Integrated soil fertility management in sub-Saharan Africa: unravelling local adaptation

Abstract

Abstract. Intensification of smallholder agriculture in sub-Saharan Africa is necessary to address rural poverty and natural resource degradation. Integrated soil fertility management (ISFM) is a means to enhance crop productivity while maximizing the agronomic efficiency (AE) of applied inputs, and can thus contribute to sustainable intensification. ISFM consists of a set of best practices, preferably used in combination, including the use of appropriate germplasm, the appropriate use of fertilizer and of organic resources, and good agronomic practices. The large variability in soil fertility conditions within smallholder farms is also recognized within ISFM, including soils with constraints beyond those addressed by fertilizer and organic inputs. The variable biophysical environments that characterize smallholder farming systems have profound effects on crop productivity and AE, and targeted application of agro-inputs and management practices is necessary to enhance AE. Further, management decisions depend on the farmer's resource endowments and production objectives. In this paper we discuss the "local adaptation" component of ISFM and how this can be conceptualized within an ISFM framework, backstopped by analysis of AE at plot and farm level. At plot level, a set of four constraints to maximum AE is discussed in relation to "local adaptation": soil acidity, secondary nutrient and micronutrient (SMN) deficiencies, physical constraints, and drought stress. In each of these cases, examples are presented whereby amendments and/or practices addressing these have a significantly positive impact on fertilizer AE, including mechanistic principles underlying these effects. While the impact of such amendments and/or practices is easily understood for some practices (e.g. the application of SMNs where these are limiting), for others, more complex processes influence AE (e.g. water harvesting under varying rainfall conditions). At farm scale, adjusting fertilizer applications to within-farm soil fertility gradients has the potential to increase AE compared with blanket recommendations, in particular where fertility gradients are strong. In the final section, "local adaption" is discussed in relation to scale issues and decision support tools are evaluated as a means to create a better understanding of complexity at farm level and to communicate appropriate scenarios for allocating agro-inputs and management practices within heterogeneous farming environments.

Nutrients in the nexus

“…The nutrient use efficiency (NUE) of fertilizer applications on these depleted soils is quite low, yielding only 12 kg of grain per kg of N applied (Dorward et al 2008). Part of the low uptake by the crop is due to fertilizer recommendations that are not appropriate for the crop or soil system or other soil factors limiting crop growth, including possible soil compaction, micronutrient deficiencies, and the need to apply lime (Vanlauwe et al 2015). This low use efficiency means a waste of very limited funds paid by smaller holder farmers.…”

“…Only when these soils receive organic resource management to address the causes of their non-responsiveness to fertilizers, can yield be expected to increase in these soils, as indicated by the dotted line upward steps to the right of position C fertilizer and improved crop germplasm, such as seed sources and propagules for planting. Organic inputs, whether they are animal manures, green manures such as legume cover crops or trees, or composts, can increase NUE, water storage and availability in the soil, and build soil organic matter Vanlauwe et al 2015). Most agriculture in sub-Saharan Africa is rainfed.…”

“…After the initial boost of nutrients with mineral fertilizers that is needed to increase crop production in these nutrient depleted soils, a more holistic stepwise approach can be taken to improve NUE and crop productivity. This approach is called integrated soil fertility management or ISFM (Vanlauwe et al 2015;Fig. 6 Soils are classified either as "responsive soils" shown as the solid line path or "less responsive soils" depicted as the dotted line path.…”

Soil nutrient maps of Sub-Saharan Africa: assessment of soil nutrient content at 250 m spatial resolution using machine learning

“…7,8) that are 'naturally': (1) deficient, (2) adequate and (3) in excess relative to specific land-use requirements; and pair these with the nutrient-specific agronomic interventions required to achieve critical crop thresholds. Such usage could help optimize the use of soil resource and possibly (major) agronomic interventions across African countries (Vanlauwe et al 2014). These agronomic interventions could consist of: targeting degraded areas that are suitable for restoration projects, and/or targeting areas for agricultural intensification and investment by modeling crop suitability and yield gaps at the regional scale (Nijbroek and Andelman 2016), and/or assessing the nutrient gaps to predict fertilizer nutrient use efficiency.…”

Dilemma of nitrogen management for future food security in sub-Saharan Africa – a review

“…Soil acidification, poor organic matter content, deficiencies of various nutrients and reduced microbial activities are among factors affecting crop responses to applied N (Fairhurst 2012;Nezomba et al 2015). Adequate diagnosis of the factors limiting application of integrated soil fertility management (ISFM) is required to optimise AE N (Giller et al 2011) and increase the sustainability of agricultural intensification (Vanlauwe et al 2015).…”