The effect of Acacia albida on tin-mine spoil and their possible use in reclamation

Abstract

From a number of studies in Africa, Acacia albida has been shown to have some extremely beneficial effects on the soil lying within the confines of its canopy shadow. On the Jos Plateau of Nigeria, Acacia albida is a natural colonizer of spoil mounds created as a result of opencast tin mining. Observation of the ground vegetation growing in the vicinity of a number of isolated Acacia albida indicated that the ground vegetation growing below the canopy was taller, more lush and a darker green than that growing outside the influence of the canopy, thus suggesting that the beneficial effect of Acacia albida on soils reported from elsewhere in Africa was operating at these sites. Analyses of soil samples collected from sites beneath and outside the canopy of a single tree confirmed this tentative hypothesis. Organic carbon, total nitrogen, pH, percentage base saturation and exchangeable calcium, magnesium, sodium and potassium were all significantly higher in soils from beneath the canopy than in those from outside. The soil beneath the tree also had a better-developed and stronger structure and a more active population of soil fauna. Having established that the Acacia albida did indeed have a beneficial effect on the spoil soils, the study was extended to compare the effect of Acacia albida on the spoil soil with that of the eucalypt plantations established on areas of reclaimed mine land. The sites under Acacia albida were found to have significantly higher levels of exchangeable calcium, pH and base saturation than those under the eucalypts. This difference is of some importance, since it has recently been shown that the eucalypt plantations are causing a gradual degradation of the soils rather than the amelioration hoped for by the unit that established the plantation policy in the 1960s. Having established that Acacia albida has a more beneficial effect on the spoil soils than the eucalypts, consideration is given to the possibility of using the native Acacia albida as an alternative to the exotic eucalypts. It is concluded, however, that a more detailed study is required into the effect that Acacia albida has on the spoil soils before any firm recommendation can be made, particularly as the eucalypts have much greater economic value than Acacia.