Study of variable nutrient responses in cotton grown in Vertisols of North-Eastern Karnataka

Cotton is intensively cultivated in the north eastern dry zone of Karnataka. The production area for this crop has been increasing over the past half a decade, while the on-farm productivity is declining. The current investigation studied the response of cotton to nutrient omissions and site-specific nutrient management (SSNM) in the farmers' fields during the monsoon season of 2016 at Raichur, Karnataka. The soils of the experimental sites were deep black, alkaline (pH 7.5 to 8.4), non-saline (EC 0.19 to 0.32 dS/m), low to medium in organic carbon (0.37 to 0.6%), low in available nitrogen (N) (70 to 235 kg/ha), medium to high in available P₂O₅ (42 to 75 kg/ha), and high in available K₂O (295 to 675 kg/ha). The study in six farmers' fields comprised of five treatments, namely SSNM (320-112-114 kg N-P₂O₅-K₂O/ha, respectively) for a yield target of 4 t/ha, omission of N, phosphorus (P) and potassium (K) from the SSNM treatment, and farmers' practice (FP).

The results revealed that the SSNM treatment recorded significantly higher seed cotton yield (4.4 t/ha) over the FP (4.1 t/ha). A significantly lower seed cotton yield was recorded with N omission (3.7 t/ha). Yield increases with SSNM over the omission of N, P, and K were 18, 9, and 16%, respectively, indicating that N followed by K were the most limiting nutrients restricting the yield of Bt cotton. The higher yield in SSNM over FP could be attributed to lower application of K in the FP, as similar N levels and slightly higher P levels were applied in the FP as compared to the SSNM treatment. Response of Bt cotton to K application (600 kg/ha) was similar to the N response (680 kg/ha) and higher than the P response (380 kg/ha). It was interesting to note that Bt cotton responded to the applied K fertilizer even when the experimental soil was high in available K. This study corroborates earlier findings that Vertisols with high K content still need external K application to achieve and maintain high attainable yield of crops. Significantly higher gross and net returns were recorded with SSNM treatment (Rs. 188526 and 136183/ha, respectively) over the FP, while returns were lower in the N omission, followed by the P and K omission treatments. Consequently, the Benefit:Cost ratio was higher with SSNM (3.6) followed by FP (3.4), and the lowest B:C ratio was observed with N omission (3.1). The study revealed that a yield target of 4 t/ha for cotton could be achieved through SSNM, and balanced nutrition with adequate K application is necessary to improve yield and economic return of cotton farmers in Karnataka.