Deformation and Shrinkage Effects on the Soil Water Release Characteristic

Abstract

The soil water release characteristic is controlled by the soil pores and so alteration of the pore system will have an effect. We sought to examine the effects of various pore deformations in a range of arable and grassland soils from the UK. Sieved topsoil materials were compressed to 50 or 200 kPa, or remolded to simulate shear deformation at the plastic limit. They were then subjected to matric potentials from 0 to -1500 kPa using conventional tension and pressure plate apparatus. Volume changes were monitored to assess shrinkage. Further samples in the compressed state were subjected to x-ray computed tomography scanning to nondestructively characterize the soil pore system. Increasing the compression from 50 to 200 kPa mainly affected the >30-μm pores, changing a dual porous system of inter- and intraaggregate pores to one mainly dominated by intraaggregate pores, as confirmed by gravimetric water release characteristic data and the scans. Shear-deformed soils retained more water than the compressed soil and shrank more, such that they remained tension saturated at low (negative) matric potentials. We developed a function to predict the soil saturation state as a function of matric potential and porosity. This explained 28 to 63% of the variance, irrespective of the initial structural state, but up to 94% of the variation when one of the fitted parameters was allowed to vary for the different initial states. This was sufficiently systematic to suggest that a general model of the effects of soil deformation and shrinkage on the water release characteristic may be possible.