Effects of Newbouldia laevis on soil subjected to shifting cultivation in the Ibadan area, southwestern Nigeria

Newbouldia laevis is the most abundant tree in shifting cultivation farms in the Ibadan area of southwestern Nigeria, where shifting cultivation is undergoing intensification. The physical and chemical properties of the soil under and outside the canopies of N. laevis were compared in order to evaluate the effects of the tree on the soil. Thirty Newbouldia trees were studied in 10 plots that measured 25 x 25 m. There was no significant increase in organic matter, total nitrogen, exchangeable calcium, magnesium, potassium and cation exchange capacity and extractable manganese, iron, and zinc in the 0-10 cm layer of soil under the canopy compared to the soil outside the canopy. The minimal accretion of organic matter and nutrients in the soil under the canopy is mainly attributable to site burning prior to cultivation (which destroys plant litter) and to the prolonged cultivation of the farms. Available phosphorus, however, was significantly higher in the soil under the canopy of trees. This is presumably due to the mobilization of inorganic phosphorus from the subsoil and its subsequent recycling to the topsoil by the trees. Soil physical status was improved under the canopy. Soil bulk density was lower and total porosity higher under the canopy due the effects of the concentration of plant roots near the stem of the trees. Copyright (C) 2002 John Wiley Sons, Ltd.