Fly Ash Amendments Reduced Heavy Metal Availability and Uptake in Leafy Vegetables Grown in Agricultural Soils From Selibe Phikwe, Botswana

Poor waste management of mining operations in Selibe Phikwe, Botswana, has resulted in elevated levels of heavy metals (HM) in farmlands and crops near the mining regions, contaminating the food chain and posing a health risk. To mitigate HM contamination in food crops grown in this area, different soil amendments were evaluated for their potential to reduce heavy metal uptake and accumulation in harvested crops. A two-factor complete randomized design was used to conduct a pot experiment in which varied rates of fly ash (FA) were applied to HM-contaminated soil in combination with compost and/or NPK inorganic fertilizers. On this setup, two successive cropping of rape and spinach crops were established. Changes in soil chemical characteristics and heavy metal availability, as well as yield and heavy metal content of harvested plants, were monitored. Our findings showed that the addition of FA from 5% to 15% (w/w) had significantly improved the yield, and reduced the concentration of arsenic (As), chromium (Cr), copper (Cu), nickel (Ni), manganese (Mn), and lead (Pb) in shoots for both spinach and rape. Additional application of compost and/or inorganic fertilizer further improved the yield but the optimum yield was obtained through the combination of 10% FA + compost for both spinach and rape. The highest reduction of As, Cr, Cu, Ni, Pb, and Mn in the shoots of spinach and rape was obtained in soils amended with 10% FA + compost + NPK fertilizer. Improvement of growth due to different amendments could be attributed to their ability to modify soil chemical properties, reduce available heavy metals in soils, and provide essential nutrients for plants. Optimum results based on growth and heavy metal mitigation can be attained by the application of 10% FA and the addition of compost + NPK fertilizer for both spinach and rape. Further verification and efficacy tests under field conditions are recommended.