Nitrogen and the 2013 Corn Crop

The NASS report indicated that corn planting in Illinois was 89% complete by May 26. This leaves more left to plant than we’d like, and it’s still wet in some areas, so we expect a long “tail” to corn planting this year, unless some of the acres intended to corn get switched to soybeans.

The rainfall that delayed planting across Illinois this year is also affecting nitrogen fertilizer management. For many, the rush to get the crop planted meant abandoning or modifying plans to apply N before or after planting or before tillage. Some ammonia was applied last fall or in early April, and some producers were able to broadcast UAN before or after planting. But in many cases planting took precedence, and where it remains wet it’s likely that a lot of acres still have not had N applied.

Later-applied N has little time in which to be lost before plant uptake begins. This means lower overall loss potential and so less need to apply “insurance” amounts of N. As an example, a plan to split-apply N – right after planting and again at sidedress – might, if the first application couldn’t be made before the crop emerged, now be modified to apply less total N but in a single application.

It is important to get N applied before the crop becomes deficient. But the chance of having N deficiency early in corn planted into warm soils is fairly small. One reason for this is that small plants take up little N – by the time the corn crop reaches V5 (5 leaves fully emerged) it has taken up only about 10 lb of N or so per acre. The other reason is that mineralization – the production of plant-available N from soil organic matter – is getting underway as soils warm up. Hence we tend to see little sign of N deficiency when temperatures are warm early in the season, even where little or no N fertilizer has yet been applied.

I wrote in early April about spring sampling for N to see how much we might have left after last year’s drought and the higher than normal amounts of soil N found last fall. Volunteers this spring sent in about 60 samples taken in the same place and to the same depth as samples last fall. As we can see in Figure 1, nitrate-N levels dropped from the fall to the spring, especially in the top foot. Ammonium-N levels tended to be close to what we might consider a “baseline” of about 5 ppm in both fall and spring.

Figure 1. Soil N in 60 Illinois fields sampled in both fall 2012 and early spring 2013.

If we calculate plant-available N as the totals (ppm x 4 for each foot of depth) of nitrate-N and ammonium-N in the top 2 ft, we find that N dropped from about 180, 150, and 180 lb N/acre in northern, central, and southern Illinois last fall, to about 140, 105, and 112 lb N/acre this spring, respectively. This represents disappearance of about 23, 30, and 38 % in northern, central, and southern Illinois, respectively, or about 42, 46, and 68 lb N/acre.

Should there be any adjustment of this year’s N rate based on the amount of N in the soil this spring? In the earlier Bulletin article I suggested making adjustments only if there were more than 10 ppm nitrate-N in the top foot in spring samples. Most of the spring samples had less than 10 ppm, and it’s likely that even some of that N was moved out of the top foot by rainfall after sampling. Thus we doubt that any adjustments should be made based on amounts of soil N present this spring.

On the large scale, increases in nitrate-N concentrations in river-fed reservoirs now being reported are largely the result of loss of nitrate-N from soils through tile lines. With the large amounts of nitrate in the soil found last fall and with rainfall at or above average over the past three months in Illinois, it’s no surprise that some of the leftover soil N from last year entered tile lines and moved out of fields. While fall-applied N might have contributed in a small way to this, the large amount of soil nitrate present at harvest last fall is certainly the major source of nitrate in drainage water. It’s also certain that N in drainage water is truly “lost” to this year’s crop.

Sampling results from Dan Schaefer of the Council on Best Management Practices provide some hints about availability of N from fall fertilizer applications. One Champaign County field that was in drought-affected corn in 2012 had 16 ppm of nitrate-N and 3 ppm of ammonium-N in the top foot of soil last fall, and 15 and 4 ppm, respectively, in the second foot, for a total of 152 lb. N. The producer applied 100 lb. N as NH3 in November using N-Serve; adding these amounts gives a total of 252 lb. N/acre in the soil last fall. Samples taken in early May this spring had 10 and 13 ppm nitrate-N and ammonium-N in the top foot, and 14 and 4 ppm in the second foot, respectively, for a total of 164 lb. N/acre. Most of the 88 lb N/acre that disappeared from the top two ft of soil probably came from the 124 lb. of nitrate-N present last fall. The 10 ppm difference in ammonium-N would suggest that about 40 percent of that applied in the fall is still in this form.

In a second field, 170 lb N was applied as NH3 with N-Serve on soybean stubble last November. In early May, it had 16 ppm nitrate-N and 10 ppm ammonium-N in the top foot and 16 and 5 ppm, respectively, in the second foot. Based on finding about 5 ppm of both nitrate and ammonium from outside the application band, we called that a baseline amount and subtracted it to calculate that about 108 lb. of N (64 percent) remained in the top two ft from last fall’s application, and that about 80 percent of it was in the nitrate form.

