Building critical SOC concentration as a major pathway for improving nutrient use efficiency in sub-Saharan Africa

Patrick Musinguzi; Peter Ebanyat; John Stephen Tenywa; Twaha Ali Basamba, Moses Makooma Tenywa

Department of Agricultural Production, Makerere University, Kampala, Uganda

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Building and maintenance critical Soil Organic Carbon (SOC) concentrations in tropical soils could be the greatest soil fertility challenge in sub-Saharan Africa (SSA). Measures that can boost SOC restoration to critical levels remain less understood. A study was conducted on a Ferralsol in sub-humid Uganda to explore the critical range of SOC concentrations for optimal response of maize to added N fertilizer. Computations were made to estimate the amount of carbon required for SOC restoration using the available organic C materials. Maize grain yield response to N rates was assessed with 0, 25, 50, and 100 kg N ha-1 in 30 fields of low fertility (SOC<1.2%), medium fertility (SOC=1.2-1.7%) and high fertility (SOC>1.7%). Non-linear regression models predicted 1.9-2.2% SOC as the critical concentration range for high yields. Theoretical projections suggest that high quantities of organic materials (19-65 t ha-1) are needed every year to build SOC to critical levels. Some organic materials can be potentially applied continuously 10 to 12 times in a year such as compost, bean-trash and mucuna pruriens, and as low as 2 times for biochar.  The projections demonstrated the difficulty in restoring SOC to optimal levels due to scarcity of materials especially among the resource constrained farmers in SSA.