Soil enrichment by Acacia mellifera subsp. detinens on nutrient poor sandy soil in a semi-arid southern African savanna

The study was conducted in a semi-arid savanna of South Africa, dominated by an almost pure stand of Acacia mellifera subsp. detinens (Black Thorn). Due to its invasive habits and suppression of the herbaceous layer, this woody species is viewed by land owners as a serious threat. Clearing of all A. mellifera trees in order to restore the production potential of the herbaceous layer is thus a well-established practice. In view of possible beneficial effects of A. mellifera, a study was undertaken with the objective of quantifying differences in soil nutrient status under A. mellifera canopies (canopied subhabitat) in comparison to the open areas (uncanopied subhabitat). The results of the study confirmed the existence of differences in the soil nutrient status between the various subhabitats, which occurred in a specific spatial gradient from the stem base of the plants towards the open, uncanopied areas. These differences were statistically significant (p<0.05) for total N, % organic matter and Ca, with the highest values recorded in the area surrounding the stem base. Though statistically non-significant (p>0.05) the P and K contents, and to a lesser extent Mg, were also higher under the tree canopies, while pH was lower. No differences (p>0.05) were established for Na. Some notable differences to similar studies in other areas were established, of which the reduction of soil pH is an example. The practical significance of these results in terms of land management practices is discussed, which will invariably necessitate a change in attitude of land owners in their approach to the management of A. mellifera.