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.

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%).

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.