Thermodynamic Concept of Water Retention and Physical Quality of the Soil

Andrey V Smagin

doi:10.3390/agronomy11091686

Abstract

The physical quality of the soil is determined by its interfacial interactions in conditions of variable water content. In this regard, water retention characteristics in the form of functions of water content and its thermodynamic potentials are used as indicators of physical quality and its dynamics in the soil. The combination of centrifugation and thermodesorption methods allowed for the first time the assessment of soil water potentials in the entire range of variation from 0 to 106 J/kg for a representative database (more than 400 samples) of the main genetic types of Eurasian soils, grouped into 5 FAO/USDA soil texture classes. The main fundamental achievement of the research is a physically based diagnosis of the critical values of water content and its thermodynamic potentials that separate the areas of dominance of various forms of soil water, physical forces, and mechanisms of interfacial interactions on the WRC-diagrams of the physical quality of the soil. Theoretical and experimental results of the study are of practical interest of sustainable agronomy for determining the optimal ranges of water content in the soil during plant cultivation, water saving, and salt protection in irrigation, mechanical tillage, and other technological operations.

Highlights

Water retentive, rheological, and structural-mechanical properties are controlled mainly by the mechanism of disjoining pressure (Figure 2C) according to Deryagin [24,25] in a wide range of soil water content [8,34,43]. This range corresponds to the stable state of the particles of the colloidal-dispersed complex separated by water films. It is determined by the ratio of the surface molecular adhesion forces and the forces of ion-electrostatic and structural repulsion, which protect the particles from coagulation and preserve their high surface energy, which is spent on water retention [8]
The experimental data of the diagram are approximated by the empirical model of van-Genuchten [32] and the fundamental model of disjoining pressure (8), which extends to the linear range of the water retention curve (WRC) with the dominance of surface forces of water retention
On the vertical axis of diagram 4B, in addition to the absolute values of the matrix pressure of soil water, the critical pressures are plotted to estimate soil-hydrological parameters (MSCW or field water capacity (FW), maximum molecular water capacity (MMW), wilting point (WP), maximum adsorption water capacity (MAW)) from water retention curves. They are represented by a system of dashed lines in the form of average capillary rise lines (|P| = Hcap ) according to [8], Richards and Weaver’s [49] constants (|PFW | = 33.3 kPa; |PWP | = 1500 kPa), and Voronin’s secants, which increase depending on the dispersity of the solid phase [7]

Summary

Introduction

Most of the properties and processes that determine the physical quality of the soil depend on the interactions of the solid, liquid, and gas phases, as well as on changes in their ratio with variable water content in the soil. A more complex thermodynamic concept of the physical quality of the soil is based on the equality of the specific energies (potentials) of the interacting physical phases in the state of thermodynamic equilibrium and uses the water retention curve (WRC) to quantify interfacial interactions in soil with a variable water content (W) [4,6,7,8,9,10,11,12]. The WRC is the basis for computer models of energy-mass transfer in soil and, for predicting the dynamics of its physical quality [13,14]

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Conclusion