Is the role of building SOC in crop production overestimated in sub-Saharan Africa?

Dr Leonard Rusinamhodzi1

1CIMMYT, Kathmandu, Nepal

The importance of soil organic carbon (SOC) in mediating soil processes is not in doubt, but has the role to support crop productivity been overestimated under the low-input crop production systems of sub-Saharan Africa? Majority of crop production happens below 1% SOC in Africa yet a recent meta-analysis suggests that increases in crop yields plateaus at SOC concentration of 2% (Oldfield, Bradford et al. 2019). What is the most practical pathway for poorly resourced farmers managing degraded fields: (a) building SOC in the long-term to achieve 2% SOC or (b) short-term in-season integrated nutrient management?  This paper reviews literature on the relationship between the amount of SOC and crop productivity focusing on rain-fed crop production in sub-Saharan Africa (SSA). Given the suggested time period of up to 50 years needed to attain a new equilibrium in disturbed systems, focusing on building SOC entail higher costs, and SOC build-up is characterized by a hysteresis – outcome lags behind the pace of changes in the factors causing it. For example, to increase the total soil organic matter by just 1 percent through dairy manure applications, it will require a total application of 222 tons (or about 20 tons applied per year for 10 years). Majority of food producers in SSA cannot achieve this amount of manure, and also given the sandy soils SOC saturation characteristics– achieving such a high target of  SOC build-up is out of reach. Results in literature suggest that in-season nutrient management based on 4Rs (source, time, place and rate) with combined application of chemical and organic fertilizers, may lead to immediate improved crop responses by applying nutrients when they are needed to reduce losses than a deliberate focus on long-term SOC build-up.

Biography: Scientist within the Sustainable Intensification Program of CIMMYT based in Kathmandu, Nepal. Leonard Rusinamhodzi holds a PhD in Production Ecology and Resource Conservation from Wageningen University and Research Center, The Netherlands. His work involves advanced analyses to study the effects of sustainable intensification on system productivity, soil quality, water and nutrient use efficiency across east and southern Africa. Leonard has extensive experience spanning more than a decade in participatory research in smallholder farming systems, with a deep understanding of the complex barriers to improved crop productivity. He has published extensively (more than 30 high impact) in the field of agronomy and targeting sustainable intensification options to the biophysical and socio-economic conditions of smallholder farmers. Leonard is particularly interested in learning and managing science that develops and uses tools that combine social, economic and bio-physical aspects of farmers to improve productivity and reduce hunger for smallholder farmers in Africa and beyond.

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