The effects of biochar and sewage sludge application on spinach (Spinacia oleracea L.) yield and soil NO3- content were investigated in typical soils of Botswana (Luvisol, Cambisol) under field conditions. Ten treatments with 3 levels of biochar (0, 2.5, 5 tons ha-1) and sewage sludge (0, 6, 12 ton ha-1) were applied in 2 subsequent seasons. Significant (p < 0.05) yield increase on the Luvisol occurred if sewage sludge was added at 12 Mg ha-1 with or without biochar. A combination of 6 Mg ha-1 sludge and 5 Mg ha-1 biochar application resulted in the highest crop yield over 2 seasons. On the Cambisol, only marginal yield increase occurred upon high rates of sole organic amendments and chemical fertilizer, while co-applications decreased yields. Decrease in soil NO3- content caused yield declines in the second season, while P uptake increased significantly (p < 0.05). Correlations between yields, soil NO3- and leaf N contents were insignificant (p > 0.05). On the Cambisol, a significant regression model for sludge and soil NO3- was determined. Therefore, one – time combined application of 6 Mg ha-1 sewage sludge and 5 Mg ha-1 on the Luvisol, and 12 Mg ha-1 sewage sludge are recommended for spinach production on the Luvisol and Cambisol, respectively. In subsequent seasons, crop productivity could be maintained by application of mineral N in order to mitigate over-application of P.
Key words: Biochar, sewage sludge, soil NO3-, luvisol, cambisol.