Effects of gypsum on hydrological, mechanical and porosity properties of a kaolinitic crusting soil

Numerous papers have shown the effectiveness of gypsum addition in reducing crust formation on sodic or generally non-acid soils, while the effect on acid soils has been less frequently studied and is less known. The present paper reports the results of a series of field rainfall simulations on an acid red soil (Rhodic Kandiustalf) in Northeast Zimbabwe. In these experiments, gypsum effect on hydrological behaviour in subsequent rainstorms was verified, and the mechanical properties of the crusts so formed were assessed by a modified hand penetrometer. The results showed the ability of gypsum to improve overall infiltration, mostly by reducing the rate of infiltration decay in the non-steady portion of a Horton-type infiltration curve; such rate of infiltration decay was shown to be not constant in subsequent rainstorms, with a well defined increasing trend, that was, however, less marked for gypsum-treated plots. Effect on final infiltration rate was much less significant. Surface crusts formed on gypsum-treated plots were shown to be significantly harder than those formed on untreated plots. Both results could be explained by the observation that a substantial loss of clay from the soil surface took place in untreated plots, while this process was significantly reduced by gypsum treatment. This lesser clay depletion would be effective in slowing structural collapse during wetting, and was also observed to allow the regeneration of a better developed soil structure on drying, so further slowing soil response in subsequent rain-storms. The greater clay content in the surface layer could also induce a higher degree of cohesion, so explaining the greater surface crust strength induced by gypsum treatment.