Soil physical properties were measured on field runoff plots established on a tropical Alfisol in Western Nigeria. Evolution of soil physical properties was assessed over a period of 6 years beginning in 1982 (when soil was cleared offits secondary regrowth) till 1987. Changes in soil physical properties were measured for six systems including plow-till, no-till, contour hedgerows of Leueaena leueoeephala established 2-and 4-m apart, and contour hedgerows of Gliricidia sepium established 2-and 4-m apart. Soil physical properties were measured once every year during the dry season following the harvest of second season crops.
Over the &year period, there were no significant differences in relative contents of textural separates of sand, silt and clay for the surface 0-5 and 5-10 cm layers. The gravel concentration of the surface 0-5 and 5-10 cm layers, however, increased significantly due to plowing and mixing of the surface and subsoil layers. Soil bulk density of 0-5 and 5-I0cm layers, respectively, increased in all treatments from initial values of 1.02 and 1.16gcm 3 in 1982 to 1.43 and 1.65 g cm -3 at the end of cropping cycle in 1986. The maximum increase in soil bulk density was observed for the no-till treatment. Accordingly, there was an increase in penetration resistance of the surface 0-5cm layer from an average value of 25.3kPa in 1982 to 210.7 kPa in 1986. The highest penetration resistance (353 kPa) of 5-10cm layer was recorded for the no-till treatment. In accord with total porosity, the gravimetric soil moisture retention at zero suction was the lowest for the no-till and the highest for a Gliricidia-based system. There were significant improvements in available water capacity (AWC) of the soil by both Leucaena and Gliricidia-based systems. In comparison with the no-till system, increase in AWC by Leucaenaand Gliricidia-based systems, respectively, was 42 and 56 percent by weight for 0-5cm depth and 12 and 58 percent by weight for 5-10cm depth. Alterations in pV curves by agroforestry-based systems were attributed to improvements in soil structure and structural porosity.