Yield, nutrient and heavy metal uptake of leafy vegetables grown in sewage sludge and poultry manure amended soils

Abstract

Urban farmers in Harare grow vegetables in soils fertilised with poultry manure (PM) and sewage sludge (SS). Feed and storage management influence nutrient supply of these organic amendments. Nitrogen mineralisation of PM and SS were determined in a non-leaching and aerobic incubation experiment. Effects of these amendments on yield, nitrogen (N) and heavy metal uptake by vegetables grown in soils treated with 150 kg N ha⁻¹ from compound mineral fertiliser (7 N:14 P₂O₅:7 K₂O), PM (2.26% N) and SS (3.26% N) were studied. Phosphorus (P) and potassium (K) were added at rates of 50 kg P ha⁻¹ and 60 kg K ha⁻¹, respectively. A second crop was grown without adding amendments. Poultry manure mineralised faster than SS. Yield was significantly higher (p < 0.05) in mineral fertiliser amended soil for the first crop whilst organic amendments resulted in significantly higher yield in the second crop. First-crop nitrogen uptake increased by 53% and 100% (Brassica napus), 92% and 158% (Brassica juncea) over the control for SS and PM, respectively. Zinc, copper, cadmium and nickel uptake was higher with SS than in the other treatments and their concentrations were lower than European Union permissible limits. Poultry manure can be used in place of mineral fertiliser, whilst SS requires early or supplementary application of mineral N for early plant growth.

Keywords:

• Brassica
• biomass yield
• heavy metal uptake
• N mineralisation
• organic amendments