Site-Specific Fertilizer Nitrogen Management in Cereals in South Asia

Abstract

In cereal crop production in South Asia, nitrogen fertilizers are generally managed as blanket recommendations formulated on the basis of crop response data averaged over large geographic areas. Blanket recommendations do not take into account the spatio-temporal variability in nitrogen supplying capacity of soils. In developed countries, improved synchronization between crop nitrogen demand and supply of nitrogen from different sources is achieved through variable rate fertilizer applicators in large fields. During the two last decades, research focussed on sitespecific nitrogen management strategies suitable for small fields owned by relatively resource poor farmers. We have reviewed the evolution and development of field specific strategies based on leaf colour chart, chlorophyll meter and optical sensor for need-based application of nitrogen fertilizers in rice, wheat and maize. The major outcomes are: (1) Rather than using soil tests and plant tissue analysis, assessment in real-time of the nitrogen needs of crops by estimating absorbance, and reflectance of light by intact leaves has been successfully translated into fieldspecific nitrogen management strategies; (2) Gadgets like chlorophyll meters, optical sensors and leaf colour charts have been standardized to apply fertilizer nitrogen as per need of the crop already growing in the field; (3) The site-specific nitrogen management approaches as evolved in South Asia are based on the estimation of the fertilizer nitrogen dose; as well as the time of application as per threshold greenness of first fully opened leaf from the top of the crop plants as measured by chlorophyll meter or leaf colour chart; (4) Simple and inexpensive leaf colour chart for guiding field specific nitrogen doses in rice is increasingly being adopted by farmers and it can increase agronomic efficiency from 5 to 16 kg grain kg N−1 over the farmers’ fertilizer practice (5) Optical sensor guided field-specific nitrogen management takes into account both nitrogen status and biomass of the crop to increase fertilizer use efficiency; (6) Defining time of application and doses of fertilizer nitrogen prior to application of site-specific doses is very crucial; and (7) Computer- and web based nutrient decision support tools can also help manage fertilizer nitrogen in cereals on a field-specific basis.