Nutrient cycling by Acacia erioloba (syn. Acacia giraffae) in smallholder agroforestry practices of a semi-arid environment in the North West Province, South Africa

Abstract

Acacia erioloba is an ecologically important indigenous leguminous tree in semi-arid areas of Southern Africa because of the many benefits it offers to local communities. However, little quantitative plant and soil data exist to explain its ability to enhance soil fertility under local conditions. Paired soil samples taken under and beyond A. erioloba tree canopies were analyzed to quantify the concentration of nutrients in two local agroforestry practices defined as trees dispersed on cultivated and grazing lands. In both practices, there was a significantly higher (p<0.05) concentration of N, P, Ca, Mg, Zn and Mn in soils collected from under A. erioloba canopies compared with those collected beyond the canopies. The nutrient concentrations were consistently higher in soil from trees that were located in grazing land than croplands. An assay of soil nutrient availability was done using wheat as a bioassay plant in pots and showed that plant height, dry matter yields and the concentrations of N, P, Ca, Mg, and Zn in the tissues of wheat were related to soil nutrient concentrations. This was confirmed by the significant correlation coefficients between soil and plant tissue nutrients. A combination of leaf-fall, continuous presence of domestic livestock near the trees and increased soil moisture were considered to have contributed to the enhanced plant nutrient concentration, plant growth and dry matter yields of wheat grown in soil from underneath the canopies of A. erioloba trees. The perspective that a local indigenous tree can contribute to improved fertility of soils in both cultivated and grazing lands is valuable as it can be used to increase the productivity of crops and grass for grazing animals in smallholder agricultural systems. It is recommended that the integration of A. erioloba in existing farming systems should be intensified in order to enhance sustainability.

Keywords: Acacia erioloba, agroforestry, nutrient cycling, smallholder, tree canopy

Southern African Forestry Journal Vol. 208, 2006: 23-30