Neem (Azadirachta indica) oil coated urea improves nitrogen use efficiency and maize growth in an alkaline calcareous soil

Two independent experiments were conducted to assess neem oil coated urea (NOCU), as a natural nitrification inhibitor and to improve nitrogen use efficiency in maize. In first experiment, soil incubation with increasing levels (i.e. 1%, 2% and 3% w/v) of neem oil coating was done for 28 days to monitor the release of NH₄⁺ and NO₃^-contents. Neem oil coated urea slowed down the release of mineral N with maximum nitrification inhibition (40%) at 3% level of neem coating as compared to 2% and 1%, hence selected for next study. In the subsequent pot study, NOCU was applied according to recommended dose (250 kg ha^-1) of urea at three different levels (50%, 75% and 100%) for maize crop. There was a significant increase in N contents of shoot and grains that ultimately provoked high protein contents in maize grains. Application of NOCU enhanced the N uptake due to its increased availability for longer period of time, resulting in higher grain yield. Maximum agronomic efficiency (24.38 g g^-1) of applied N was observed with 75% dose of NOCU, while maximum recovery efficiency (0.44 g g^-1) was recorded with 100% dose of NOCU. Conclusively, application of NOCU saves 25% N application compared to ordinary urea.