Refinement and Validation of STCR–IPNS Fertilizer Prescription Equations for Rainfed Cassava on Typic Haplustalf

Rainfed cassava (Manihot esculenta Crantz) cultivated on Alfisols is highly responsive to balanced nutrient management, yet fertilizer recommendations remain generalized and site-nonspecific. This study aimed to refine Soil Test Crop Response (STCR)-based fertilizer prescription equations under NPK-alone and Integrated Plant Nutrition System (IPNS) approaches for rainfed cassava grown on Typic Haplustalf and to validate them across multiple farmer-managed locations. Unlike existing fertilizer recommendations developed predominantly for irrigated conditions, the refined equations account for nutrient requirement and soil, fertilizer, and organic-source contributions under rainfed conditions, providing a locally calibrated framework for Typic Haplustalf. The crop nutrient requirement was quantified as 0.31 kg N, 0.17 kg P2O5, and 0.67 kg K2O per quintal of tuber yield, indicating a high potassium demand. Incorporation of farmyard manure (12.5 t ha−1) in the STCR–IPNS approach reduced inorganic fertilizer requirement while enhancing nutrient-use efficiency. Multi-location validation demonstrated yield attainment within ±10% of targeted levels. The highest mean tuber yield (33.4 t ha−1), response ratio (67.7 kg−1), and benefit–cost ratio (2.80) were recorded under STCR–IPNS with a 32 t ha−1 target. The refined equations therefore offer a practical basis for soil test- and yield target-based fertilizer recommendations, enabling more efficient fertilizer use while sustaining cassava productivity under rainfed Alfisols.