Malawi: What Is the Role of Nitrogen In the “Green Belt” of Africa?.

Today, the African Green Revolution, as called for by former UN secretary general Kofi Annan in 2004, starting with an emphasis on overcoming soil nutrient depletion. Malawi presents a case of one country hit particularly hard by depleted soils—like many of its neighboring countries, it has faced widespread hunger in recent decades due to low crop production. Small holder farmers produce 80% of Malawi’s food, with farmlands primarily devoted to maize and with severe seasonal water limitations (unimodal rains less than 6 months a year). The major limiting nutrient there is nitrogen. To address N depletion, Malawi adopted a national fertilizer subsidy program in 2005, providing approximately 2/5 to 2/3 of rural households vouchers redeemable for two 50 kg bags of nitrogen fertilizers (NPK ratio: 23:21:0) and 3 - 5 kg of improved maize or legume seed at a discount price. National crop yield estimates increased from 0.8 to 2.0 tons/ha after the start of the subsidy.

At a well-studied field site, we have measured similar increases in maize yield with fertilization compared to areas where little or no fertilizer is not used. This study bridges the site-level and national-level crop yield estimates to understand, what are the spatial and temporal patterns between and within agroecological zones of increased yields and how do these relate to implementation of the national fertilizer subsidy program? Using coarse resolution remote sensing data (MODIS) with high temporal analysis of crop phenology, we estimate crop improvements or declines with spatially explicit, wall-to-wall detail from 2000 to 2010, covering pre- and post-subsidy. This works lends insights for attributing observed yield response to causative and confounding factors, such as the amount of fertilizer received, precipitation patterns, slope and natural soil fertility. Quantitative assessment of this type is needed to assess crop fertilizer responses, develop best regional management practices and consider future programs supporting adaption to climate change and ensuring food security.