Suitability of Morupule fly ash as amendment for two agricultural soils in Botswana

Abstract

Electricity generation in thermal power stations results in high fly ash production that, if not managed properly, will be a potential source of pollutants. In Botswana, the fly ash from Morupule Power Plant is being as an amendment for agricultural soils through a series of experiment that characterized and assessed fly ash deposits as liming material and nutrient source; assessed the effects of fly ash amendments applied at different rates in combination with the nutrient sources on the growth, yield, and quality of harvested spinach, rape, and corn successively grown in two representative soils as well as their effect on soil properties and heavy metal availability. Chemical characterization of Morupule FA showed to have an alkaline pH of 8.12 to 8.42, an EC of 2300 to 2370 mS/m, very low organic matter contents of 0.012%, and a liming potential equivalent to 98% CaCO3. It contained high concentrations of essential elements for plants such P (865 mg kg-1 ), K (1 676 mg kg-1 ), S (34 472 mg kg-1 ), Ca (119 230 mg kg-1 ), Cu (23 mg kg-1 ), Fe (36 222 mg kg-1 ), Mn (796 mg kg-1 ), Ni (66 mg kg-1 ), Zn (32 mg kg-1 ). It also contained some non essential elements for plants such as As (11 mg kg-1 ), Hg (8.6 mg kg-1 ), Ba (614 mg kg-1 ), Cr (159 mg kg-1 ), Sr (535 mg kg-1 ), Zr (374 mg kg-1 ), and Pb (51 mg kg-1 ). In Selebi Phikwe soil, the yield of spinach, rape, and corn was increased due to the addition of FA at the rate of 5 to 15%. These yields were enhanced by the addition of compost and NPK, or its combination, especially in treatments with 10% FA. For spinach, treatments with 10FA+comp, 10FA+NPK+Compost, and 15FA+compost had heavy metal concentrations below the maximum allowable limit (MAL) for human consumption; while for rape, only the treatment v Abstract of 10FA+compost had heavy metals concentrations below MAL. While the kernels harvested from 10FA+compost had heavy metal concentrations below the MAL. FA increased the pH, reduce heavy metal availability, and increased the amount of available nutrients of Selebi-Phikwe soil. In Tswapong soil, the highest dry matter yield of spinach and rape was obtained in 10FA+NPK+compost, and in 5%FA+NPK+compost for corn. Among the different treatments, spinach and rape grown in 10%FA+NPK alone or combination with compost; and corn kernels from treatment with 15%FA + Compost alone or combination with NPK had low heavy metal concentration, which are below the MAL. FA amendments increased the pH up to 8.0 and modify the heavy metal and nutrient availability of neutral Tswapong soil. Overall, the application of FA from 5% to 15% enhanced the growth of spinach, rape, and corn grown in Selebi Phikwe and Tswapong soils. For this study, the optimum FA rate was 10% FA which gave the highest dry matter yields for the series of crops grown in Selebi Phikwe and Tswapong soils with minimum heavy metal accumulation in harvestable parts of the crops.