TECHNIQUES FOR MONITORING CROP NITROGEN STATUS IN CORN

Nitrogen (N) fertilizers used in com production contribute to the occurrence of nitrate to ground water. Tissue testing is a tool that can aid in fertilizer N management. Several tissue testing techniques were evaluated for their potential to aid in N management of irrigated com in Nebraska. Leaf N content provided a traditional means of identifying N deficiencies in corn. Leaf N concentrations were difficult to interpret because of luxury consumption and the lack of consistent critical levels across hybrids and growth stages. Chlorophyll meters showed little sensitivity to luxury consumption and provided instantaneous results, but required an in-field reference fertilized with extra N. Chlorophyll meter readings were highly correlated with grain yield and provided early detection of a N deficiency. The combination of chlorophyll meter data and irrigation demonstrated the merits of a ''fertilization-as-needed'' approach to N management. The post-harvest stalk nitrate test accurately identified situations with excess N available to the corn crop. This test does not require an in-field reference to evaluate N management after the fact, and also indicates environmental implications of N fertilizer practices.