Nitrogen transformations and loss potential in the soil
Wet soil conditions in the spring create concerns that nitrogen (N) applied in early spring or earlier might be lost. When soils become too wet, the potential for N losses is directly related to the amount of N present in the nitrate (NO3-) form. With the exception of urea-ammonium nitrate (UAN) solutions that contain 25% of the total N as nitrate or ammonium nitrate that contains 50% of the total N as nitrate, most commercial fertilizers being used today are in the form of ammonium (NH4+) or forms that rapidly transform to ammonium (like anhydrous ammonia and urea). In the ammonium form, N is retained in the exchange sites of soil particles and organic matter.
The transformation of ammonium to nitrate, or nitrification, is done by soil bacteria that need warm temperatures and oxygen. In general we had cool temperatures or warm days followed by cool days that likely caused slow nitrification rates. In fields where ammonium-base fertilizers were applied within a few days before soil conditions became excessively wet, the potential for N loss from the fertilizer is minimal as there was not enough time for the fertilizer to nitrify. Further, once soils are saturated the lack of oxygen slows down nitrification and the potential for N loss from N in ammonium form. Since urea is soluble in water, the only concern would be if substantial precipitation occurred soon after urea was applied in well drained fields. In sandy soils or heavily tile-drained soils it is possible to move urea or nitrate as much as a foot for each inch of rain. On the other hand, movement is only approximately five to six inches for each inch of rain in a clay loam or silt loam soil. That said, between rain events nitrate will start to move back up as evaporation from the soil surface create an upward suction force that moves water and nitrate closer to the surface. Similarly, evapotranspiration from actively growing crops will result in a similar suction force in addition to some nitrate uptake by the crop.
Nitrogen loss will occur in fields where N from fertilizer or organic N from the soil was present in nitrate form before the soils became excessively wet. In fine-textured soils, water saturated conditions caused N lost through denitrification. Denitrification rates increase after about a day under oxygen-depleted conditions that result when soil pore space is filled with water. Under these conditions, soil microbes utilize nitrate for respiration, and N is released as a bi-product in gaseous forms that are lost to the atmosphere. For each day the soil remains saturated with water under warm soil temperatures, it is possible to lose as much as 5% of the nitrate-N in the soil. In coarse-textured soils or soils intensively tiled, N loss occurs mostly by leaching below the root zone or into tile lines.
Unlike last year where we started the season with soil profiles near saturation or already saturated, this year soils had great capacity to accumulate water and substantial precipitation was needed to recharge the soil that was still dry from the previous growing season. Once the soil is fully recharged, additional rain can start to move nitrate into tile lines or below the root-zone. While there are areas that have received heavy rains, these tend to be highly localized. In general, soil water conditions have not been conducive to substantial N loss, at least the potential has been less than normal. At the long-term drainage plots in Waseca we have measured 3.7 inches of water in the tile line. The total amount of drained water (and nitrate coming out of the field) so far is lower than most years where we see around 6 inches of drainage by this time.
Nitrogen in corn
Figure 1. Nitrogen deficiency in corn occurs in the lower leaves. Chlorosis develops along the leaf midribs, starting at the leaf tip and moving toward the stalk.
Nitrogen deficiency is characterized by yellowing along the leaf midribs, starting at the leaf tip and moving toward the stalk (Figure 1). Nitrogen deficiency will first be observed on the lowest leaves of the plant. It is important to mention here that sulfur deficiency is often confused with N deficiency. Unlike N, sulfur deficiency occurs in the new (top) leaves and the chlorosis is between veins (Figure 2). The chlorotic areas of N deficient plants will turn brown as the season progresses. Severe N deficiency will result in a poorly-developed crop canopy that will be unable to intercept all of the sunlight during grain fill. Nitrogen-deficient leaves also have a lower capacity for photosynthesis, further limiting the potential for grain fill.
Nitrogen uptake by corn from emergence through the V6 (six leaf collar) stage only represents about 5% of the total plant uptake. However, starting at about the V8 (eight leaf collar) stage, there is rapid accumulation of N by the plant, with about 60% of the total N uptake occurring between V8 and silking. Thus, it is important to detect N-deficient areas now, and that supplemental N is sidedressed on these areas as soon as possible.
Determining the need for additional nitrogen
To determine whether supplemental N should be applied in corn, consider the In-Season Corn N Calculator http://z.umn.edu/ncalculator>. For your convenience we have recently created a free app that allows you to use this tool with your smartphone or tablet: http://z.umn.edu/cropcalc.
