Soil test-based integrated nutrient management enhances cotton yield, fertilizer efficiency and soil sustainability in the vertisols of Karaikal

India’s cotton farming, reliant on chemical fertilizers, faces soil degradation and yield decline. To combat this, a sustainable Soil Test Crop Response - Integrated Plant Nutrient System (STCR-IPNS) approach was adopted in vertisols of Karaikal district (Typic Haplusterts), to develop fertilizer prescription equation for cotton. A fertility gradient experiment was conducted with rice (ADT 45), wherein three strips were established by applying phosphorus and potassium at 150 and 100 kg ha-1 respectively, based on their fixing capacities, while nitrogen was applied as per the recommended dose. A field experiment with cotton hybrid RCH 659 BG II was conducted using four levels of nitrogen (N at 0, 60, 120 and 180 kg ha-1), phosphorus (P₂O₅ at 0, 30, 60 and 90 kg ha-1), potassium (K₂O at 0, 30, 60 and 90 kg ha-1) and three levels of farmyard manure (FYM) (0, 6.25 and 12.5 t ha-1) to optimize fertilizer use efficiency and enhance soil health. Maximum yield and growth parameters were achieved with NPK + 12.5 t ha-1 FYM. Using yield data, nutrient uptake, initial soil N, P, K levels and fertilizer doses, basic parameters were computed to develop fertilizer prescription equations. Nutrient requirement for cotton followed the order K₂O > N > P₂O5. Nutrient contributions from soil (Cs) were higher for P2O5 followed by K2O and N. Fertilizer contributions (Cf) followed K₂O > P₂O5 > N, while FYM contributions (Cfym) were highest for N followed by K2O and P2O5. Using these parameters, fertilizer prescription equations and nomograms were developed for target yields (31, 33 and 35 q ha-1). Application of NPK with FYM at 12.5 t ha-1 resulted in fertilizer savings of 52, 30 and 48 kg ha-1 of N, P2O5 and K2O respectively. The STCR-IPNS approach enhanced yield, nutrient use efficiency and soil fertility, making it ideal for sustainable intensification in vertisols of Karaikal district.