Risk of Late-Season Nitrogen Deficiency

Click the map to select your location.
Or zoom in by selecting your county from the menu on the right.
The blue marker shows your selected location. The red dot shows the nearest weather center to the selected location which is present in the database.
  1. Engage farmers and agronomists in the participatory learning and educate them about the complex impact of rainfall, management, and cropping system on the risk of the-end-of season corn N deficiency.
  2. Conduct “what-if scenarios” analysis to estimate the risk of specific farmer practices to be deficient given the field management, different N rates, and observed, historical or projected spring rainfall.
  3. Enhance 4R (right rate, timing, form and method) education using large-scale data from production fields over a long period of time.
This information is also provided in the relevant sections of the tool.
  1. Select your area of interest by clicking on your county using the map in the left navigation panel or selecting it from the dropdown menu.
  2. Select your crop rotation in the left navigation panel: corn after corn or corn after soybean.
  3. Click on the “Risk based on Rainfall” tab, and in the left navigation panel enter Nitrogen Rate (lbs) using either the Slider widget or the ‘Manual N Rate:’ input box.
  4. Select the boxes in the “Management Practices” column. Multiple management practices can be selected.
  5. Review the output probability graph, paying particular attention to where the “Current Cumulative May through June Rainfall” vertical line crosses the probability line(s) for each management practice(s). The color legend on the vertical axis of the graph indicate the nitrogen deficiency risk categories: dark red, “Very High”; red, “High”, light red, “Moderate”, pink, “Low”, yellow, “Very Low”. The dark and light blue vertical bands show, respectively, the range of historical rainfall in 50% and 90% of years.
  6. Check the “Contextual Information” below the graph. The probability numbers from Step 5 are shown in the Table and indicate risk categories of N deficiency. Change the N rate to see how the risk probability changes.
  7. Click on the “Risk based on Nitrogen Rate” tab. See how the probability changes when the N rate is higher or lower by 15 or 20 lbs N/acre from your original Nitrogen application rate. Probabilities for the observed rainfall are shown by a red line; median historical rainfall for your location are symbolized by a blue line. The light blue band shows the range of rainfall in 50% of historical years.
  8. Check the “Contextual Information” below the graph. The probability numbers from Step 7 are shown in the Table and indicate the risk category.
  9. Click the “Rainfall Maps” tab. This tab contains two maps: Cumulative May through June rainfall for the current year (upper) and the deviation from the historical rainfall averages (lower).  If the tool is used between May and June, the cumulative daily rainfall will be calculated until the current date.
  10. Select another location or different county to check how probability value change.
  11. Read “Introduction” to gain more information about background, data, methodology and examples from using the on-line tool.
Please contact us if you have comments or questions.

Every year farmers play a game of risk with N management. One type of agronomic or production risk occurs when a corn field becomes deficient in N fertilizer and yield is lost. Quantifying this agronomic risk for a specific field is difficult because outcomes are dependent upon many factors. Indeed, farmers across Iowa use several N management practices that differ in N forms—several commercial types in addition to animal manure, with different timing and application methods, and of course, different rates of N across several landform regions with differing rainfall patterns and at least two common cropping systems.

Our research indicates that spring rainfall is a primary risk factor for N loss and deficient corn N status, but early and mid-summer rainfall could prevent post-spring application of N and, therefore, is a secondary risk factor should a grower delay N application.  The Iowa average (1893 – 2014) May-June (spring) and July-August (summer) rainfall are similar with slightly more rainfall in the spring (8.5”) than in summer (7.4”).  A climate risk context is given by separating the data into the current climate normal period (1981 – current) and past period (1893 – 1980).  Spring and summer rainfall are uncorrelated in Iowa (Figure 1).  Using the fact that spring and summer rainfall are independent, we identify rare years with the 95th percentile.  Prior to 1981, only seven years fall outside the 95th percentile but in the 35 years since 1981 this has happened in 11 years (32%).


State avg. rainfall for May - June and July-August, 1893 - 2014.
Figure 1. State average rainfall for May-June (spring) and July-August (summer) from 1893 to 2014. Data obtained from the State Climatologist of Iowa.

Change in excessive rainfall has implications for N risk management.  Growers should know the risk for loss of N from their fields (Figure 1).  The 11 extreme years identified in 1981 - 2014 can be grouped by whether the extreme rainfall occurred in the spring, summer, or both.  Years with extremely high rainfall in the spring (4 of 11 years) would imply higher risk for N loss from pre-spring application.  Years with extremely high rainfall in the summer (3 of 11 years) would imply the risk of inability to get into the field to apply post-spring N.  Finally, years with extremely high rainfall in both spring and summer (4 of 11 years) would imply risk to both management approaches.

The Iowa Soybean Association On-Farm Network and Environmental Programs & Services teams have been tracking effects of rainfall on corn N management using late-season aerial imagery and corn stalk nitrate surveys since 2006. When data of yield response to N are not available or difficult to collect, the annual survey based on the late-season corn stalk nitrate test guided by the aerial imagery of the corn canopy provide important post-season feedback information. Farmer participation in annual feedback surveys is as critical as going through an annual medical check-up.

