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Development of Critical Values for the Leaf Color Chart, SPAD and Fieldscout CM 1000 for Fixed Time Adjustable Nitrogen Management in Aromatic Hybrid Rice (Oryza sativa L.)

The nitrogen (N) requirement of hybrid rice is generally greater than in conventional rice varieties. Recommendations for N monitoring at regular intervals of 7–10 days through leaf greenness are available, but farmers are accustomed to apply fertilizer N at selected growth stages only. An inexpensive leaf color chart (LCC) and nondestructive chlorophyll meters were evaluated for site-specific N management strategy in world’s first aromatic rice hybrid PRH-10 at the Indian Agricultural Research Institute, New Delhi. Two field experiments were conducted on PRH-10 with four levels of N (0, 70, 140, and 210 kg ha−1) during June–October of 2010 and 2011 to determine the LCC, soil–plant analysis development (SPAD), and Fieldscout CM 1000 (CM 1000) values for achieving economic optimum grain yield at three critical growth stages (tillering, panicle initiation, and flowering). Quadratic regression between N levels and grain yield were used to determine economic optimum grain yield (6427 kg ha−1 in 2010 and 6399 kg ha−1 in 2011) corresponding to optimum economical dose of 151 kg N ha−1 (2010) and 144 kg N ha−1 (2011). Nitrogen concentration in fully expanded youngest leaf correlated significantly (P < 0.01) and positively with LCC score, SPAD value, CM 1000 value, and total chlorophyll concentration at tillering, panicle initiation, and flowering for both years. The critical LCC score, SPAD, CM 1000 values, chlorophyll concentration, and leaf N concentration obtained were at tillering 4.4, 42.3, 285, and 2.16 mg g−1 fresh weight and 3.29%; at panicle initiation 4.4, 43.0, 276, and 2.16 mg g−1 fresh weight and 3.02%; and at flowering 4.5, 41.7, 270, and 2.05 mg g−1 fresh weight and 2.83%, respectively. Corrective N application should be done when observed leaf N indicator values at a particular growth stage reach or go below the critical values.