Crop demand-driven site-specific nitrogen applications in rice (Oryza sativa) and wheat (Triticum aestivum): some recent advances

Management of fertilizer nitrogen in rice (Oryza sativa L.) and wheat (Triticum aestivum L. emend. Fiori & Paol.) in India typically consists of blanket recommendations, developed for large tracts having similar climate and land forms. It cannot help the increase N-use efficiency beyond a limit. Further improvement can be achieved only by planning strategies for fertilizer N management responsive to temporal variations in crop N demand and fieldiito- field variability in soil-N supply. Improvement in the synchrony between crop N demand and the N supply from soil or the applied N fertilizer is likely to be the most promising strategy to improve N-use efficiency. It can be achieved by following real-time N management based on periodic assessment of plant-N status and delayed application of fertilizer N until the N level goes below a critical level. Gadgets like chlorophyll-meter (SPAD-meter) and inexpen- sive leaf colour chart (LCC) have proved quick and reliable tools to decide the time when fertilizer N needs to be applied to the crop. Recently significant progress has been made in the development and on-farm evaluation of LCC-based real-time N management in rice, but there is a need to work out appropriate criteria for using need- based N-management strategy in wheat. Optical sensors offer an opportunity to take into account both plant N sta- tus (or leaf colour) as well as crop biomass for making crop-demand-driven fertilizer-N recommendations. Combi- nation of preventive location-specific split schedule with corrective LCC or SDAD-meter or optical sensor-based N management constitutes another attractive strategy for achieving high N-use efficiency in rice and wheat. The fixed-time or adjustable-dose approach seems very promising for wheat, because unlike in rice, the fertilizer appli- cations need to be synchronized with irrigation events. Appropriate preventive fertilizer-N management scenarios need to be worked out that can be used before using the SPAD meter, LCC or optical sensors to guide corrective fertilizer-N application at a given crop-growth stage.