Assessment of soil fertility status and nutrient index in major cropping systems of semi-arid regions

Abstract:

Fertility maps built decades ago still guide fertilizer recommendations across much of the semi-arid Sahel, even as cropping patterns have shifted considerably since those maps were drawn. Would a fresh soil fertility assessment across the region's four dominant cropping systems reveal the same nutrient status those older maps assume, or has the picture changed? Soil samples were collected from 84 georeferenced sites spanning cereal-cereal rotation, cereal-legume rotation, cereal-fallow, and mixed cropping systems around Bamako, Mali, and analyzed for pH, electrical conductivity, organic carbon, available nitrogen, phosphorus, potassium, and available zinc, iron, manganese, and copper. Nutrient Index Values (NIV) were computed for each parameter and cropping system following standard fertility rating procedures. Cereal-cereal rotation sites showed the poorest overall fertility profile, with 62% of samples falling in the low organic carbon category, while cereal-legume sites showed significantly higher organic carbon and available nitrogen, consistent with residual nitrogen fixation benefits carried over from the legume phase. Available zinc deficiency was widespread across all four systems, affecting 71% of sampled sites regardless of cropping history, suggesting a region-wide micronutrient gap that current fertilizer recommendations, built mainly around nitrogen, phosphorus, and potassium, do not address. A strong positive correlation between organic carbon and Nutrient Index Value for nitrogen (r = 0.74) points to organic matter management as a practical entry point for improving overall soil fertility across cropping systems. These findings support updating regional fertilizer recommendations to reflect current cropping-system-specific fertility status and to incorporate zinc management as a standard component.