Nutrient requirement of Bt. cotton (Gossypium hirsutum)

A field experiment was conducted on Vertisols of Dharwad, Karnataka during the rainy seasons of 2007 and 2008, to ascertain the nutrient requirement of Bt cotton (Gossypium hirsutum L.) with the recommended dose of fertilizer (RDF, 120–60–60 kg N P₂O₅ K₂O/ha) advocated to the conventional non-Bt cotton hybrid.The investigation focused on varying the recommended nitrogen application levels into 7 splits–10% as basal, remaining 90% in equal splits at 30, 60, 75, 90, 105 and 120 days after sowing (DAS) with entire dose of P and K coupled with the soil application of MgSO₄ @ 25 kg/ha, foliar nutrition of urea (2%), DAP (2%), MgSO₄ (1%), KNO₃ (2%), calcium nitrate (1%), multimicronutrient (1%), Cytokinin (2 ppm) and polyethylene mulching (100 micron thickness). ‘Bunny BG I Bt (NCS 145)’ cotton genotype was sown with 90 cm × 60 cm spacing. Foliar application of KNO₃ 2%, soil and foliar application of MgSO₄ @ 25 kg/ha and 1% respectively, and soil application of MgSO₄ @ 25 kg/ha alone increased the seed-cotton yield by 25.0, 24.4 and 24.3% respectively, over the RDF. Sympodial branches and total number of bolls/plant followed the similar trend. At 120 DAS, increase in leaf potash content (1.02%) was recorded with foliar application of 2% KNO₃ and the increase in leaf magnesium (1.86%) was noticed with soil and foliar application of MgSO₄ @ 25 kg/ha and 1% respectively. Significantly increased 2.5% span length (33.1 mm) and bundle strength (24.8 g/tex) were observed with foliar application of KNO₃ (2%) over the other treatments. Foliar application of KNO₃ (2%) (3 sprays on 15 October, 1 November and 31 November) significantly increased the seed-cotton yield (2.54 t/ha), gross returns ( 56.2 × 10³/ha), net returns (27.8 × 10³/ha) and benefit: cost ratio (1.98).