ANALYSIS OF SOIL TEXTURAL POROSITY RELATIVE TO SKELETON PARTICLE-SIZE, USING MERCURY POROSIMETRY

Soil textural porosity represents the pore space due to packing of soil particles. The characteristics of this pore space are affected by particle-size distribution. Binary mixtures of clay and skeleton (<2-mum and coarser particles, respectively) were studied to evaluate whether the textural-space division into clay-fabric pore space and its complementary space, lacunar pore space, could be modified by decreasing skeleton size. Skeleton particles with diameters of 400 mum (glass beads), 20, 13,6, and 4 mum (silt) were mixed in the wet state with various clay contents (5-90%), then dried. Mercury porosimetry was used to characterize textural pore space by volumetric pore density distribution curves (VPD) and volumes intruded into small pores of diameter <0.05 mum (V(SP)). For clay-bead (400-mum) mixtures, bimodal VPD curves occurred for all clay-content levels, corresponding to distinct intrusion of either lacunar or clay-fabric pores. Values of V(SP), however, were only related to intrusion into clay-fabric pore space. In clay-silt mixtures, unimodal VPD curves occurred, and V(SP) values were greater, when the clay content was >60 to 70%. Similarly, V(SP) values and the unimodal VPD curve were both explained by hidden lacunar pores, i.e., lacunar pores introduced only via clay-fabric pores. Clay-fabric pore space and lacunar pore space were always present in clay-silt mixtures, but the proportion of hidden lacunar pore space increased with clay content and at a greater rate the smaller the particle size.