Corn producers often evaluate nitrogen sufficiency in their fields by observing the leaf color or amounts of firing. Although these evaluations often are not reliable, visual rating of N sufficiency can provide important information when used appropriately. Visual rating relies on many of the same principles as tissue testing, and can be considered a form of tissue testing.
Several symptoms of nitrogen deficiency easily can observed by comparing corn plants with adequate N to plants that do not have adequate N. N-deficient plants appear to be less green. Plants with extreme deficiencies may appear yellow-green or yellow. This symptom can be seen at any stage of growth. It occurs because N deficiencies limit chlorophyll production in the plant.
Another symptom of N deficiency in corn is a specific pattern of yellow and green on individual leaves. The yellow forms a V-shape, with the widest part toward the end of the leaf and narrowest part on the midrib pointing toward the stalk. The percentage of the leaf that is yellow tends to increase with severity of the N deficiency. This pattern occurs first on lowest leaves and moves higher as severity increases. The yellow pattern occurs because N is moved from old tissues to new tissues within the plant.
A third symptom of N deficiency is firing, the premature death of leaves after they have turned yellow. The dead leaves are brown and shriveled to a fraction of their former size. Because it is normal for lower leaves to die as corn plants mature, only the amounts of firing that exceed normal can be considered a symptom of N deficiency.
Each of these symptoms can be used to evaluate N sufficiency only if you compare them with plants that have adequate N and are grown under otherwise similar conditions. Comparisons are needed for two important reasons. First, loss of greenness and firing occurs progressively as all corn plants mature, so plants must be the same physiological age. Second, plant greenness and amounts of firing tend to vary with weather, soil factors, hybrid, diseases, and other factors.
The need for comparisons when visually rating N sufficiency is not a serious problem in production agriculture. Comparisons easily can be made if a producer plans ahead and applies fertilizer so as to have narrow strips with above-normal and below-normal rates of N application in some fields. These strips should cross several different soil types to enable comparisons within areas of uniform soil conditions.
Strips with above-normal and below-normal rates of fertilization are needed because corn usually shows no visual signs of N excesses. If you find differences between normal and above-normal rates, the normal rate did not supply enough N. If you find no differences between normal and below-normal rates, the normal rate supplied more N than needed.
Extensive studies in Iowa showed that reasonable evaluations of N sufficiency were provided by a visual rating system based on firing comparisons. This system was at least as reliable as analysis of ear leaves at silking, and it required less time and money. It is described in the first 1993 volume of the Journal of Production Agriculture.
The visual rating system based on firing was developed and evaluated in research trials that also measured yields and end-of-season cornstalk nitrate concentrations. In accordance with normal practice, the studies were conducted on areas selected to have a high degree of uniformity.
Subsequent use of the visual rating system in non-uniform fields was more difficult than expected because N sufficiency varied from site to site within a field. Thus, many visual ratings were needed for each field. Other measurements, including differences in yield and stalk nitrate concentrations, confirmed the variability in N sufficiency. Aerial color photographs showed N sufficiency varied in spatial patterns related to landscape position and other factors.
Spatial variability of N sufficiency poses the same problem for conventional tissue testing as it does with a visual rating system. Analysis of a sample collected from an area with a mixture of N deficiencies and excesses provides little useful information. The fact that this usually is not recognized as a serious problem in conventional tissue testing shows an important advantage of visual rating systems; visual rating systems encourage and facilitate the dividing of fields into meaningful units before samples are collected. This is important because an essential step in moving toward site-specific management of N is learning what should be considered a site to sample.
In summary, visually rating a field's N status should not be considered too "low tech" to be useful. Effective use requires some skill, but the test can provide important information at little cost. Checking for strips of N-deficient corn that coincide with fertilization patterns can reveal problems caused by nonuniform applications of fertilizer. When strips of below- and above-normal rates of N are deliberately placed in fields, visually rating the N sufficiency can reveal how fertilizer needs vary with landscape position and other factors. Spatial patterns in corn color or firing can be used to show how fields should be divided to collect soil or plant samples.
This article originally appeared on pages 150-151 of the IC-478(19) -- July 28, 1997 issue.