Establishing indicator leaf and its threshold values for need based nitrogen management using chlorophyll meter and leaf color chart in Bt cotton

Over application of fertilizer N to cotton is not only a potential threat to environment but also leads to increased costs of cultivation. The study aimed to establish the indicator leaf and its critical greenness for in-season management of fertilizer nitrogen (N) in Bt cotton using chlorophyll (SPAD) meter and leaf color chart (LCC). The response of three varieties and N treatments viz. 0, 30, 60, 90,120, 150 and 180 kg N ha(-1) applied in two splits {(50% at thinning and 50% at first flowering) and three splits (50% at thinning, 25% at first flowering and 25% at boll formation)} was studied through split plot design. SPAD values and LCC scores of first, second, third and fourth fully opened leaves from the top of the main stem was recorded at first flowering and boll formation. The physiological efficiency and harvest index was highest for 90 Kg N ha(-1) applied in two splits. Beyond 120 kg N ha(-1), the N use efficiency parameters were higher for the N treatments applied in three split compared to the respective two split N treatments. The fourth leaf from the top in terms of SPAD values and LCC scores correlated best with N concentration compared to other leaves at all growth stages. The calculated critical SPAD values for the fourth leaf were 45 and 41 at first flowering and boll formation, respectively. Critical score of fourth leaf was 4.1 and 4, respectively at first flowering and boll formation, respectively. It is suggested that color of the fourth leaf from the top of Bt cotton can well indicate N supply from the soil and can help in need based N management.