Relationships between soil particle size fractions and infiltrability

Medinski T.V. ; Mills A.J. ; Fey M.V. (2009)


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.

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