LAND CLEARING AND USE IN THE HUMID NIGERIAN TROPICS .2. SOIL CHEMICAL-PROPERTIES

A 4-yr study was conducted to investigate the effects of land clearing and subsequent land use on chemical properties of an Ultisol at Okomu, southern Nigeria. After 1 yr of cropping, soil pH, available P, and exchangeable Ca, Mg and K in the topsoil (0-10 cm) were significantly higher in the window zones of the shear-blade-cleared plots than in the nonwindrow zones, manually cleared plots, or forested control. In manual clearing, the organic-C and total-N contents declined below that of the forested control with 3 yr of cropping. Soil pH after 4 yrs and exchangeable K after 2 and 4 yr of cropping were significantly higher in the pasture system than in the other systems. In a given system, pH, organic C, and available P decreased with cropping time. Total N decreased with cropping time in all systems but pasture where there was a little increase. The levels of exchangeable Ca+2 and Mg+2 also decreased with cropping duration in all systems except the alley cropping in which their concentrations increased. The K+ content decreased with time in all systems but the pasture in which its level remained unchanged owing to returning of the hay to the field after cutting.