PORE VOLUME CHANGES IN A STRUCTURED SILT-LOAM SOIL DURING DRYING

Shrinkage and volumetric water content of undisturbed soil cores 150 mm diam. × 100 mm high from four horizons of a silt-loam, Orthic Humic Gleysol were monitored throughout a drying cycle from saturation to oven dryness. Drying from saturation to − 100 kPa was by desorption apparatus. Subsequently the cores were subjected to controlled air drying and finally oven drying. The volume change, relative to the volume at saturation, of each core was monitored at 20 different water contents on the desorption curve, by measuring the X, Y and Z positions of 92 pins inserted in the top of each core. Total shrinkage values expressed as percentages of the cores' saturated volumes were Ap −23.4%, Bg −7.0%, BCg −11.8% and Cg −12.9%. The influence of soil structure on soil shrinkage was analyzed and the results were inconclusive because there appeared to be a complex interaction between ped size and the scale of measurement for shrinkage. Water loss during drying was partitioned between that replaced by air and that associated with pore volume reduction. For the Ap horizon from 0 to 1500 kPa, the proportions were 62% and 38% respectively. The results show clearly the dependence of soil bulk density and porosity on water potential and reinforce the contention that for comparison purposes bulk density and porosity must be expressed at a reference water content or potential. Key words: Shrinkage, pore volume, bulk density, undisturbed samples