Evaluating Plant‐Available Nitrogen in Soil‐Crop Systems

Modern soil-crop systems must supply nitrogen (N) in quantities needed for high crop productivity and ensure high N utilization by optimizing the fertilizer N rate, timing, and application methods. A description of plant-available N for any soil-crop system can be given through mass balance principles once the system is defined in space and time. The crop N requirement factor is essential to every soil N evaluation system and can be defined as the crop N uptake near maximum yield. The choice of the cost-effective N recommendation system will depend heavily on economic conditions; particularly on the cost of N. One of the main elements influencing fertilizer efficiency is soil water content, because water directly affects denitrification and leaching losses. The authors soil N evaluation systems must be undergirded by a comprehensive field calibration program that is able to document and interpret site-specific data that affect basic soil N transformations.