Submitted to: Communications in Soil Science and Plant Analysis
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 31, 1994
Publication Date: N/A
Interpretive Summary: The traditional way to tell if corn is nitrogen (N) deficient is to look for yellowing leaves. A more objective approach is to analyze leaf tissue for its N concentration. The visual approach is rapid, but very subjective while the later is time consuming and costly. Establishing a lower limit for leaf N concentration that will sustain growth and still attain good yields is nearly impossible. This is because corn hybrids differ in their N requirements and the N adequacy concentration changes with stage of growth. These difficulties cause many producers to revert back to visual means to assess crop N status. A portable chlorophyll meter was found to provide a good relative indication of crop N status, but to interpret the meter readings it was important to have a small portion of the field growing with excess N fertilizer. Comparing meter readings between plants known to have excess N and those that might be deficient provides a good indication of crop N status. This comparison technique works because chlorophyll meter readings reach a plateau when something else beside N begins to limit growth. Many times corn plants take up more N from soil than can be used effectively. At harvest, this excess N remains in the stalk tissue and can be detected, which implies excess N was available to the crop during the growing season. These techniques can be used by producers to determine if N was deficient or excessive so that future management practices can be adjusted accordingly.
Technical Abstract: Nitrogen fertilizers used in corn production contribute to the occurrence of nitrate in 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 corn in Nebraska. Leaf N content provided a traditional means of identifying N deficiencies in corn. Leaf N concentrations in corn leaves 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 an 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.