Benchmarking Fertilizer Recommendations for Maize ( <i>Zea mays</i> L.) and Validating Formula‐Derived Phosphorus Rate for Soybean ( <i>Glycine max</i> L.) With Microbial Inoculation on Volcanic Soils of Buea, Cameroon

Chemical fertilizer improves crop productivity, but indiscriminate blanket application can degrade soil fertility and distort nutrient balances. This study was organized around two connected soil fertility management components. Firstly, a simplified site‐specific nutrient‐gap formula was developed from measured soil nutrient stock, bulk density, sampling depth and crop nutrient demand and used to benchmark existing blanket nitrogen (N), phosphorus (P) and potassium (K) fertilizer recommendations for maize on the volcanic soils of Buea, Cameroon. Secondly, a field trial was conducted to evaluate a formula‐derived P‐fertilizer rate on soybean and economic performances in an integrated approach with microbial amendments. Maize benchmarking revealed that blanket fertilizer rates were either oversupplied with 17.2% N and 69.2% K or undersupplied at 14.7% and 41%. Field validation of the formula‐derived P‐rate on soybean revealed that the prescribed P‐fertilizer rate with or without microbial amendments jointly influenced rhizosphere activities, soybean yield and profitability. Beneficial microbes, P‐rates and microbes ∗ P‐rates significantly influenced plant‐available P in soybean rhizosphere, soybean nodulation, AMF root colonization, soybean yield and profitability. The formula‐derived 58 kg P ha −1 enhanced available P and nodulation under Bradyrhizobium  +  Kosakonia  + AMF, promoted AMF symbiosis and functionality, produced the highest soybean grain yield (1.2 t ha −1 ) and increased net income by 35.5% relative to blanket lower and higher rates of 29 and 87 kg P ha −1 , respectively. These findings demonstrate agronomic and economic benefits of soil test–based nutrient‐gap formula‐guided fertilization strategy over blanket fertilizer approach that is either over‐ or underapplied. Overall, this study offers a cautious, field‐grounded basis for using soil test nutrient‐gap estimates to guide fertilizer management in low‐input subsistence farming systems.