Inseason Nitrogen Recommendation Methods: A Review.

Early in-season loss of nitrogen (N) continues to be a problem in major commercial crops. The primary cause of N loss is through nitrate leaching or denitrification from excessive rainfall. Low NUE of present N management practices is partly due to the poor synchrony between the N application and crop demand. The efficiency of a single pre-plant N application decreases with the rate of N fertilizer applied, in contrast, in-season N application results in improved NUE as compared to pre-plant N application. There are problems in N recommendation on the basis of yield expectations. Several studies have confirmed that the N recommendation is not correlated with the EONR. Therefore, the yield expectation method not a reliable tool for N recommendation because in many cases it gives an overestimated value of N application. Use of tissue analysis for N management is a common practice in many crops; wheat, corn, and potato but the approach of critical N at the early growth stage does not provide a reliable estimate crop N status and it is very laborious, time consuming, and expensive technique. It could be useful for small scale farming, not practical in large-scale commercial agriculture. Several studies have been conducted to measure the efficacy of two commercially available active optical sensors, GreenSeeker^® Model 505 (Trimble, Inc., Sunnyvale, CA), and Crop Circle (Holland Scientific, Inc., Lincoln, NE) in predicting the corn, wheat, sugar beet, and sunflower yield. The basic differences between the two sensors are the use of different wavelengths to calculate NDVI.  The ground based active optical (GBAO) sensors have important advantages over the chlorophyll meter, satellite images and aerial photographs in managing corn N nutrition at a field scale. They are faster and less labor intensive than the chlorophyll meter. The GBAO sensors also does not require full canopy or ultra-high resolution as do aerial photographs.