A comparison of selected heavy metals in soils mixed with domestic and industrial sludges and assessment of effects of the sludge pollutants on oxidative stress markers of the African kale (Brassica oleracea var acephala) grown using sewage sludge manure

Sewage sludge is used as fertiliser and contains nutrients required for plant growth. It also contains contaminants that can leach into crops. The effects of sewage sludge on kale plants were investigated. Mixture ratios of 50:50 and 20:80 for soil and sludge were prepared. The soil-sludge blends were analysed for selected metal residues. Kale seedlings of 10-13 cm were planted on the different soil-sludge combinations. Leaf lengths were measured 21 days post transplanting for four weeks. After sixty days, the leaves were analysed for metal residue levels and antioxidant enzyme activities. The results showed higher metal concentrations in soil blended with industrial sludge than in soil mixed with domestic sludge. The highest growth of plants was observed after 28 days in plants grown on 50% soil-industrial sludge mixtures. Superoxide dismutase and glutathione peroxidase activities were higher in plants grown on soil applied with sewage sludge when compared to enzyme activities in plants grown on sludge-free soil. The high levels of metals and enhanced antioxidant enzyme activity observed were attributed to the contaminants in the sewage sludge. Preassessment of sewage sludge to be used as soil manure is recommended to safeguard the health of plants and, indirectly, humans who consume the crops. were added to 0.5% thiobarbituric acid made in 20% trichloroacetic acid, and the mixtures were heated at 95°C for 30 min. After rapid cooling on ice, the mixtures were centrifuged at 10,000 × g for 10 min. The absorbance of each sample was measured at 532 nm.