article · Environmental Health Insights
A field trial in Zomba, Malawi, compared the impacts of bio-slurry, chemical fertiliser, and an unamended control on soil properties and heavy metal accumulation in maize, rape, and Chinese cabbage. Application of bio-slurry significantly enhanced soil organic carbon, organic matter, and exchangeable bases, including calcium, sodium, magnesium, and potassium. In contrast, chemical fertiliser and the control resulted in a notable drop in soil pH. Across all treatments, heavy metal concentrations in both soil and harvested crops stayed well beneath international permissible safety thresholds. Iron was the most abundant metal recorded, whereas cadmium was the least abundant. Despite these chemical and agronomic benefits, the raw bio-slurry exceeded safety limits for coliforms, moulds, and mesophilic aerobic bacteria. This microbiological contamination presents a food-safety hazard, meaning bio-slurry requires sanitisation or proper hygienic handling before field use.
Organic soil amendments like bio-slurry can enhance soil quality without causing dangerous heavy metal buildup in staple foods and vegetables. However, using untreated bio-slurry introduces severe microbiological hazards to crops. Understanding these specific trade-offs helps agricultural practitioners and policymakers implement safe organic fertilisation methods that protect both soil productivity and public food safety.
The research supports the use of bio-slurry as an organic fertiliser alternative to chemical inputs for vegetable and grain growers. Because the raw slurry exceeded microbiological safety thresholds, commercial adoption requires downstream processing solutions, such as sanitisation or pre-treatment methods, alongside safe handling protocols. The findings represent applied field research demonstrating agricultural efficacy, though practical deployment remains dependent on establishing effective microbial decontamination systems.
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This study evaluated the effects of bio-slurry and chemical fertiliser application, relative to an unamended control, on soil physicochemical properties and the concentrations of selected heavy metals (Fe, Mn, Zn, Cu, Pb, Cd, Cr, and Hg) in soil and in maize ( Zea mays L. ), rape ( Brassica napus ), and Chinese cabbage ( Brassica pekinensis ) grown in Zomba, Malawi. A randomised complete block design with three treatments (100% bio-slurry, 100% chemical fertiliser, and an unamended control) and three replications was used. Soil samples were collected before planting and after harvest, and soil, bio-slurry, and crop samples were analysed for heavy metals by atomic absorption spectrometry. Bio-slurry significantly increased soil organic carbon, organic matter, and exchangeable bases (Ca, Na, Mg, K) relative to chemical fertiliser and the control, whereas chemical fertiliser and the control were both associated with a significant decline in soil pH. All measured heavy metal concentrations in soil and crops remained below FAO/WHO permissible limits, with iron (Fe) the most abundant and cadmium (Cd) the least abundant metal overall. However, the bio-slurry itself exceeded CODEX microbiological limits for mesophilic aerobic bacteria, moulds, and coliforms, indicating a microbiological food-safety concern that requires further treatment or hygienic handling before field application. The findings indicate that bio-slurry improves soil fertility more effectively than chemical fertiliser without increasing heavy-metal risk in the crops tested, but that its microbiological quality must be addressed before it can be recommended as an unqualified safe soil amendment in Malawi.
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DOI: 10.1177/11786302261484022
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