Locations of more than 3500 corn fields evaluated for post-season corn N status during a period from 2006 through 2014.
Figure 2. Locations of more than 3500 corn fields evaluated for post-season corn N status during a period from 2006 through 2014. The digital color aerial imagery of the corn canopy was used to select three sampling areas (1, 2, and 3) within three predominant soil types to characterize the average field N status. Corn stalk sample 4 was collected within a target deficient area that looked deficient or yellow. Observations from sampling area 4 were not used in analyses of this study.

Data collected by agronomists and farmers from more than 3,500 corn fields across Iowa between 2006 and 2014 were used to develop equations describing the relationship between rainfall, N rate and management This analysis enables researches to estimate the risk to a specific field of experiencing deficient status N by the end of the season (more detailed in a manuscript “Integrating Field and Climate Data for N Risk Management”).

The data collected over the last 10 years indicate that different management practices, a combination of timing and N form, will require slightly different N rates for achieving optimal corn N status (Figure 3). The distribution of N rates shown as box plots in Figure 3 indicate that the spring and sidedress applications require slightly lower rates that those in the fall. Also, the variability in N rates needed to produce the optimal N status with fall-injected swine manure applications are slightly higher than that with commercial N sources such as UAN and AA.

It can be noticed also that a range of rates for SD N (UAN or AA) and Spring UAN for both corn after corn and corn after soybean are in the same ballpark as those produced by the Iowa State University N Calculator. The value of the ISA data, however, is that inferences about other practices such as Fall AA and Spring AA can be made.

The deficient N status suggests that the supply of N from the soil and fertilizer was likely not adequate, and plants will likely respond to additional N with above break-even yield response.

The optimal N status suggests that the supply of N from the soil and fertilizer matched plant demand, and additional N will produce above break-even yield response less than 50% of the time.

The excessive N status suggests that the supply of N from the soil and fertilizer exceeded plant demand, and additional fertilizer will not increase yield response.


Distribution of N rates used by farmers to produce the optimal corn N status.
Figure 3. Distribution of N rates used by farmers to produce the optimal corn N status within XXX fields surveyed across Iowa from 2016 through 2014.
  • Fall AA: fall-applied anhydrous ammonia
  • Fall SM: fall-injected swine manure
  • SD UAN/UAN: sidedress urea ammonium nitrate solution or anhydrous ammonia
  • Spring AA: spring-applied anhydrous ammonia
  • Spring UAN=spring-applied UAN
This information would be adequate for risk neutral farmers, who ignore the effect of rainfall on N management.

For the risk neutral farmers vthose who ignore the risk and do not want to gamble, the data in Figure 2 would be enough to guide their N management. These farmers will ignore the effect spring rainfall and other factors.

For risk-averse and risk-tolerant farmers the N calculator should help quantify the risk of being deficient for a range of rainfall, N rates, management and cropping system scenarios. The value of the risk calculator to the risk-averse or risk-taker farmer is shown in Table 1.


Example from Southern Iowa

Total rate,
lb N /acre
May through June rainfall, inches
10 (normal) 13 16 19 22
Fall Anhydrous Ammonia
130 0.39 0.50 0.60 0.67 0.75
150 0.36 0.46 0.56 0.65 0.72
160 0.35 0.44 0.55 0.64 0.71
Fall Injected Swine Manure
130 0.42 0.52 0.62 0.70 0.77
150 0.39 0.49 0.59 0.78 0.75
160 0.37 0.47 0.57 0.65 0.73
Sidedress UAN or Anhydrous Ammonia
130 0.55 0.65 0.73 0.80 0.85
150 0.52 0.63 0.71 0.78 0.84
160 0.51 0.61 0.70 0.77 0.82
Spring Anhydrous Ammonia
130 0.26 0.35 0.44 0.53 0.62
150 0.24 0.32 0.41 0.50 0.59
160 0.23 0.30 0.39 0.48 0.57
Spring UAN
130 0.43 0.53 0.63 0.71 0.77
150 0.40 0.50 0.60 0.68 0.75
160 0.38 0.48 0.58 0.66 0.73
Table 1. Probability of late-season deficient corn N status for different combination of timing, forms and N rates based on 690 corn after soybean fields surveyed within the southern half of Iowa between 2006 and 2014. This information is critical for risk-averse and risk taking farmers.

Aggregated data of annual N status surveys clearly shows a greater probability of N deficiency with each additional inch of May through June rainfall, which has a much larger impact (ten to 16 times greater) on N deficiency than an additional pound of N fertilizer. Simply put, both rainfall and N rates impact the likelihood of N deficiency but these effects are in different directions, with the overwhelming impact being from early season rainfall (Table 1, representing southern Iowa). The data in Table 1 also clearly shows that, in general, starting with more N does not rapidly decrease risk values.

