AGGREGATE STABILITY AND ASSESSMENT OF SOIL CRUSTABILITY AND ERODIBILITY .2. APPLICATION TO HUMIC LOAMY SOILS WITH VARIOUS ORGANIC-CARBON CONTENTS

Surface crusting results from aggregate breakdown under raindrop impac t. It reduces the infiltration rate and may induce erosion by increasi ng runoff, Soil crustability and erodibility generally increase as org anic carbon content decreases. Samples of topsoil were collected from fields cropped continuously for maize after land clearance at various dates. Organic carbon content ranged 4-30 g kg(-1). Aggregate stabilit y was assessed by measuring fragment size distribution Samples were al so subjected to simulated rainfall. The size of fragments forming the seal when the rain ceased and the infiltration rate during the rainfal l were measured to characterize seal structure and hydraulic propertie s, Development of the seal and infiltration capacity were related to a ggregate stability, which was itself a function of organic carbon cont ent. The relations between aggregate breakdown, crusting and infiltrat ion enabled us to predict soil physical behaviour from measurements of aggregate stability which are easier to make than direct measurements under rainfall. The proposed tests offer the opportunity to evaluate actual or potential soil physical degradation and erosion risks withou t extensive field measurements.