Evaluating Nitrogen Nutrition Index of Wheat Fields in Gorgan

This study was carried out to evaluate the N nutrition status of 16 selected wheat fields in Gorgan during 2006-2007 growing season. Based on the obtained results, dry matter yields of fields were varied from 0.38 ton ha- at the beginning tillering to 12.51 ton ha- at the harvesting maturity. The mean of N concentrations (%) in plant dry matter were 3.09 at beginning tillering, 2.60 at complete tillering, 1.62 at at stem elongation, 1.56 at boot stage, 1.14 at the beginning watery ripe stage and 1.08 at harvesting maturity. These results indicate that the most reduction in N concentration occurred in stem elongation stage, and that N concentration in plant dry matter decreased as plants developed. Actual N concentrations in dry matter were substantially less than critical N concentrations at all development stages; The means of critical N concentrations at above mentioned development stages were 4.38, 4.33, 2.88, 2.32, 1.95 and 1.72, respectively. Then, the means NNI for 16 wheat fields were very lower than optimum (NNI=1) at all development stages; 0.71 at beginning tillering, 0.59 at complete tillering, 0.57 at stem elongation, 0.68 at boot stage, 0.59 at the beginning watery ripe stage and 0.61 at harvesting maturity.