Is This Corn Nitrogen Deficient?
|Emerson D. Nafziger|
When asked, most producers don’t recall seeing widespread N deficiency in their cornfields, except in cases where corn “fires” under dry conditions, or when wet soils in low-lying areas of fields result in yellow plants. There are occasional reports of strips receiving extra N and producing more yield than the rest of the field, and though such reports are often anecdotal, many people realize that the N fertilizer applied to a field might not always be adequate to produce maximum yields.
Fig.1 Corn Yield response to N rate run for seven years in the same field at Urbana
Our recent work on N rates for corn has shown clearly that applying enough N to assure that a field is never deficient is not economically sound. Figure 1 shows N response data for a study at Urbana where corn has followed corn for 7 years. The economically optimum N rate, where the last lb of N was just paid for by the increase in yield, is 194 lb N per acre, but it ranged from 125 to 225 (the maximum rate used). High corn yields in a year did not automatically mean that a lot of N was needed, though there was a tendency for higher yields to need more N.
How do we deal with the uncertainty that the N rate we in the same field at Urbanachoose for a certain year might not be adequate for that field for that year? Because we can’t know what the weather and soil conditions will be before the season, we don’t have a good way to guess at a “best” rate, hence we should use the rate that on average provides the best return. It can also be helpful, though, to learn to look at the corn crop with regard to N nutrition, and to recognize the signs of inadequate N.
Fig. 2 Corn with high N (top) has both darker green leaves and more leaf area than the N deficient corn (bottom) Photo by Bryan Young
The most common way we try to assess N deficiency is by seeing how dark green the leaves of the crop are. Greener leaves have higher chlorophyll content, and so they can do photosynthesis faster, thereby making more sugars available to feed the stalks and roots and to fill grain. Because of N available from soil organic matter, young corn might not show deficiency symptoms even where no N has been applied. In older corn, as shown in Fig. 2, low N rates cause smaller and paler leaves, leading to less photosynthesis and lower yield.
When N deficiency is severe, it’s easy to see by eye, though it’s not easy to judge how deficient a crop is. One way to put numbers on N deficiency is to use the SPAD meter, developed by Minolta. This device measures the amount of light the leaf absorbs, which is related to the leaf thickness and chlorophyll content. As shown in Fig. 3, SPAD readings taken after pollination often correlate very well with yield. While SPAD thus helps us “see” N deficiency, it usually works only after corn plants are 3 to 4 feet tall, and its ability to predict yield response to N gets better after that, peaking at about the time of pollination.
Fig. 3 Relationship Between SPAD readings after pollination
and grain yield in corne at Urbana in 2001
The corn crop usually takes up about 80 to 85 % of its total N by the time pollination is over. Even if there is no shortage of N through pollination, dry conditions later can cause what appears to be N deficiency, with lower leaves showing the yellowing that we associate with N deficiency. These leaves may drop toward the ground as well. This “firing” of lower leaves of older plants is often seen as a signal that plants ran out of N, and there is often regret that more N was not applied. This is a mistake. Root systems don’t grow much after pollination, and so they can’t tap “new” N. If soils around the roots are dry, and there is not enough water to move N toward the roots, then the plant will simply not take up enough N to keep up with movement of N to the grain, and the lower leaves will often give up some of their N in order to make up the difference. These lower leaves are shaded anyway, so the loss of their area may not be a big problem.
While we know that unexpected problems such as weather and insects can limit the amount of N available to a corn crop, once we set an N rate based on sound data, we should not worry excessively that we have made a mistake by not applying more N. We know from our research that it would take at least 250 lb of N in most fields in a given year to be 95% sure (100% sure is unreasonable) that the crop has no shortage of N. This is very costly “insurance”. Instead, we need to be ready to see minor deficiency symptoms from time to time, and to chalk it up to events that we can’t control. The effect on yield is likely to be fairly minor.