So some of the N that was present last fall moved deeper in the soil, and some moved out of fields in drainage water. Still, a considerable amount of N is present this spring, and it is not clear that the amounts “lost” are greater than normal. Applications were made in a way that minimized nitrification last fall, and soil temperatures were normal this winter. So while any nitrate-N present last fall would have been able to move with rainfall starting in late winter, it’s not likely that fertilizer N would have been mobilized (in nitrate form) earlier than normal. It is possible that some of last fall’s N was also taken up as crop residue started to decompose; this would look like a “loss” but such N might become available this season as residue breakdown continues.

A major concern, especially in those parts of Illinois that have been the wettest over the past three months, is whether or not N applied last fall or early this spring will still be available for the crop this spring. Because so much depends on what happens during the season, there’s not a clear answer to this question, but we can consider some of the factors that will affect N availability to the crop.

With soil temperatures now in the mid- to upper 60s in central and southern Illinois, the biological processes that convert ammonium to nitrate are gearing up, though they are still only at about half-speed compared to what they will be when soil temperatures reach the upper 70s or lower 80s over the next month. Conversion to nitrate only means increased potential for loss; loss still requires enough water to move the N out of the rooting zone.

In the more saturated parts of the state, denitrification – the conversion of nitrate to gaseous forms of N – may be causing some loss of N at this point, though we wouldn’t expect losses from this to be as large as they would be with warmer soils. The length of time water stands on the soil will directly affect how much N is lost.

Even with the movement of some N to more than two feet deep and some N loss from the field altogether, it is difficult to predict that we’ll see N deficiency if normal N fertilization practices are used. As mentioned above, N produced from soil organic matter is an important source of N for the crop. We normally expect about 2 percent of the N contained in soil organic matter to be mineralized over the course of the season. Soil organic matter is about 5 percent N, so a soil with 3 percent organic matter in the top foot (about 4 million lb. of soil) contains about 6,000 lb of organic N. At 2 percent, about 120 lb of N would mineralize in a season.

Mineralization occurs whenever there are favorable temperatures and moisture conditions. So it is a source of N for the crop throughout the season, but the mineralization rate doesn’t keep up with the N needs of the crop once rapid uptake begins. In fields that were in corn the previous year we also expect some of the soil N, including mineralized N, to be tied up as microbes continue to break down old stalk and root tissue for some weeks after soil temperatures warm up.

We would expect the N uptake rate to exceed the rate of mineralization at some point in vegetative growth, perhaps at about stage V6 or V7. With late planting into warmer soils this year, mineralization might have gotten a little bit of a “running start” so might provide enough N to carry the crop a little longer. Warm temperatures also tend to bring the crop to each vegetative stage with a little less dry weight, so N demands may be lowered slightly during plant development. On the other hand, rainfall can move some mineralized N deeper and out of reach of the roots.

Another important factor in determining how much N will be available to the crop is how well the crop will be supplied with water. With the rush to plant this year, some fields were planted before the soils were as dry as would have been ideal. This means that there was some soil compaction, and possibly some sidewall smearing. The fact that it has not dried out much since planting means that nodal root penetration is not likely to be a problem this year like it was in 2012, however.

Wet soils will help roots proliferate well enough, but they will tend to stay shallow due to higher oxygen levels at the surface. Nitrogen moves to roots with water and is taken up as water enters the plant. Under wet conditions, water uptake comes from shallow roots, and N deeper in the soil can remain unavailable. Once the surface starts to dry out, however, roots will grow deeper and water will move towards the roots, carrying N from deeper in the soil.

Movement of N from deeper in the soil under drying conditions could considerably improve the N supply in cases where the N has moved deeper but is still in the soil. How much of this might happen this year depends on the weather in the coming weeks. If we get a stretch of dry weather for a week or so, we can expect rapid root growth with increased soil oxygen. But with soils holding maximum amounts of water now, it could take several weeks of relatively dry weather before water and N start to move up from deeper in the soil. If it stays wet, this might never happen, or at least not in time to stave off N deficiency.

So have we “lost” enough N to justify reapplying some amount, or applying more than we had planned to apply? In parts of western Illinois where May rainfall has been well above normal, losses through tile lines might be considerable, and it may make sense to increase the sidedress rate to replace some of any N previously applied. The replacement rate should be tied to how much of the previously-applied N is likely to be, or to have been, in the nitrate form before rainfall events. The earlier the application and the more N applied as nitrate, the greater the potential for loss up to now. For ammonia applications made in early April, there’s little reason to expect that a lot of N has been lost from the field.

Nitrogen loss is likely to be low or moderate in areas with normal rainfall in May. In these areas, it might be prudent to wait to see what the weather does over the next few weeks before applying additional N to those fields where N went on early. If it dries out and warms up, crop growth should take off and N supply should improve. If it returns to wet weather, especially warm and wet, then some supplemental N might be considered.

In cases where most of the N still needs to be applied, application should be made as soon as possible, with whatever form works best, using normal rates, or even a little less than previously planned. Do not apply solution UAN over the top of the emerged crop, and inject UAN or use urease inhibitor with urea or UAN applied on the surface, especially if the weather is warm and dry at the time of application. Then hope that rainfall returns to normal so the applied N can be taken up and used well by the crop.