Iowa State University's new nitrogen fertilizer recommendations for corn (outlined in Pm-1714) encourage use of the end-of-season test for cornstalk nitrate, and not the ear-leaf test at silking. This deserves explanation because this test at silking has been a common practice for decades.

The ear-leaf test for N has two uses, and their differences must be recognized. The first use is to help interpret the results of research trials. The second use is to evaluate N sufficiency at various sites in production agriculture.

The ear-leaf test can provide useful information when used in research trials. An example is a trial in which various levels of animal manure are applied to a field and yields of grain are measured. Although yields show overall effects of the manure, they do not show whether the effects were caused by nitrogen, phosphorus, or other factors related to manure application. The ear-leaf test, however, reveals the effects of the manure on N sufficiency for crop growth.

When the ear-leaf test is used in this way, results can be interpreted without use of critical concentrations determined at other sites. Interpretations can be based solely on differences observed among treatments within a site. If numerous rates of N are applied, the results can be used to identify a critical concentration of N that is appropriate for the conditions studied.

Successful use of the leaf test in research trials seems to have prompted use of this test as a general diagnostic tool to evaluate N sufficiency of corn growing at specific sites in production agriculture. The test often is used where a problem is suspected and a more rigorous evaluation of N sufficiency is desired. Use of the ear-leaf test for this purpose, however, relies on the assumption that an appropriate critical concentration is known.

Numerous calibration trials conducted in Iowa during the past decade have revealed several problems associated with its use as a general diagnostic tool in production agriculture. The following four points summarize what was learned.

1. Corn ear leaves at silking show "poverty adjustment," but they show little tendency for "luxury uptake" of N (see the previous ICM newsletter for how these characteristics are determined). Therefore, the tissue test does not distinguish between optimal and excessive levels of plant-available N. Lack of luxury uptake makes it difficult to define a critical concentration of N for this test.
2. Many factors (weather, hybrid, etc.) influence the concentration of N in ear leaves when plants have adequate but not excessive amounts of N. The breakpoint concentration between inadequate and optimal concentrations of N in the ear leaves, therefore, tends to differ substantially among sites.
3. Published critical concentrations of N for this test tend to be too high. Use of published critical concentrations for the leaf test may incorrectly show deficiencies in plants that had great excesses of available-N. These excesses were confirmed by measuring plant yield responses to applied N and concentrations of nitrate in cornstalks at the end of the season.
4. The unrealistically high critical concentrations reported for this test should be attributed to reasonable errors in the methodology used to determine critical concentrations. The published critical concentrations were best estimates derived from small datasets. The recent studies involved rigorous analysis of large datasets.

The ear-leaf test was first described more than 50 years ago. The rationale was that leaves are the primary site of photosynthesis and, therefore, should provide reliable detection of situations where N deficiencies limited rates of photosynthesis and crop growth. The latest research confirms most of the observations made by the early scientists. The early scientists, however, were addressing nitrogen management problems that were important before the advent of commercial fertilizers.

Advances in fertilizer technology have made it economically feasible for modern corn producers to essentially eliminate N deficiencies during corn production. Fertilizer N often is applied at above-optimal rates. The key nitrogen management problem today, therefore, is learning how to use N fertilizers to maximize the benefits for producers (and society as a whole) while minimizing environmental problems.

Current efforts to improve N management practices require use of diagnostic tools that reliably measure levels of N excesses as well as N deficiencies. The end-of-season test for cornstalk nitrate can provide these measurements, but the ear-leaf test cannot. In light of recent findings, it is time to re-evaluate use of the ear-leaf test at silking to measure N sufficiency in production agriculture.