You can also access the tool as a printable worksheet: http://z.umn.edu/
Since the tool is most accurate when corn is at V5-6 and currently most corn in the state is around those stages, this would be an ideal time to use the calculator. For entire fields or portions of fields meeting the requirements for supplemental N, this calculator recommends a rate of 40 to 70 lb N/ac, depending on the situation. However, with the high cost of N fertilizer and lower corn prices, producers planning to apply supplemental N based on this tool need to have a clear idea about how much N they should apply. Research from Minnesota showed that supplemental N applied at 30 to 40 lb N/ac is sufficient for corn following soybean on most soils or corn following corn on medium-textured soils such as silt loams. In contrast, higher rates, possibly up to 60 to 70 lb N/ac may be needed on heavy (finer textured) soils where corn follows corn and high amounts of surface residue are present. If you determine that additional N is needed, it would be important to make the application as soon as field conditions are fit. Delaying applications not only risks needing high-clearance equipment (as corn can grow very quickly at this time), but also research has shown that delaying applications past the V6 development stage can result in yield reduction under Minnesota conditions.
Applying additional nitrogen
If additional N is needed, then the next question is how to apply it. Injection into the soil or dribbling the N fertilizer between rows are the best ways to sidedress because these applications can reduce volatilization of urea and protect the crop from foliar damage. If anhydrous ammonia is used for the application, it is important to watch soil conditions to ensure the knife track closes properly to avoid foliage damage by free ammonia escaping to the atmosphere.
When injecting or dribbling are not viable options, broadcast applications of N would then be the next alternative. Urea granules will have the least impact on leaf burn compared to UAN or dry products such as ammonium nitrate or ammonium sulfate. To minimize adhesion of dry products to the leaves, it is best to apply when the foliage is dry. Remember, though, that urea is subject to volatilization if at least a ¼ inch of rain does not fall within 3 to 4 days after application. As much as 30% of broadcast urea can volatilize if there is no rainfall within approximately 10 days after the application. You may want to consider using urea plus a urease inhibitor such as NBPT (Agrotain) to “buy” more time for rain to incorporate the product before volatilization losses occur.
If UAN solution is broadcast over corn when plants are small (about 6 inches) likely the damage will not result in yield loss. Even when plants are bigger (V4 development stage), the foliage damage caused by a rate as high as 90 to 100 lb. N/acre typically does not cause significant yield reduction. One ways to reduce damage from UAN is to apply in advance of rain. If rain falls within a few hours after application, it will wash the fertilizer off the foliage and will also reduce the potential for volatilization of urea. If a broadcast application of UAN is the only option available, try to do it as soon as possible because the smaller the plant, the lesser the potential for foliar damage. If you plan to include herbicide with your UAN application, make sure you read the herbicide label first to make sure such application is allowed. Additionally, be aware that including herbicide with the UAN solution can intensify leaf burn. For example, research conducted in Minnesota showed, addition of 2 lb. atrazine/acre at a rate of more than 90 lb. N/acre at V3 development stage caused severe leaf burning. Applying 2 lb. atrazine/acre at 60 lb. N/acre causes similar leaf burning as applying 120 lb. N/acre with UAN alone.
At sidedress time the use of slow-release products (such as polymer coated urea) is not a good practice because you want N to be available to the crop immediately. While the coating can protect urea from volatilization (just like a urease inhibitor does), it will take time for the coating to breakdown and release N, resulting in a further delay in N availability to the crop. Our research has consistently shown yield reduction when slow release fertilizers are used for sidedressing.
If my crop is too tall already, can I apply N “over the top”?
In some fields crops are getting tall. If N is needed, application of dry products, such as ammonium nitrate and urea, “over the top” can result in foliar damage observed as small lesions when the granules fall into the whorl or leaf axil of the corn plant. Also, as the leaf emerges from the whorl, the margin might be white due to excess N in the leaf. Typically, though, this damage is an aesthetic concern and rarely translates into yield reduction.
As mentioned earlier, “over the top applications” of UAN are the least desirable way of applying N. However, if this is the only alternative and the plant needs more N, the yield benefit from the additional N will likely outweigh the leaf burn caused by the application. Research has shown yield reduction when a rate of more than 60 lb. N/acre was applied at V8 development stage. To avoid extensive foliage damage, when N applications are needed later than the V8 development stage, it would be very important to fit the high clearance equipment with drop hoses so UAN is applied directly on the soil surface without touching the crop canopy.