Development and validation of STCR-based fertilizer prescription equations for field bean under Alfisol conditions of the Eastern Dry Zone of Karnataka

Field bean (Lablab purpureus L. Sweet) is an important multipurpose legume in the Eastern Dry Zone of Karnataka, but fertilizer recommendations are often not adjusted for soil fertility variation or yield targets, resulting in sub-optimal productivity. This study aimed to develop and validate soil test crop response (STCR)-based targeted-yield fertilizer prescription equations for field bean grown on Alfisols in Karnataka. A three-phase investigation was conducted during 2023–2025 at AICRP–STCR, ZARS, UAS Bengaluru, comprising a fertility gradient experiment with fodder maize, a main STCR calibration experiment with field bean, and validation trials at two locations. Targeted-yield equations were derived using nutrient requirement and contributions from soil, fertilizer and farmyard manure (FYM) based on N, P and K uptake and yield data under NPK-only and NPK + FYM systems. The gradient experiment established distinct low-, medium- and high-fertility strips for available N, P and K. In the validation trials, STCR-based treatments targeting 12 q ha −1 produced the highest seed yields (13.42 and 13.21 q ha −1 under NPK + FYM), exceeding the general recommended dose by 20.2–23.7% and the soil fertility-rating approach by 35.1–38.8%. Observed yields under the four STCR treatments exceeded the specified targets by 7.8–17.6%, indicating positive prediction errors rather than uniform agreement within ± 10%.Integration of FYM reduced calculated fertilizer N, P 2 O 5 and K 2 O requirements by approximately 8–15% at comparable soil-test levels, while improving agronomic efficiency and apparent recovery, particularly for potassium (K apparent recovery up to 2.74–2.81 kg kg −1 under NPK + FYM compared with 1.82–1.89 kg kg −1 under NPK-only). This study establishes and validates crop- and soil-specific STCR fertilizer equations for field beans in Karnataka's Eastern Dry Zone. The equations provide a soil-test-based alternative to blanket recommendations, enabling site-specific nutrient management and improved fertilizer efficiency.