developed by

Gouy [30], “The distribution of ions in the solution at the solid phase surface is determined by two opposite effects. On the one hand, the thermal motion tends to uniformly distribute ions so that similar numbers of positive and negative ions are in each element of solu tion volume. On the other hand, because of the excess of similarly charged ions at the interface, the electro static forces originated from the solid phase surface act so that the elements of solution volume located near the interface contain an excess of oppositely charged ions. The solid phase surface attracts the oppositely charged ions and repulses the similarly charged ions.

The equilibrium established due to these two forces (thermal motion and electrostatics) is analogous to the equilibrium of gaseous molecules in the atmosphere under the impact of gravity. The excess of oppositely charged ions occurring at the surface decreases with the distance from the interface in accordance with the barometric law” [4].