Navigating Nitrogen Management in Wet Spring Conditions – Estimating Nitrogen Losses

May 28, 2024 4:55 PM
Blog Post

This is part two of a three-part series on nitrogen management this spring. Read part one, “Navigating Nitrogen Management in Wet Spring Conditions – Nitrogen Loss Mechanisms,” and part three, “Navigating Nitrogen Management in Wet Spring Conditions – Does the corn need more nitrogen?”.

Image of saturated soils with water damaged corn.

While part one of this series addressed mechanisms for nitrogen loss, farmers are most interested in how to estimate and account for nitrogen losses. It is important to note that guidelines for N application rates for corn in Iowa inherently account for "normal" N losses, as N rate research trials are conducted in real-world field conditions. These trials incorporate variations in soil N supply and climatic conditions, ensuring a robust foundation for N rate recommendations. However, estimating N loss necessitates various approaches, each offering insights into the dynamic interplay between soil, weather, and crop uptake.

Estimates of Nitrate-N Production and Loss

Back in 2007 and again in 2014 my predecessor, John Sawyer, produced excellent articles outlining methodologies for quantifying nitrate-N formation and loss. These approaches consider factors such as soil saturation duration, temperature, and fertilizer application timing to assess the risk of N loss events. By quantifying the extent of nitrate-N production and potential N losses, farmers can make informed decisions regarding supplemental N application and other management adjustments.

Table 1. Approximate time for different fertilizer sources to convert to ammonium and nitrate.

Fertilizer Source

Approximate time

until Ammonium

Approximate time

until Nitrate

AMS, 10-34-0, MAP, DAP

0 weeks

1 to 2 weeks

Anhydrous Ammonia

0 weeks

3 to 8 weeks

Urea

2 to 4 days

1.25 to 2.5 weeks

UAN

50% from urea in 2 to 4 days

25% is ammonium, 0 weeks

50% in 1.25 to 2.5 weeks

25 in 1 to 2 weeks

25% is nitrate, 0 weeks

Source: Dan Quinn, Purdue University. Adapted from other sources including Havlin et al., 1999

 

Table 2. Approximate percent nitrate-N lost per day via denitrification at different soil temperatures under saturated conditions.

Soil Temperature

Nitrate-N loss per day

°F

%

55 – 65

2 – 3

>65

4 -5

Adapted from Shaver et al., 2023 and Torbert et al., 1993

Let’s consider an example scenario using these methods and the information provided in tables 1 and 2. Let’s assume that 200 units of N per acre as anhydrous ammonia was applied without a nitrification inhibitor in early April. We can safely assume that at least 50% of the applied N has converted to nitrate. Now let’s assume that after the rainfall events over the past two weeks portions of our field are saturated and have been saturated for the past 7 days. Remember that it takes about two days for soils to become anaerobic and denitrification processes to escalate. The average soil temperature over the past past 5 days has been approximately 65°F. Based on these assumptions, we can estimate that approximately 20 to 25 pounds of N per acre have had the potential to be lost via denitrification from the saturated portions of the field over the past 5 days. Using the same scenario except assuming the anhydrous application occurred in the fall and 100% has already converted to nitrate the potential N losses via denitrification over the 5 days would be estimated at 40 to 50 pound of N per acre. If the soils remained saturated for an additional 5 days at 65°F the potentials N losses via denitrification would be estimated at 40 to 50 pounds of N per acre for the spring application and 80 to 90 pounds of N per acre for the fall application. Remember that these estimates do not include potential leaching losses of N via tile drainage which is likely to be occurring under these saturated conditions; thus, the actual estimate of total potential N losses would be greater. So remember that, while these calculations can provide insight, they are merely educated estimates as there are countless sources of variation that will affect actual N losses.

Spring Rainfall Triggers

Details of this approach were originally presented in this 2016 ICM News article and update in this 2018 ICM News article. Based on long-term N rate trials conducted in Iowa from 1999-2016, criteria for triggering supplemental N applications have been established based on springtime rainfall accumulation. The data suggests that springtime precipitation can be used to predict the likelihood of needing an Economic Optimum Nitrogen rate of at least 20 lbs N ac-1 greater than the Maximum Return to Nitrogen (MRTN) rate recommended by the Corn Nitrogen Rate Calculator (CNRC) (Table 3). Why at least 20 lbs N ac-1? Because the cost of application would likely outweigh the benefit of applications less than 20 lbs N ac-1.

