Towards a model for soil structural behavior

Abstract

Clay particle interaction as mediated by water is considered within the context of attractive and repulsive inter-particle forces-van der Waals forces, ion correlation forces responsible for the potential minimum between Ca-clay particles, cation hydration, ion induced structural forces and diffuse double layer forces especially in relation to the threshold concentration concept. In relation to diffuse double layers, the surface potentials, as determined by co-ion exclusion, are shown to be near constant with respect to concentration. Consideration is also given to the nature of the matrix within which the interparticle forces operate, that is the domain and quasicrystal entities. The particle separations within these entities are defined by the application of the Kelvin equation to the low temperature N2 desorption isotherms. The close agreement between the surface areas obtained from the application of the BET equation to the nitrogen adsorption isotherm and the areas obtained from the desorption isotherms leads to the concept of intrinsic failure; that is the presence of pores external to domains and quasicrystals which permit ready access to the total surface area. The role of electrolyte concentration in controlling the permeability of sodic soils (the threshold concentration concept) is reviewed and discussed with respect to recent studies on the behaviour of red-brown earth soils. At a macroscopic level some consideration is given to the role of oxides and organic matter in stabilizing soil aggregates against slaking. The interaction of rainfall, structure and wheat yield for red-brown earth soils is discussed in terms of a critical rate of wetting which causes incipient failure. In relation to incipient failure the hypothesis is advanced, that the role of organic mater in protecting against slaking, is in strengthening coarse pores; it appears that the pores involved are those with a high probability of continuity and which are responsible for the rapid attainment of field capacity.