Assessing the Potential for Nitrogen Loss from Heavy Rainfalls
Carrie Laboski, Extension Soil Fertility Specialist, Department of Soil Science, UW-Madison
Several inches of rainfall over the past week and a half have many growers and agronomists concerned about the potential for N loss. The amount of N loss is dependent on soil moisture and drainage along with the form and timing of N. Nitrogen loss can occur through denitrification and leaching.
Denitrification is the process whereby nitrate is converted to the gases dinitrogen or nitrous oxide and subsequently released to the atmosphere. This conversion is carried out by soil bacteria. Denitrification can be a significant mechanism for N loss on medium- and fine-textured soils. It is generally not an issue on coarse-textured soils because they do not remain saturated for any length of time. There are several environmental factors that determine if denitrification occurs and to what extent.
- Nitrate. Nitrate must be present for denitrification to occur. If nitrate is not present or is in low concentrations, denitrifiaction losses will be minimal.
- Soil water content and aeration. Denitrification occurs in wet soils with low oxygen concentrations. Denitrification increase with the length of time the soil is saturated. Standing water may result in a greater percentage of nitrate being denitrified.
- Temperature. Denitrification proceeds faster on warmer soils, particularly when soil temperature is greater than 75°F. Table 1 shows the combined effect of soil temperature and days of saturated soil on N loss.
- Organic matter. Denitrification occurs because soil bacteria are breaking down organic matter under low oxygen conditions and the bacteria use nitrate in a biochemical process. Soils with low soluble organic carbon will have less potential for denitrification than soils with high soluble organic carbon. Thus, nitrate that resides deeper in the soil profile (eg. below 12 inches) where there is less organic matter will have a greatly reduced or minimal probability of being denitrified.
- Soil pH. Denitrification is negligible in soils with a pH < 5.0. Thus, pH likely doesn’t limit denitrification on most of our cropland in Wisconsin.
It is important to keep in mind that nitrate must be present for denitrification to occur. So N losses will depend on the form of N that was applied and the time between application and saturated soil conditions. Table 2 provides estimates of the time it takes for various N fertilizer materials to transform to nitrate. Conversion of ammonium based fertilizers to nitrate takes 1 to 2 weeks. Urea must first be hydrolyzed to ammonium before it is converted to nitrate. If a urease inhibitor (eg. Agrotain) was used with urea, then the length of time that it takes for urea to convert to ammonium may be extended 10 to 14 days depending upon the rate of inhibitor used. Injection of anhydrous ammonia increases the soil pH for several weeks, which in turn limits the amount of ammonium that is converted to nitrate. If a nitrification inhibitor (eg. Instinct, NServe, DCD) was used, it will also extend the time it takes for ammonium to convert to nitrate.
Here’s an example of how to estimate the amount of nitrate that might have been lost through denitrification. Let’s assume:
2) The soil remained saturated for five days.
3) The average soil temperature at a four inch-depth has been 75°F.
In this situation only a minimal amount of the urea and ammonium forms will have been converted to nitrate in four days before the soil became saturated. Thus, only the nitrate portion of UAN is susceptible to N loss. This is 30 lb N/a (120 lb N/a x 25% as nitrate) (Table 2). Approximately 75% of the nitrate could be expected to denitrify in the five days when the soil was saturated resulting in a potential for 22.5 lb N/a to be lost (30 lb N/a as nitrate x 75% of nitrate denitrified) (table 1). Please note that these are estimates of N loss, and should not be considered exact.
Nitrate is the form of N that can be leached when precipitation (or irrigation) exceeds the soil’s ability to hold water in the crop root zone. Leaching is a much bigger issue on sandy soils that typically hold 1 inch of water per foot of soil compared to medium- and fine-textured soils that hold 2.5 to 3 inches of water per foot of soil. To determine if nitrate could leach out of the root zone, compare the rainfall totals in your area to the number of inches of water that your soil can hold in the crop root zone. The amount of N loss from leaching is dependent not only on rainfall, but also on the amount of N in the nitrate form. Use the information in Table 2 to estimate how much nitrate may have been leached.
Urea is highly water soluble. If the leaching rainfall occurred before urea had time to hydrolyze (2 to 4 days), then urea may have leached. However, if there were more than 4 days between urea application and the leaching rainfall, then it is likely that all of the N would have converted to ammonium and remains within the root zone.
Supplemental N applications
If all of the N was applied prior to the heavy rainfall, try to determine how much N loss may have occurred. The next step is to decide whether or not you need or want to apply supplemental N fertilizer to your corn crop. When making this decision, compare the amount of N loss (in lb N/a) that you think may have occurred to MRTN rate and profitable range of N rates for your N:corn price ratio. For example let’s say that corn follows soybean on a high yield potential soil and you applied 150 lb N/a preplant and now estimate that you lost 25 lb N/a. If your N:corn price ratio is 0.05, then the profitable range of N rates is 125 to 160 lb N/a. Thus, even with some N loss, you might still be within the profitable range of N rates. If you are uncertain how much N may have been lost and the corn is clearly deficient in N, then application of 50 lb N/a should result in profitable yield increases.
Where the entire crop N requirement has not yet been applied, sidedress or other postemergence applications should contain the balance of the crop N requirement plus 25 to 50% of the fertilizer N that was already applied.
Options for applying supplemental N, when it is needed, include traditional sidedressing with anhydrous ammonia or N solutions. UAN solutions can also be applied as a surface band. Dry N fertilizers (urea, ammonium sulfate, or ammonium nitrate) can be knifed in or broadcast applied to the crop. Leaf burning from solution or dry broadcast applications should be expected. Appling the dry materials when foliage is dry will help minimize burning. Broadcast N rates should be limited to 90 lb N/a for corn with 4 to 5 leaves and to 60 lb N/a for corn at the 8-leaf stage. Under N deficient conditions, corn will respond to supplemental N applications through the tassel stage of development if the N can be applied.