In both the 2008 and 2009 growing seasons, excessive spring rainfall across the Corn Belt and other areas of North America created concerns that much of the nitrogen (N) applied in the fall and spring may have been lost. Not only does lost N reduce grower profits, but wet weather also presents challenges for re-supplying a deficient crop. This Crop Insights will discuss assessing and managing nitrogen losses in corn due to excess rainfall.
Nitrate Form at Risk of Loss
Nitrate (NO3-) is the form of nitrogen most readily taken up by the plant, but it is also the form with the greatest risk for loss. In fact, with heavy rainfall and highly saturated soils, N loss potential is directly related to the amount of nitrogen in the nitrate form. In such situations, nitrate may be lost from the soil either by leaching or denitrification, depending primarily on soil characteristics. Coarse-textured soils allow water and nitrates to move readily downward through the soil profile. When this leaching places nitrates below the root zone, they are of no use to the plant and are essentially lost (Figure 1). Fine-textured soils, on the other hand, have capillary pores that hold water tightly, restricting its downward movement. In this situation, saturated soils and anaerobic conditions may result in nitrate being lost upward to the atmosphere through denitrification (Figure 1).
Nitrogen Reactions in Soil Determine Losses
When nitrogen is applied to a field in the ammonia (NH3) or ammonium (NH4+) form, it is temporarily protected from water-induced losses. This is because it binds to soil particles, which are negatively charged. Denitrification and leaching are not a risk until the ammonium is converted to nitrate in a bacteria-mediated transformation called nitrification. This occurs in a two-step process - in the first step, the bacterium Nitrosomonas converts NH4+into nitrite (NO2- ) and in the second step, Nitrobacter bacteria convert NO2- to NO 3-.
The activity of nitrifying bacteria is minimal at temperatures below 50 F. The bacteria also need aerobic (unsaturated soil water) conditions to nitrify ammonium. Thus, the amount of nitrification that occurs in the soil depends largely on soil temperature and the time elapsed from application until the soil becomes saturated with water. Further, the nitrification process can be reduced with the use of inhibitors that will lower the activity of these bacteria and allow N to stay in the ammonium form for a longer period. The use of nitrification inhibitors is most important in fall since the time elapsed between application and crop uptake is longest for fall applications.
It is important to note that only the portion of the applied N that is in nitrate form is subject to denitrification or leaching. The fact that N is in the nitrate form does not mean that N is lost; rather, it means it is susceptible to loss.
Sidedress N application allows for more efficient fertilizer use since the nitrogen is applied close to the period of maximum nitrogen uptake for corn (Figure 2).
As Figure 2 demonstrates, maximum N uptake in corn occurs in periods of maximum growth - roughly from growth stages V9 to V18 (the "exponential growth" stages of vegetative development). The risk of N loss is lowest during this period.
All common N fertilizers can be used for sidedress applications. Dr. Fabian Fernandez at the University of Illinois ranks N fertilizers from most to least desirable for sidedress application (Fernandez, 2008):
The advantages of using a meter to evaluate N status are that the meter is fast, non-destructive, repeatable, and can detect a deficiency long before it is visual to human eyes.
Aerial images are another way to assess the potential nitrogen needs of a corn crop, allowing growers quickly to assess many fields in a short period of time (Figure 4).
Aerial photographs as a tool for diagnosing deficiency work best if taken when the corn is waist high or taller, so that soil color is not an issue. However, this limits rescue N application options to high-clearance equipment or airplanes. Aerial photographs also work best after the field is no longer saturated and the corn has time to take up whatever nitrogen is left. Corn will appear nitrogen-deficient (and will actually be nitrogen-deficient) in saturated soils even if the soil nitrogen has not been lost.
Once corn is too big to pass through with a tractor and sidedress applicator, nitrogen application options are somewhat restricted due to machinery limitations, nitrogen source options, and dependency on rain to move the nitrogen into the soil profile. UAN solution dribbled between rows with high clearance equipment is an effective rescue N treatment, but broadcast UAN solution causes severe yield loss due to leaf burn.
Another option is a broadcast application of granular N (ammonium nitrate or urea) by airplane, with urea being the far better choice when corn is two feet or taller in height. Ammonium nitrate has one advantage over urea - it will not be subject to volatilization losses on the soil surface like urea. However, ammonium nitrate can cause much greater leaf burn to large corn, especially to plants that are wet from rain or dew. This occurs as moisture on the leaves allows some of the nitrogen fertilizer to stick to the leaves, resulting in tissue burn (Figure 5). With aerial application, it is safest to keep rates below 125 lb N/acre.
In research at the University of Missouri, Dr. Peter Scharf found broadcast urea causes almost no yield loss due to leaf burn. In his studies, 150 lb N/acre as urea was applied on corn up to four feet tall, and the yield difference between broadcast and in-row placement of urea was measured. The difference was generally less than 4 bu/acre in favor of in-row placement. Ammonium nitrate, on the other hand, caused substantial leaf burn and yield losses of almost 20 bu/acre when broadcast over 3- or 4-foot corn, 8 bu/acre when broadcast over 2-foot corn, and no yield loss when broadcast over 1-foot corn. This work indicates that urea may be a better N source than ammonium nitrate for aerial application on all but the smallest corn. University of Missouri studies have also shown that full or nearly full yield can be achieved if rescue N is applied by the time the corn tassels.
The disadvantage of urea broadcast on the soil surface is that it reacts with the urease enzyme present in plant material. The reaction releases the ammonia in urea, which may then be partially lost to the air. Rainfall is needed to move the urea into the soil to prevent these losses. Urease inhibitors, such as Agrotain, can be used with urea to slow the conversion to ammonia. However, even with use of Agrotain, rainfall is still needed to move the urea into the soil for plant uptake.
In most dryland areas, there is less likelihood that corrections to deficiencies can be successfully made after pollination. Nitrogen could be aerially applied but rainfall is necessary to move the applied N into the soil profile for root uptake. Root systems are also starting to decline in size and activity once ears reach the R3 stage. This combination of factors makes it much less likely that application of a foliar-safe form of N will remediate a late-season nitrogen deficiency.