Evaluating Nitrogen Loss
There are several ways to evaluate N losses after excessive rainfall events:
- taking soil tests
- using crop sensors
- gathering other information to make a good estimate
Soil tests can be a useful tool for helping to determine supplemental N needs in the spring, even though the predictive value of these tests is affected by weather conditions that impact N losses and mineralization. Because of wet spring conditions in broad areas of the Corn Belt in recent years, several universities have written articles on use of soil sampling to help determine N loss and crop needs. See list below.
University resources for determining N loss.
Local conditions, including soil type, drainage characteristics and average temperature and rainfall greatly affect N-loss potential. For this reason, growers are encouraged to review their own state's recommendations.
When soil sampling for N, it is important that samples be taken to a depth of at least 12 inches. This is because heavy rains can move nitrate deeper into the soil profile, so sampling to a depth greater than 12 inches may provide a better idea of how much nitrate remains in the root zone. After collecting the samples they should be frozen or spread out to air-dry before being sent to a reliable soil lab.
Of all soil-testing procedures for N, the test most commonly recommended and used is the Pre-Sidedress Nitrate Test (PSNT), also known as the Late Spring Soil Nitrate Test (LSNT) (Camberato and Nielsen, 2010). However, this test also has limitations and specific applications, so adjustments may be required. The test was originally designed to estimate the amount of N available in fields where manure has been applied or a legume such as alfalfa or clover has been plowed down. In these situations, the test is an indicator of how much N is currently available and how much is expected to be released. Because of this, the critical level may need to be adjusted above 25 ppm when using the PSNT for determining N availability following heavy rains.
Recently improved optical sensors can help assess N deficiency and the amount of N needed to optimize crop response (Shanahan, 2010). These crop sensors, mounted on N-application equipment, work by emitting modulated light of the appropriate wavelength onto plants then measuring how much of that light is reflected back to the sensor. This measure of “crop greenness” correlates with plant chlorophyll content. Estimating chlorophyll content also estimates crop N status, as the two are closely linked.
The sensing and application process begins by calibrating the sensor using a sufficiently fertilized “reference strip” area of the field. First, sensor readings are collected from the well-fertilized area and the values are recorded in an on-board computer. Then, as the applicator traverses the rest of the field, the sensing, rate calculations and application all occur at once. For more detailed information concerning crop sensors, read the Crop Insights on this subject.
In addition to crop sensors, aerial imagery and chlorophyll meters are also good tools for evaluating the N needs of a growing corn crop.
Estimating N Loss
In lieu of soil testing or crop sensing, nitrogen loss can be estimated, and this estimate used as a basis for deciding if more nitrogen should be applied. When estimating N loss the key questions to answer are:
- When was the N applied?
- What form was used?
- How much was applied?
- What were the field conditions following the application?
Greater quantities of N fertilizer are converted to nitrate as time goes by and soil temperatures increase. Knowing when the N was applied and what fertilizer was used enables the estimation of the quantity of N in the nitrate form when rainfall occurred (Table 1).
Table 1. Amount of nitrogen fertilizer in the nitrate-N form 0, 3 and 6 weeks after application.
Adapted from Lee et al., 2007.
Figure 3. Anhydrous ammonia applied just prior to periods of prolonged soil saturation is less prone to denitrification than urea or UAN. Photo courtesy of Deere & Company.
Just because N was in the nitrate form does not mean all of it was lost. The soil temperature and duration of soil saturation are two key factors affecting denitrification. The warmer the soil and the longer it was saturated both increase denitrification losses (Table 2).
Table 2. Estimated denitrification losses as influenced by soil temperature and days of saturation.
From Bremner and Shaw, 1958.
Another way to estimate whether a rescue N application is needed is to use the Nitrogen Loss Scoresheet developed at the University of Missouri (Scharf, 2008). After estimating the amount of nitrate lost by either method above, a better decision can be made as to whether a rescue N application is needed and how much additional N is required.