As shown in Table 1, when all other factors are equal and the total N rates used by farmers range from 130 to 160 lb N/acre, corn after soybean fields in the Southern Iowa with 13 inches of May-June rainfall will have a 26 to 60 % chance of late-season N deficiency; with 16 inches, a 40-70 %; and with 19 inches, a 50-80 % chance of N deficiency.

Based on historical data, fields in the southern Iowa that received Spring AA may have about 15-30% less chance to be deficient than other form and timing combinations.

The online calculator estimates clearly indicate the higher risk of deficient corn N status for fields for corn after soybean compared to corn after corn. While this survey does not allow us to draw a clear cause and effect relationship, other studies suggest that it is likely that corn after soybean fields lose more N (references) than corn after soybean. These differences are likely because soybean residue have a lower C-to-N ratio and mineralize much faster that corn residues. It is also likely that some fields with corn on corn have a longer history of manure applications, and therefore, higher yield potential and demand for this nutrient.

Similar to other N diagnostic tools, the stalk nitrate test outcomes can be variable. The data collected between 2006 and 2010 from on-farm replicated strip trials with two N fertilizer rates (Agronomy Journal, 2012, 104:1284–1294) indicate the change of receiving above break-even yield response to the application of an extra 50 lb N/acre when the stalk nitrate test is deficient is between 55 -75%. This probability is about 30-60 % when the stalk nitrate test is optimal and less than 30% when stalk nitrate test shows the excessive stalk nitrate value of larger than 6000 ppm. The false positive (when the test suggest deficient status when it is truly not) can arise when corn plants exactly use up all N at the time of physiological maturity thus maximizing yield, so that additional N would not produce economic response.


Probability of economic yield response to an additional 50 lb N/acre for different corn N status.
Figure 4. Probability of economic yield response to an additional 50 lb N/acre for different corn N status when corn and N prices deviate by 30% from their long-term averages. Data from 125 on-farm replicated strip trials conducted between 2006 and 2010 across Iowa were used in developing the risk equations (Agronomy Journal, 2012, 104:1284–1294 link).

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Contextual Information

Reading this Graph

When you read this plot, pay particular attention to where the “Current Cumulative May through June Rainfall” vertical line crosses the probability line(s) for each management practice(s). The color legend on the vertical axis of the graph indicate the nitrogen deficiency risk categories: dark red, “Very High”; red, “High”, light red, “Moderate”, pink, “Low”, white, “Very Low”. The dark and light blue vertical bands show, respectively, the range of historical rainfall in 50% and 90% of years.

Risk Table

The probabilities shown on the y-axis in the graph above are provided in the table, and indicate risk categories of N deficiency. Change the N rate to see how the risk of N deficiency changes. Table cells which are colored indicate higher risk.

Contextual Information

Reading this Graph

See how the probability changes when the N rate is higher or lower by 15 or 20 lbs N/acre from your original Nitrogen application rate. Probabilities for the observed rainfall are shown by a black line; probabilities for the median historical rainfall for your location are shown by a blue line. The dark blue band shows the range of rainfall in 50% of historical years, and the light blue band shows the range of rainfall in 90% of historical years. For your selected N rate, consider the risk at the top and bottom of the blue band: in 90% of years, your risk will fall into this range if you apply fertilizer at the selected rate.

Risk Table

The probabilities shown on the y-axis in the graph above are provided in the table, and indicate risk categories of N deficiency. Change the N rate to see how the risk of N deficiency changes. Table cells which are colored indicate higher risk.

Explanation

The first map shows average historical rainfall in May and June, computed for 4km x 4km regions within the state, between 1997 and 2017. In May and June, this historical rainfall is cumulative over the period between May 1 and today, otherwise, the map shows the full cumulative rainfall during both May and June.

The second map shows the deviation from the historical average rainfall for each region. In May and June, this map will update daily, and rainfall totals should include all but the last 24-48 hours. Otherwise, the map shows the most recent May-June period.

All rainfall totals are gathered from Iowa Environmental Mesonet .

Sponsored by:

Iowa Soybean Association

Iowa Soybean Association Logo

On-Farm Network

On-Farm Network Logo

Environmental Programs & Services

Iowa Soybean Association Environmental Programs & Services Logo

Partially Funded By:

Integrated Farm Livestock Management Project from the Soil Conservation Division of the Iowa Department of Agriculture and Land Stewardship

USDA Natural Resource Conservation Service through Conservation Innovation Grants


Informed By:

Iowa Environmental Mesonet

This applet uses aggregate climate information as well as GIS data to provide rainfall estimates for specific locations.


Developers:

  • Dr. Susan Vanderplas
  • Dr. Chris Anderson, Iowa State University
  • Iowa Soybean Team

Acknowledgement:

We are very thankful to all farmers, agronomists, technical service provides, partner industry professionals and university scientists who participated and contributed to the on-farm field and environmental studies used to develop this tool.