Studies on crusting in a kaolinitic soil from Zimbabwe and the effect of different conditioners

Abstract

Raindrop impact energy and c1ay dispersion are known to promote crust formation in many soils. Appropriate management practices and/or application of high ionic strength electrolytes or polycations have been suggested to reduce soil susceptibility to crusting. On this basis, the effects of gypsum, basic Al polychlorosulfate, and iron sulphate on a Rhodic-Kandiustalf soil from Zimbabwe were studied, on small runoff plots. Other plots were prepared from untreated soil (control), mulched surface and material from the Bt horizon replacing the top 15 cm of the Ap horizon . A11 conditioners had similar effects in reducing runoff in comparison with the control (15% and 30% respectively for storms ofmoderate intensity). Runofffrom all conditioner plots increased with time and intensity of rainfall approaching the control values . Bt material significantly reduced runoff in comparison to the conditioners. The effectiveness of mulching (constant runoff about values of 5%) emphasises the importance of raindrop impact on the disintegration of soil aggregates. Laboratory investigations suggested that c1ay content, formation of phosphate-metal complexes, and presence of low molecular weight humic substances also contributed substantially to crust formation.