Issue No. 8, Article 14/May 18, 2007
Nitrogen Loss from Surface Application of Urea-Containing Fertilizer
A number of producers in Illinois applied urea-containing fertilizer on the soil surface after corn was planted this spring, under the not-unreasonable assumption that it would rain within a few days to move the fertilizer into the soil. Where it did not rain for a week or more, there is concern that some of the N has been lost. Some are even considering applying more N now, while the crop is still small.
The fertilizer material we are concerned about is urea, including both dry urea and the urea that makes up half of the nitrogen in 28 or 32% UAN solutions. The other half of the N in UAN is in the form of ammonium nitrate, which is not subject to loss by volatilization into the air. So at most, only half of the N in UAN can be lost. Because most of the surface-applied material in Illinois this spring has been UAN, we will only concern ourselves with this material.
Urea can be attacked and broken down into ammonia, carbon dioxide, and water through the action of an enzyme called urease. If this reaction takes place on the soil surface, the ammonia is susceptible to loss via volatilization. There is a lot of urease in crop residue, and its action is speeded up when temperatures are high and when the soil surface is moist. High pH also increases loss of ammonia from urea. Application to warm, wet soils with corn residue and high pH on the surface can mean loss of up to half of the N from urea within a week. On the opposite end of the scale, in fields where corn follows soybean and where tillage was done, there may be little or no loss of N from surface-applied urea even when it stays dry after application.
Incorporation of UAN or urea is the most common way to prevent such volatilization loss. It is possible to include with urea or UAN a urease inhibitor, such as Agrotain, to prevent volatilization. This is effective, but the need for, and cost-effectiveness of, urease inhibitor will vary depending on conditions after urea application. The best protection against loss of N from UAN surface-applied without an inhibitor is to have a quarter inch or more of rainfall soon after application. This moves the urea into the soil where urease activity is lower, and where released ammonia can be trapped in the soil water.
Lack of rain after application, moist soil surfaces, high temperatures, and a lot of crop residue on the surface all increase the potential for N loss. When surface soil pH is less than 7, which is common in Illinois, losses are reduced. While we have reason to worry when it stays dry and warm after surface application of UAN, work in several states has shown that N losses from urea seldom exceed 20%, and that even a small amount of rainfall (a tenth of an inch) is enough to reduce N loss.
Some people have thought that incorporation using a rotary hoe might help incorporate urea and thus reduce loss. Such incorporation is not very thorough, but even a small amount of incorporation might help protect some of the urea, so use of the rotary hoe is probably better than nothing. The cool temperatures after May 16 will reduce loss rates as well, but some N loss will continue where there has been no rain.
Because plants are taking up N slowly now, and because the chances that N loss from surface-applied UAN might be less than many people expect, adding more N now is not appropriate. Remember that the roots in dry areas are probably unable to take up N that was applied to the surface, and that half of the N in UAN is not subject to loss. So deficiency symptoms may not mean that a lot of N has been lost, only that the plant does not have access to it.
Those who feel a need to do more than use a rotary hoe might consider applying 50 or 60 lb of N (injected) in a few strips in a field, and then watching to see if the rest of the field stays lighter green than the added-N strips. Even if the field is slow to green up, no action should be taken to correct possible N loss unless plants continue to show deficiency into late vegetative stages (chest-high or so, perhaps stage V12). Research has shown that mild N deficiencies can be corrected without yield loss up to late vegetative stages, so there will be time to do this if necessary.
Finally, remember that N is only one factor that affects corn growth and yield. If corn was planted when soils were too wet, if soil or sidewall compaction are restricting root growth, or if there are disease or insect problems, then adding more N will probably not be effective. Be sure that the problem is really N deficiency before attempting to use expensive N to fix it.--Emerson Nafziger, Bob Hoeft, and Fabián Fernández