The influence of uniaxial compression upon pore size distribution in bi-modal soils

Abstract

Transport of water, of dissolved and non-dissolved chemicals, of gas, the rooting of plants and the conditions for all soil biota are influenced by the soil porous system and by the soil pore size distribution. We have analyzed the change of the soil porous system due to the short time compression in ranges from 0 to 300 kPa in Entisols, Alfisols, and Vertisols. The procedure was based upon the evaluation of the soil water retention curves. The equation of log-normal pore size distribution in bi-modal soils was applied for the analysis. The obtained physical parameters describe the soil porous system and its change due to the compression. The minimum pressure head h A between two peaks of pore size distribution represents the boundary between the structural and matrix domains of the soil porous system. The value of h A was in broad ranges from h A = −136 up to h A = −585 cm. Pressure head corresponds to equivalent pore radius from r = 10.9 to r = 2.5 μm. The rate of change of h A due to the applied compression was not the same in all examined soils. h A decreased in some soil taxons and in other soils it slightly increased when the compression stress rose. It follows that the boundaries between the soil pore categories cannot be taken as a fixed value for all soils and for all compression stresses. The increase of the compression stress caused a decrease of the total porosity, but the decrease of the structural porosity was much more expressed. The ratio of the decrease of structural porosity due to compression was in ranges between 0.91 and 0.48. The change of the matrix porosity has not the same tendency in all soils. In majority of instances it was increasing, but in two soils it was slightly decreasing with the increase of compression stress. The characteristics of the pore size distribution did not change in a uniform way either. The pores size distribution curve is more flat and more broadly distributed due to compression in the structural domain. The tendency is opposite in the matrix domain. A generally valid rule on the change of the pore size distribution due to compression does not exist for all soil taxons. The change was great in both domains of soils with a low stability of aggregates. The change was relatively small in soils with a well-developed structure where it was more related to the structural domain. The log-normal model does not fit in all instances to pore size distribution in the structural domain. A profound study of the soil micromorphology may result in a better description of the soil porous system and its internal change during compression.