Abstract
Poor and variable crop responses to fertilizer applications constitute a production risk and may pose a barrier to fertilizer adoption in sub-Saharan Africa (SSA). Attempts to measure response variability and quantify the prevalence of non-response empirically are complicated by the fact that data from on-farm fertilizer trials generally include diverse nutrients and do not include on-site replications. The first aspect limits the extent to which different studies can be combined and compared, while the second does not allow to distinguish actual field-level response variability from experimental error and other residual variations. In this study, we assembled datasets from 41 on-farm fertilizer response trials on cereals and legumes across 11 countries, representing different nutrient applications, to assess response variability and quantify the frequency of occurrence of non-response to fertilizers. Using two approaches to account for residual variation, we estimated non-response, defined here as a zero agronomic response to fertilizer in a given year, to be relatively rare, affecting 0–1 and 7–16% of fields on average for cereals and legumes respectively. The magnitude of response could not be explained by climatic and selected topsoil variables, suggesting that much of the observed variation may relate to unpredictable seasonal and/or local conditions. This implies that, despite demonstrable spatial bias in our sample of trials, the estimated proportion of non-response may be representative for other agro-ecologies across SSA. Under the latter assumption, we estimated that roughly 260,000 ha of cereals and 3,240,000 ha of legumes could be expected to be non-responsive in any particular year.
Highlights
Low agricultural productivity, recurrent food shortages and high prevalence of food insecurity in subSaharan Africa (SSA) have led to repeated calls to intensify agriculture, with a particular focus on addressing the widespread soil fertility depletion in agricultural lands (UN Millennium project 2005; Sanchez 2010; Shapouri et al 2010; Andriesse and Giller 2015; Binswanger-Mkhize and Savastano 2017)
While there is a diverse literature reporting on response variability observed in on-farm trials performed at different spatial scales across SSA (Tittonel et al 2007; Kihara et al 2016; Zingore et al 2007; Ronner et al 2016; Njoroge et al 2017; Ichami et al 2019; Roobroeck et al 2021; Garba et al 2018; Wortmann et al 2017), few quantify the proportion of fields that fail to show an appreciable response in a given year
Variations in the absolute response for different studies are shown for cereals and legumes in Fig. 1, and Supplement Figures S1 and S2
Summary
Recurrent food shortages and high prevalence of food insecurity in subSaharan Africa (SSA) have led to repeated calls to intensify agriculture, with a particular focus on addressing the widespread soil fertility depletion in agricultural lands (UN Millennium project 2005; Sanchez 2010; Shapouri et al 2010; Andriesse and Giller 2015; Binswanger-Mkhize and Savastano 2017). While the accessibility to fertilizers remains a main constraint to the widespread use of fertilizers by smallholder farmers, the production risk associated with poor crop responses caused by variable weather conditions (Mafongoya et al 2007) and/ or by local edaphic constraints (e.g. limited soil rootable zone or water holding capacity and soil organic matter) i.e. the so-called non-responsive soils (Vanlauwe et al 2010), could discourage farmers to invest in fertilizers (Holden 2018; Schut and Giller 2020). Determining the rate of incidence of non-response to fertilizer is needed to understand the magnitude of the problem, and this requires on-farm observations on the variability in yield responses. Quantifying inadequate yield response in a dataset on fertilizer
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