Abstract
Use of mineral fertilizers is essential to enhance crop productivity in smallholder farming systems of Sub-Saharan Africa, but various studies have reported ‘non-responsiveness’ where application of inorganic fertilizers does not lead to satisfactory yield gains. This phenomenon is not well defined nor are its extent and causes well understood. In order to close these knowledge gaps, we assessed the effects of commonly recommended nitrogen (N), phosphorus (P) and/or potassium (K) fertilizer inputs on maize grain and soybean production on farmer fields across prevalent land slope and/or soil texture gradients (2 × 2 matrix) in four agroecosystems over two growing seasons. The extent of the problem in the two cropping systems was compared by decomposing frequency distributions into various ranges of fertilizer effect sizes that represent specific degrees of non-responsiveness and responsiveness. Key soil properties and rainfall variables for field trials were also determined to identify the factors that are limiting crop yield increases by mineral fertilizer input.Significant differences were found in mean fertilizer effect on crop productivity and frequency of non-responsiveness among the study areas and growing seasons, with some explicit contrasts between maize and soybean. The application of mineral fertilizers failed to increase maize yields by more than 0.5 t ha−1 in up to 68 % of farmer fields and soybean yields by more than 150 kg ha−1 in up to 65 % of farmer fields for specific study areas and/or growing seasons, while for others crop responses exceeded those levels. Unlike hypothesized, there were no consistent differences in crop fertilizer responses between the soil texture and land slope classes at any of the study sites. The variation in fertilizer effects on maize grain productivity across the study areas and growing seasons was most strongly related to the soil silt and clay content, and exchangeable cation balances of calcium (Ca), magnesium (Mg) and K, whereas fertilizer effects on soybean were most strongly influenced by the evenness in rainfall during growing seasons, and the soil silt content, extractable P, and ratio of total C and total N. Findings from our study emphasize that non-responsiveness by maize and soybean crops in African smallholder agroecosystems is dependent on multiple interacting factors, and requires careful scrutiny to ensure returns on investments.
Highlights
Enhancing the usage of mineral fertilizer by farmers in Sub-Sahara Africa (SSA) is paramount to intensify crop production and overcome food insecurity (Sanchez, 2010; Vanlauwe et al, 2014; Andriesse and Giller, 2015)
Our analysis indicates that the larger yield increases by PK fertilization in DRCongo compared to Kenya and Nigeria were related to the higher Total carbon (TC):total nitrogen (TN) ratio of soils in DRCongo
Based on our findings we argue that the possible influence of TC:TN on fertilizer uptake and responsiveness should be taken into ac count by future research
Summary
Enhancing the usage of mineral fertilizer by farmers in Sub-Sahara Africa (SSA) is paramount to intensify crop production and overcome food insecurity (Sanchez, 2010; Vanlauwe et al, 2014; Andriesse and Giller, 2015). Modest application rates of such fertilizers have led to a doubling or tripling of crop yields beyond the baseline in some locations (Sanchez et al, 2007; Denning et al, 2009; Nziguheba et al, 2010) These and other studies found major variation in maize yield increases by NPK fertilizer at field, farm and regional level, including little or no response (Tittonell et al, 2005a, 2005b, 2010; Vanlauwe and Giller, 2006; Wopereis et al, 2006; Zingore et al, 2007a, 2007b; Sileshi et al, 2010). Few studies have systematically investigated the occurrence of fertilizer non-responsiveness in maize and soybean cropping systems across SSA, and even fewer the biogeochemical factors that cause it (Kihara et al, 2015, 2016; Ronner et al, 2016; Njoroge et al, 2018; Shehu et al, 2018; Ichami et al, 2019)
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