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Core Ideas Mean maize response to N is similar in Kenya, Tanzania, and Rwanda. Maize is relatively more responsive to nitrogen in Malawi and Rwanda. Maize is responsive to phosphorus and potassium in Rwanda. There is little potential for profitable response to phosphorus and potassium in most areas of East Africa. Maize response to Mg–S–Zn–B needs more investigation in Rwanda. Information is scarce for maize (Zea mays L.) response to nutrient application for many production areas in tropical Africa. Research was conducted to determine macronutrient response functions and to diagnose Mg–S–Zn–B deficiencies. Site–year × N‐rate interactions within countries often accounted for little variation in yield relative to the N‐rate effect. Country mean grain yield responses to N‐rate were curvilinear to plateau, but linear in Malawi. Although mean yields differed, the response to N was similar for Kenya, Tanzania, and Zambia with a mean yield increase of 0.94 Mg ha−1 due to 50 kg ha−1 N compared with 1.59 Mg ha−1 for Malawi and Rwanda. Response to N was related to yield with no fertilizer applied (r = 0.40). Only Rwanda had mean responses to P and K with respective yield increases of 0.99 and 0.22 Mg ha−1 due to 15 kg ha−1. Application of Mg–S–Zn–B caused a mean yield increase of 0.73 Mg ha−1 in Rwanda but had no effect in other countries. Application of affordable fertilizer to twice as much land at 50% compared with 100% of the economically optimum rate results in mean gains of 50% for production and agronomic efficiency and 72% for profit/cost ratio. Soil test results were not related to response to applied nutrients but historical yield appears to be weakly predictive of N response. The determined country‐level P and K response functions can be widely applied, except for Kenya, in consideration of other available information. The response to Mg–S–Zn–B in Rwanda needs further investigation.

C assava (Manihot esculenta Crantz) is an important crop of smallholder farmers in Tanzania, Kenya and Ghana (Harvest Choice, 2011). In Ghana, cassava is second to maize in terms of production area and an important cash crop (MOFA, 2010; FAO, 2015). In Tanzania and in Western and Coastal Kenya, cassava is second to maize in importance as a food crop (Mwango'mbe et al., 2013). However, mean cassava fresh storage root yield was estimated to be 13.5, 6.2 and 14.4 Mg ha-1 for Kenya, Tanzania and Ghana, respectively (FAOSTAT, 2017) while cassava yield has reached 80 Mg ha-1 (FAO, 2013). Pests and diseases, the use of poor cultural practices, and low soil fertility status are among causal factors for low cassava yields (Harvest Choice, 2011; Ezui et al., 2016). Several studies have shown that the soil nutrient balance is negative due to nutrient removal at harvest, minimal or no use of fertilizers, and soil erosion (Asadu and Nweke, 1999; Shekiffu, 2011; Fermont 2009). Few soil fertility studies have been conducted in cassava production in Tanzania, Kenya and Ghana and very few farmers apply fertilizer to cassava (