A low input approach to vegetation establishment on mine and coal ash wastes in semi-arid regions. II. Lagooned pulverized fuel ash in Zimbabwe.
A low input approach to the establishment of vegetation on mine and coal ash wastes is illustrated by application to a pulverized fuel ash (PFA) dam in a hot semi-arid part of Zimbabwe. Chemical analysis indicated that salinity, very high pH, moderately high concentrations of boron, and moderately low levels of K and Mg may limit plant growth on the PFA. Glasshouse trials demonstrated that N was extremely deficient, but that other essential nutrients were present in non-limiting amounts. Plants grown on PFA, with N fertilizer application, grew less than on a soil given similar fertilizer applications, further suggesting the presence of toxic factors. Comparison of numerous species on PFA receiving fertilizer N with those on soil resulted in relative shoot growth ranging from 0 to 56%. In general, herbaceous legumes were more sensitive to the PFA than were grasses or trees. Field trials confirmed N deficiencies observed in pot trials, but to a lesser degree. Covering the PFA dam wall with a thin layer of fire-clay subsoil led to the growth of an abundance of aggressive annual weeds which out-competed many planted species. These weeds were not suitable for stabilizing the dam walls since their roots were confined to the fire-clay and because they died annually. The indigenous leguminous tree Acacia gerrardii was the best performing species on the site probably because, apart from being N-fixing, it was well adapted to the PFA (its roots, therefore, could penetrate the deeper moist layers) and because its tall perennial growth habit allowed it to out-compete annual weeds. Moderately PFA-tolerant stoloniferous perennial grasses, preferred by engineers for dam stabilization and erosion control, were successfully established on the site using a combination of pre-plant herbicide and nitrogen application. The low input approach did not work as well on the PFA as in concurrent experiments on tin mine tailings because of the greater toxicity problems and constraints imposed by the active status of the lagoon. In the longer term, after the lagoon is no longer active and weathering can reduce the toxicities, it is envisaged that this approach may yet lead to the successful establishment of a self-sustaining ecosystem.