Table 3. Corn Nitrogen Rate Calculator recommendations for Iowa using a 0.1 fertilizer to grain price ratio.

CNRC Iowa Region

Crop Rotation

MRTN, lbs N ac-1

Main

Corn following Soybeans

145

Corn following Corn

187

Southeast

Corn following Soybeans

154

Corn following Corn

208

Current guidance is to monitor rainfall totals from April 1 – June 30 for most of Iowa, but for Southeast Iowa, rainfall totals should be monitored from March 1 – June 30. Rainfall totals exceeding 15.5 inches for most of Iowa or 17.8 inches for Southeast Iowa may indicate potential N losses significant enough to create an N deficit requiring a supplemental N application. Based on these rainfall totals, there is a 76% chance of accurately estimating the need for a supplemental N application, which isn't perfect but does provide fairly good odds of correctly determining the need for supplemental nitrogen. Waiting until the end of June to assess rainfall totals isn't necessary; in fact, ongoing monitoring allows for proactive planning. Current statewide precipitation maps (Figures 1 and 2) suggest that some regions in Iowa are approaching these critical rainfall thresholds, but local rainfall measurements will provide more precise data.

Map of accumulated precipitation in Iowa from April 1, 2024 to May 28, 2024.
Figure 1. Accumulated precipitation for Iowa from April 1, 2024 to May 28, 2024. This is the map that the majority of Iowa would reference. Source: Midwest Regional Climate Center

Map of accumulated precipitation in Iowa from March 1, 2024 to May 28, 2024.
Figure 2. Accumulated precipitation for Iowa from March 1, 2024 to May 28, 2024. This is the map that Southeast Iowa would reference. Source: Midwest Regional Climate Center

The thresholds mentioned above are informative but not foolproof due to variations in N supply and loss. Factors such as soil type, weather conditions, and rainfall intensity contribute to uncertainty around N application thresholds. For example, short-term high-intensity rainfall events that cause runoff rather than infiltration may have a reduced impact on soil N losses. On the other hand, sustained rainfall events leading to soil saturation and surface ponding can increase losses due to denitrification. The timing of rainfall events can also significantly influence N losses. High precipitation levels in the fall could worsen N losses where fall anhydrous ammonia applications were made, potentially leading to the need for supplemental N applications. Conversely, early spring precipitation in fields where N applications have yet to be made may not result in significant N loss due to colder soils resulting in less nitrate buildup. Additionally, rainfall totals from a single month (e.g., April or May) do not yield the same accuracy in estimating supplemental N needs as including June rainfall. Therefore, variation in N loss, additional N need, and potential crop response will vary among fields. It's important to note that the thresholds described above are linked to suggested MRTN N rates from the CNRC. If lower or higher N rates have already been applied to the field, then the likelihood of needing a supplemental N application based on the thresholds increases or decreases, respectively.

Consider a supplemental N application if current rainfall totals at your farms (until the end of June) are near or above the thresholds and your current N application is less than or equal to the suggested MRTN rate. However, note that relying solely on precipitation totals may not accurately determine the supplemental N rate needed. While directionally correct (i.e., more precipitation above the indicated totals implies greater N need), this method cannot be precisely calibrated. It is generally suggested to limit supplemental N applications to 50 lbs N ac-1 or less if you've applied close to the MRTN rate. Adjustments should be made based on your applied N relative to the MRTN rate; if significantly below, more than 50 lbs N/acre could be considered, while if well above, the likelihood of an N deficit is reduced.

Late Spring Soil Nitrate Test (LSNT)

The Late-Spring Soil Nitrate Test (LSNT) is a valuable tool for estimating soil nitrogen availability for corn in Iowa. According to Iowa State University Extension and Outreach (CROP 3140), the LSNT should be conducted when corn is 6 to 12 inches tall, typically from late May to early June. Due to this year’s wet conditions and planting delays, corn may not reach this height by early June. However, we still advise collecting LSNT soil samples in the first two weeks of June, regardless of corn height. The LSNT is not calibrated for and should not be used for interpretation of samples taken beyond this time frame.

