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Abstract

Abstract Spatially explicit soil fertility diagnosis is still difficult in data sparse paddy cultivated areas because single soil test indicators cannot separate soil chemical condition, primary nutrient supply, micronutrient balance and yield relevance. This study developed a soil-test driven, domain-balanced Soil Fertility Index (SFI) framework using 64 georeferenced long-term paddy-field samples from Hanumakonda district, Telangana, India. Ten laboratory indicators (pH, electrical conductivity, organic carbon, available nitrogen, phosphorus, potassium, zinc, iron, copper, and manganese) were transformed into agronomic scores and aggregated into Chemical Condition, Macronutrient, Micronutrient, and Overall Soil Fertility indices. Regularized regression models were used to approximate the derived fertility-index targets from the original soil-test variables and to identify the conditional contribution of individual indicators within each index formulation. Ridge, Lasso and ElasticNet regression models were evaluated using five-fold cross-validation within the training dataset ( n  = 51) and independent test-set evaluation ( n  = 13). Among the evaluated models, Lasso provided the best prediction of Overall SFI (R 2 = 0.594, RMSE = 0.033, RPD = 1.63), whereas ElasticNet performed best for the Micronutrient Index (R 2 = 0.683, RPD = 1.85) and showed stable prediction of the Macronutrient Index (R 2 = 0.577, RPD = 1.60) Relative zone mapping showed that low to moderate priority classes dominated the predicted overall SFI surface, while the Micronutrient Index produced the sharpest local management contrast. The predicted fertility surfaces were mapped across approximately 140,387 ha within the empirical point-covered sampling domain using a 30 m spatial analysis grid. Independent yield validation supported the agronomic relevance of the framework. Available nitrogen and organic carbon were strongly correlated with yield. Yield validation showed that predicted MNI relative zones significantly separated productivity classes, with mean yield increasing from 22.52 to 26.81 q acre −1 (Kruskal-Wallis H = 17.52, p  < 0.001), whereas Overall SFI, MiNI and SFI 5core zones did not show significant yield separation. The proposed framework provides an interpretable and reproducible decision-support approach for prioritizing soil testing and site-specific nutrient management in data-sparse paddy landscapes.