Infiltrability in soils from south-western Africa: effects of texture, electrical conductivity and exchangeable sodium percentage

Abstract

The effects of particle size, electrical conductivity (EC) and exchangeable sodium percentage (ESP) on infiltra-bility were investigated in 35 soils from Namibia and western South Africa. Samples were grouped as follows: sand (5–9% clay + silt), loamy sand (16–20%) and silty loam (73–80%). Subgroups included: low EC and low ESP (LL); low EC and high ESP (LH) and high EC and ESP (HH). Infiltrability was measured using a rapid laboratory syringe method. To investigate dispersion and flocculation processes the effect of four mobile phases on infiltrability was examined. Mobile phases included distilled water (W), a gypsum solution (G), a 1:5 soil suspension in gypsum solution (GS), and a 1:5 soil suspension in water (WS). Infiltrability was highest in the sand group, and lowest in the silty loam group. Electrical conductivity and ESP effects varied between particle size groups. In the silty loam group, EC and ESP effects were not significant due to swelling of clay minerals and rapid blockage of soil pores with dispersed particles. In the sand group, soils with high ESP (22) had significantly lower (p<0.05) infiltrability than soils with low ESP (4) levels. The application of gypsum in these soils enhanced infiltrability and minimized the negative effect of high ESP. The ameliorating effect of gypsum was apparent in treatment GS, but not treatment G, which highlights the dominant effects of mechanical disturbance and clay dispersion on crust formation and infiltrability.