The effectiveness of coal fly ash in the amelioration of acid soils of the South African highveld: a comparison with conventional liming materials

Abstract

Soil acidity is one of the greatest limitations to crop production in South Africa, while the country’s coal-fired power plants generate millions of tonnes of alkaline coal ash per year. A field experiment was carried out to evaluate the liming potential of fly ash. Fly ash, dolomitic lime and Calmasil (a calcium silicate slag) were applied to acidic sandy loam soils in the presence or absence of gypsum. Based on the lime requirement of the soil and the Calcium Carbonate Equivalence (CCE) of the materials, both Calmasil and lime were applied at rates of 0, 1, 2, and 4 tonnes/ha, and rates of 0, 7, 14 and 28 tonnes/ha were used for ash. Gypsum was applied at a rate of 4 tonnes/ ha. Compared with Calmasil and lime, ash had the lowest effect on pH and soil acidity. However, ash caused a 48% reduction of topsoil acidity and its reduction of subsoil acidity was enhanced by gypsum. Ash (28 tonnes/ha) increased bean yield in season one from 958 kg/ha in the control plots to 1 724 kg/ha, and maize yield in season two from 5 569 to 7 553 kg/ha. These yields were similar to those derived from Calmasil and lime.