Peer review

Fertilizer nitrogen (N) is one of the most important nutrient inputs in global crop production. The general fertilizer N management practices in field crops consist of applying preset N doses at specified growth stages in multiple splits. Blanket or soil-test-based recommendations ignore temporal and spatial variability in soil N supply and crop demand for N and thus could not help improve N use efficiency beyond a certain limit. Synchronizing plant N demand and fertilizer N supply is a proven fertilizer management approach to improve N use efficiency. In-season plant growth comprehends the total N supply to plants from different sources, thus in-season plant N status and plant biomass could be a better indicator of the N availability to crops than soil testing. Optical sensors have emerged as efficient diagnostic tools for estimating crop N status and yield of the crops and thus help guide site-specific need-based fertilizer N topdressings. Relationships between spectral properties measured using optical sensors and plant N concentration, total N uptake, various agronomic and yield parameters of major field crops have been extensively studied. This chapter reviews the results of investigations carried out for assessing plant N status and developing rational fertilizer nitrogen management strategies using different kinds of optical sensors in wheat, rice, maize, and cotton.

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