Unravelling causes of poor crop response to applied N and P fertilizers on African soils

In most of sub-Saharan Africa (SSA), crop production is practiced on highly variable soils, which have different biophysical constraints (Jones et al., Reference Jones, Breuning-Madsen, Brossard, Dampha, Deckers, Dewitte, Hallett, Jones, Kilasara, Le Roux, Micheli, Montanarella, Spaargaren, Tahar, Thiombiano, Van Ranst, Yemefack and Zougmore2013). A growing number of studies have shown that crops respond favourably to nitrogen (N), phosphorus (P) and potassium (K) fertilizers on some soils; but they do not respond in any significant manner on other soils, which are now called ‘non-responsive soils’ (Ichami et al., Reference Ichami, Shepherd, Sila, Stoorvogel and Hoffland2019; Kihara et al., Reference Kihara, Nziguheba, Zingore, Coulibaly, Esilaba, Kabambe, Njoroge, Palm and Huising2016; Nziguheba et al., Reference Nziguheba, van Heerwaarden and Vanlauwe2021; Roobroeck et al., Reference Roobroeck, Palm, Nziguheba, Weil and Vanlauwe2021; Vanlauwe et al., Reference Vanlauwe, Kihara, Chivenge, Pypers, Coe and Six2011). The problem of non-responsive soils is both widespread and severe in some countries. For example, in diagnostic trials across sites in Kenya, Malawi, Mali, Nigeria and Tanzania, 25% of the fields were non-responsive, whereas 28% showed low response to any nutrient amendment (Kihara et al., Reference Kihara, Nziguheba, Zingore, Coulibaly, Esilaba, Kabambe, Njoroge, Palm and Huising2016). In nutrient omission trials conducted on farmers’ fields across three states of Nigeria, Shehu et al. (Reference Shehu, Merckx, Jibrin, Kamara and Rurinda2018) found 9–16% of the maize fields to be non-responsive to any nutrient application. In a recent meta-analysis, non-responsive soils constituted 14% for Kenya and 11% for SSA (Ichami et al., Reference Ichami, Shepherd, Sila, Stoorvogel and Hoffland2019). In a study by Roobroeck et al. (Reference Roobroeck, Palm, Nziguheba, Weil and Vanlauwe2021), NPK fertilizers also failed to increase maize yields by more than 0.5 t ha⁻¹ in up to 68 % of the farmers’ fields in western Kenya, eastern DR Congo, west-central Tanzania and northern Nigeria. Using data sets from 41 on-farm fertilizer response trials across 11 countries, Nziguheba et al. (Reference Nziguheba, van Heerwaarden and Vanlauwe2021) estimated the proportion of non-responsive fields at 4.9% for cereals and 17% for legumes. Nziguheba et al. (Reference Nziguheba, van Heerwaarden and Vanlauwe2021) further estimated that roughly 0.3–2.5 million ha of cereal and 3.2–4.6 million ha of legume fields could be non-responsive in any particular year. Poor and variable crop responses to nutrient inputs may pose a barrier to fertilizer adoption (Nziguheba et al., Reference Nziguheba, van Heerwaarden and Vanlauwe2021). However, non-responsiveness is neither well-defined nor its extent and causes fully understood in SSA (Roobroeck et al., Reference Roobroeck, Palm, Nziguheba, Weil and Vanlauwe2021; Ichami et al., Reference Ichami, Shepherd, Sila, Stoorvogel and Hoffland2019).

Sileshi, G.W.; Kihara, J.; Tamene, L.; Vanlauwe, B.; Phiri, E.; Jama, B.