Puneet-Sharma, Varinderpal-Singh and Bijay-Singh

Department of Soil Science, Punjab Agricultural University, Ludhiana, INDIA

Over-application and inefficient splitting of fertilizer N in rice (Oryza sativa L.) lead to low N use efficiency (NUE) and risk of N ending up in the environment. Currently, fertilizer N in rice is managed following blanket recommendations for large tracts. This cannot help to increase NUE beyond a limit. Improvement in synchrony between crop N demand and N supply from soil is likely to be the most promising strategy. Gadgets like chlorophyll meter (SPAD) and optical sensors can help in monitoring crop N status and guide fertilizer N applications. Site-specific, need-based fertilizer N management strategies based on applying 30 kg N ha^-1 when leaf colour was less than the SPAD values of 35.0, 37.5, 40 or 42.5 were tested on rice two cultivars PR118 and PR120. Another set of treatments consisted of applying 30 kg N ha^-1 when ratio of NDVI as measured by optical sensor in the test plots and an over-fertilizer reference plot was less than 0.80, 0.85 or 0.90 at 42 days after transplanting (DAT) of rice. A fixed dose of 30 kg N ha^-1 and 45 kg N ha^-1 was applied at planting and at 21 DAT. Results revealed that application of N whenever leaf colour intensity was lower than SPAD value of 40 was the best option for both of the varieties. In the optical sensor based strategy, application of N at NDVI ratio less than 0.85 proved a better option. This study shows that SPAD meter and GreenSeeker optical sensor can be successfully used to manage N in a site-specific manner.