Soil organic carbon, texture and oxide-associated processes regulate phosphorus availability across Nigerian agroecosystems

Abstract Background and aims Phosphorus (P) limitation constrains productivity in tropical soils, yet the combined roles of soil organic carbon (SOC), texture, reactive minerals, and acidity in regulating plant-available P remain insufficiently quantified across diverse agricultural soils. This study assessed the relative importance of soil properties and tested hypothesised direct and indirect relationships regulating P availability across Nigerian agroecosystems. Methods A national dataset of 554 soil samples was analysed. Plant-available P was measured using Mehlich-3 extraction. Random Forest analysis was used to identify dominant predictors and explore non-linear responses, while structural equation modelling was used to test hypothesised direct and indirect relationships among SOC, texture, oxides, acidity, and available P. Results SOC was the strongest predictor of available P. Available P increased with SOC, although the response showed diminishing returns at higher SOC levels. The structural equation model explained 29% of the variation in available P. SOC had the strongest positive direct association with available P. Texture had contrasting effects: finer-textured soils were positively associated with SOC and oxide contents but showed a negative total association with available P. Oxalate-extractable Fe and Al were positively associated with Mehlich-3 extractable P, indicating their role as P-retaining surfaces and potential reservoirs of extractable P. Stronger acidity was negatively associated with available P. Conclusions Available P across Nigerian agroecosystems is shaped by the combined effects of SOC, texture, Fe and Al oxides, and acidity. These findings provide national-scale evidence for targeting established soil fertility practices according to dominant soil constraints across contrasting agroecological zones.