EVALUATION OF SOME PHYSICAL-PROPERTIES OF AN OXISOL AFTER CONVERSION OF NATIVE SAVANNA INTO LEGUME-BASED OR PURE GRASS PASTURES

This study evaluated the effects of improved tropical grass and grass- legume pastures on the physical condition of an oxisol, previously cov ered by native savanna. Two long-term pasture experiments with differe nt grass and legume species and with different stocking rates were use d. Measurements were made of bulk density, penetrometer resistance, wa ter retention characteristics, total porosity, pore-size distribution and water infiltration rate. The bulk density of the 0-5 cm and 6-11 c m layers was not affected by any of the treatments. Penetrometer resis tance was affected by presence of a legume and stocking rate, but diff erences were small and of little practical importance. There was no di fference in water retention curves between the pure grass and grass-le gume treatments. The decline in water content with increasing pF occur red more gradually in the low-stocking-rate treatments than in the hig h-stocking-rate treatments or the native savanna. Consequently, the am ount of plant-available water was smaller in the former treatments. Th e water infiltration rate was higher under grass-legume than under pur e grass pastures and decreased with increasing stocking rate under bot h types of pastures. Differences in soil physical properties are expla ined by differences in root systems between the two types of pastures, and by differences in biomass, composition and distribution of soil f auna, especially earthworms. The implications of this work for decreas ing soil erosion are stressed.