An evaluation of the effectiveness of coal ash as an amendment for acid soils

[摘要] Soil acidity is one of the greatest limitations to crop production in most soils of theworld. The increasing high costs of conventional liming materials have made itnecessary to explore the possibilities of using cheaper substitutes. In South Africa, 16million hectares of land are naturally acid while on the other hand, the country's coalfiredpower plants generate 28 million tons of mostly alkaline coal ash per year,disposal of which is increasingly becoming difficult. The use of coal ash as anagricultural soil amendment while solving the liming needs of local farmers, may alsopresent a safe and more economical disposal option. This study was carried out toevaluate the effectiveness of coal ash as an agricultural liming material. A greenhouseexperiment was conducted using maize as test crop. A field experiment was alsoestablished on Beestepan Farm in Middelburg, Mpumalanga Province using dry beansas test crop for the first season. In both experiments, fresh unweathered coal ash fromDuvha power station (CCE 10%), dolomitic lime (CCE 77%) and calmasil (calciumsilicate slag, CCE 99%) were applied to acidic sandy loam soils in the presence orabsence of gypsum. Both calmasil and dolomitic lime were applied at equivalent ratesof 0, 1, 2, and 4 tons/ha, and rates of 0, 7, 14 and 28 tons/ha were used for ash.Gypsum was applied at a rate of 4 tons/ha. All treatments were applied in threereplications. Results showed that liming increased soil pH, improved soil nutrientstatus and plant uptake of base cations, and enhanced yield. In the greenhouse, coalash decreased exchangeable acidity from 13.0 mmolc/kg to 6.67 mmolc/kg, increasedCa levels from 200 mg/kg to 379 mg/kg, and increased Mg levels from 25.9 mg/kg to42.0 mg/kg. Nitrate levels were also raised from 4.4 mg/kg to 14.8 mg/kghypothetically as a result of the increase in the activity of nitrifying bacteria followinga decrease in soil acidity after ash application. Maize yield in the greenhouse was notsignificantly affected by ash or by other liming materials, and the sufficient wateringand consequent elimination of aluminium-induced drought stress is put forward ashaving masked crop responses to acidity. In the field, coal ash reduced exchangeableacidity from 10.0 mmolc/kg to 5.88 mmolc/kg, increased Ca levels from 71 mg/kg to132 mg/kg, and increased Mg levels from 7.3 mg/kg to 17 mg/kg. The increase inbean yield from 958 kg/ha to 1724 kg/ha by ash was similar to that realized bydolomitic lime and calmasil. Gypsum had little effect on soil acidity, but itsubstantially improved soil Ca and sulfate levels, and enhanced bean yield in the field experiment. The study demonstrated that coal ash could be effective as a limingmaterial, and underscores the need for a cost-benefit assessment of ash usenecessitated by the relatively higher rates of ash required to obtain significant soil andplant responses.

[发布机构] Stellenbosch University