Relationships between soil particle size fractions and infiltrability
The influence of particle size fractions on infiltrability was investigated in soils sampled across Namibia and western South Africa. Infiltrability was determined using a laboratory technique calibrated with rainfall simulation, which measures the passage of a suspension of soil particles through a packed soil column. Water-dis-persible soil particle size fractions were determined using a high definition digital laser particle size analyser. Total (calgon-dispersed) particle size fractions were determined by hydrometer. Dispersion of soil particles resulting in crust formation on the soil surface appeared to be a main mechanism reducing infiltrability. Water-dispersible clay and fine silt determined by laser analyser showed higher correlation with infiltrability (r =-0.43 for clay and-0.47 for fine silt) than total clay and fine silt determined by hydrometer (r2 =-0.30 and-0.28, respectively). Clay, fine silt, coarse silt, very fine sand and fine sand fractions (<120 μm) showed a probable plasmic role in soil crusts. At a content of these fractions > 5% infiltrability was inevitably restrained. The 120-200 μm fraction showed no clear relationship with infiltrability. It played either a plasmic or skeletal role, depending on its ratio to the <120 μm and >200 μm fractions. Fine, medium and coarse sand fractions (>200 μm) showed a probable skeletal role in soil crusts, I.e. forming pores that enhanced infiltrability. At levels >50% of these fractions, infiltrability was potentially maximal. This potentially maximal infiltrability was also explained by the concomitant decrease in plasmic fraction content with an increase of the skeletal fraction.