The LSNT measures nitrate-N in the top foot of soil to guide sidedress nitrogen application. It accounts for residual nitrate, mineralized organic matter, and any nitrate converted from fall or early spring-applied nitrogen. Because nitrate can leach below the top foot with excess rainfall, test reliability varies, particularly with the wet spring we have had this year. To use the LSNT results, first compare them to the state’s calibrated critical level of 25 ppm. If results are above the critical level, no additional N application is suggested. If below, while not a perfect calibration, we can estimate supplemental N rates by subtracting the test result from the critical level and multiplying by eight. For example, if the test result is 15 ppm, subtract 15 from 25 to get 10 ppm, then multiply by eight to get a suggested application rate of 80 lbs N ac-1.

Given this spring's wet conditions and the effect they may have on soil nitrate levels, consider adjusting the critical value to 20-22 ppm, especially if rainfall at your farm has been more than 20% above normal from April 1 to the sampling time (Figure 3).

Map of accumulated precipitation in Iowa from April 1, 2024 to May 28, 2024 as a percent of the average from 1990 to 2020.
Figure 3. Percent of average rainfall for Iowa from April 1, 2024 to May 28, 2024. The average used for comparison is based on data from 1999 - 2020. Values less than 100 indicate less than average rainfall while values greater than 100 indicate greater than average rainfall. Source: Midwest Regional Climate Center

As we conclude part two of our series on nitrogen management in wet spring conditions, we've explored methodologies for estimating nitrogen losses, crucial for informed decision-making in unpredictable weather, and now turn to the pressing question of whether additional nitrogen is needed for corn. Join us in part three as we navigate the complexities of determining supplemental nitrogen requirements and explore practical approaches for enhancing corn production in challenging spring conditions.

This was part two of a three-part series on nitrogen management this spring. You can read part one, “Navigating Nitrogen Management in Wet Spring Conditions – Nitrogen Loss Mechanisms,” if you missed it, or continue the series with part three by reading “Navigating Nitrogen Management in Wet Spring Conditions – Does the corn need more nitrogen?”.

References:

Bechman, T.J., 2024. “Determine how much nitrogen is left.” Indiana Prairie Farmer, May 6, 2024. https://www.farmprogress.com/corn/determine-how-much-nitrogen-is-left

Sawyer, J., 2007. “Estimating nitrogen losses – early spring 2007.” Iowa State University Extension and Outreach Integrated Crop Management Encyclopedia, May, 14, 2007. https://crops.extension.iastate.edu/encyclopedia/estimating-nitrogen-los...

Sawyer, J., 2014. “Estimating Nitrogen Losses in Wet Corn Fields.” Iowa State University Extension and Outreach Integrated Crop Management Blog, June 17, 2014. https://crops.extension.iastate.edu/cropnews/2014/06/estimating-nitrogen...

Sawyer, J., 2016. “Precipitation and nitrogen this spring.” Iowa State University Extension and Outreach Integrated Crop Management Blog, June 6, 2016. https://crops.extension.iastate.edu/blog/john-sawyer/precipitation-and-n...

Sawyer, J., 2018. “Potential Nitrogen Loss- 2018.” Iowa State University Extension and Outreach Integrated Crop Management News, June 22, 2018. https://crops.extension.iastate.edu/cropnews/2018/06/potential-nitrogen-...

Sawyer, J.E., and Mallarino, A.P., 2017. Use of the Late-Spring Soil Nitrate Test in Iowa Corn Production. Iowa State University Extension and Outreach Publication CROP 3140. https://store.extension.iastate.edu/Product/5259

Shaver, T.M., Wortmann, C.S., Shapiro, C.A., Hergert, G.W., Anderson, B.E., Pavlista, A.D., 2023. Nutrient Management for Agronomic Crops in Nebraska. UNL Extension Publication EC155. https://extensionpubs.unl.edu/publication/9000016363764/nutrient-managem...

Torbert, H.A., Hoeft, R.G., Vanden Heuvel, R.M., Mulvaney, R.L. and Hollinger, S.E., 1993. Short‐term excess water impact on corn yield and nitrogen recovery. Journal of production agriculture6(3), pp.337-344.