Rainfall totals over the past week (June 19 to 26) in the southern half of Wisconsin range from 1 to 15 plus inches. Many soils are saturated and some fields have had or still have standing water in all or part of the field. The million-dollar question is: How much nitrogen (N) loss should I expect from denitrification or leaching and what should I do about it? To answer this question, we’ll consider each situation independently.

Denitrification

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 soil. 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.

1. Nitrate. Nitrate must be present for denitrification to occur.
2. 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.
3. Temperature. Denitrification proceeds faster on warmer soils, particularly when soil temperature is greater than 75°F.
4. 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. 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.
5. 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.

Table 1 shows the combined effect of soil temperature and days of saturated soil on N loss. Current soil temperatures vary throughout the state, but have been in the 60 to 72°F range at many locations over the last week. Thus, there is the possibility for significant N loss if soils remain saturated for more than three days and soil temperatures stay warm.

Table 1. Estimated N losses from denitrification as influenced by soil temperature and number of days the soil is saturated. (From Shapiro, University of Nebraska)

Here’s an example of how to estimate the amount of nitrate that might have been lost. If 120 lb N/a as UAN was applied after planting corn and four days before saturated soil conditions existed and the soil remained saturated for five days, you might expect 20-25 lb N/a to have been denitrified. 120 lb N/a x 25% = 30 lb N/a in the nitrate form, assuming minimal conversion of ammonium and urea to nitrate (Table 2). 30 lb N/a as nitrate x 75% of nitrate denitrified over 5 days = 22.5 lb N/a lost. Please note that these are estimates of N loss, and should not be considered exact.

Another method that could be used to assess the N status of your fields is to use the pre-sidedress nitrate test (PSNT). If the concentration of N in this one-foot soil sample is greater than 21 ppm, then there should be adequate N for the crop. There are a couple caveats when using the PSNT in this manner. First, it will work best if N was broadcast rather than band applied. Soil samples collected from fields where N was banded, may not accurately represent the N status of the field. Second, even in medium- and fine-textured soil, nitrate may have moved into the second foot of soil. In this case, the PSNT won’t measure all of the N that is in the root zone and available for the crop.

If all or most of your N for corn is coming from an organic N source (manure and/or forage legume), then the PSNT can still be used to estimate N credits that are subtracted from your selected maximum return to N (MRTN) N rate. Note: when average May-June soil temperatures are more than 1°F below the long-term average, the N credit is often underestimated. The PSNT is not recommended on coarse-textured soils or where corn follows soybean. For more details on how to use the PSNT see UWEX Publication A2809 Nutrient application guidelines for field, vegetable, and fruit crops in Wisconsin.

Remember that the PSNT measures the amount of N in the nitrate form only; ammonium is not measured by the PSNT. If a nitrification inhibitor was used when N was applied, it is possible that N will still be in the ammonium form and thus available to the crop. Testing for ammonium in a PSNT sample could be useful to determine the N credit if a nitrification inhibitor was used. The total concentration of nitrate plus ammonium (in ppm), can be compared to the PSNT N credit table on page 48 of UWEX Publication A2809 Nutrient application guidelines for field, vegetable, and fruit crops in Wisconsin. Most soil testing laboratories will measure ammonium as well as nitrate in a PSNT sample if requested.

If all of the N was applied prior to the heavy rainfall, try to determine how much N loss may have occurred using one or a combination of the methods just described. 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 the 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 130 lb N/a preplant and now estimate that you lost 25 lb N/a. If your N:corn price ratio is 0.10, then the profitable range of N rates is 105 to 130 lb N/a. Thus, even with some N loss, you might still be within the profitable range of N rates. For more information on the MRTN, see UWEX Publication A2809 Nutrient application guidelines for field, vegetable, and fruit crops in Wisconsin.

Remember the greatest yield increase comes from the first 50 lb N/a applied to the crop. 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. If you estimate that 100 lb N/a or more may have been lost then apply supplemental N at a rate equal to about 50% of the amount of N lost.

Where the entire crop N requirement has not yet been applied and where N loss is suspected, sidedress or other postemergence applications should contain the balance of the crop N requirement plus 25 to 50% of the amount of 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 or as a broadcast spray over the growing crop. Dry N fertilizers (urea, ammonium sulfate, or ammonium nitrate) can also be 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.  Approximately ¼ inch of rain is needed within 2 days to minimize ammonia volatilization from surface applied urea containing fertilizers. If this rainfall is uncertain, a urease inhibitor can be applied with the urea containing fertilizer to minimize volatilization losses by extending the time before rainfall to 7 to 10 days.

Leaching

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. Rainfall totals over the past week may have caused nitrate leaching out of the root zone for corn (~36 inch root zone) grown on sandy soils. 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. Using the information in Table 2, it is possible 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.

Nitrogen best management practices for corn on sandy soils is to sidedress or split apply N. If sidedress N applications have not yet occurred, then growers should proceed as planned. If split N applications have occurred, supplemental N should be applied and should equal the approximate amount of nitrate that may have leached out of the root zone. Corn grown on irrigated sandy soils are highly responsive to N fertilization. On non-irrigated sandy soils, water (usually too little) limits crop yield more than N. Under N deficient conditions, corn will respond to supplemental N applications through the tassel stage of development if the N can be applied.

For irrigated fields, N solutions can be injected into the irrigation water (fertigation). Water application rates should not exceed the infiltration rate of the soil and should not exceed the soil’s ability to hold the water in the root zone of the crop. Thus, if the soil profile is full of water, you may need to wait a few days before fertigating. The key is to manage the water so that the N fertilizer that is being applied is not leached.

Summary

In the southern half of Wisconsin, some N losses may be expected on fields were N has already been applied. The amount of N loss will vary with soil texture, amount of rainfall, form of N applied, use of nitrification and urease inhibitors, and time elapsed between N application and rainfall. Therefore, each field may need to be assessed independently to estimate N loss and determine a course of action.