To reach the goals set by the Iowa Nutrient Reduction Strategy, consider utilizing split fertilizer that can increase nutrient efficiency and reduce nutrient loss. Additionally, there is less investment in the field if forced to replant to soybeans after weather-related losses or planting delays. This may allow time to gather additional information about the year’s market and growing conditions. The cost of making a second fertilizer application trip across the field and potential for rainy weather are deterrents to spring nitrogen application.
Often, when switching to split application, the general plan is to apply 50-60 percent of the nitrogen recommendation in the fall or early spring, and then side-dress the remaining 40-50 percent into the growing crop. An alternative approach is to determine the side-dress amount using the Late Spring Nitrate Test (LSNT), a nitrate-only soil test. It should be done when the plant is 6-12 inches in height. This allows for spring nitrogen losses or gains to be reflected. Soil samples are taken at a depth of 12 inches. The test provides information on plant-available nitrogen concentrations in the soil before the plant begins rapid uptake of nitrogen.
Research recommends 25 ppm of nitrate-nitrogen in the top 12 inches of soil to produce maximum yield; however, the interpretation of the results vary with cropping system, manure history, and even weather conditions prior to and after sampling. One of the challenging parts of using the LSNT is determining the “critical” soil nitrate concentration you are trying to achieve.
|Recommended N Rate|
|Soil Test||Normal Rainfall||Excess Rainfall|
|ppm NO3-N||lb N/acre||lb N/acre|
In manured fields, a “critical” soil nitrate-nitrogen concentration of 15-20 ppm is recommended. This is lower than non-manured fields because manure application provides additional organic matter that will be mineralized after the time of soil sampling and becomes available to plants later in the season. Using the results, calculate the amount of nitrogen that would be recommended to side-dress. The formula for calculating nitrogen application is if the soil test was greater than 20 ppm then 0, otherwise (20 ppm – soil test nitrate) * 8 = lbs. of N/acre to apply. Alternatively, Table 1 provides a way to select a side-dress nitrogen application rate. In this table, excess rainfall would be May precipitation that exceeded 5 inches; normal rainfall should be used for other cases.
As with any new fertility management program, first-time users are encouraged to experiment with the test in small areas before using it to guide fertilization on all their fields. As with most recommendations, this test is intended to maximize profits when used across many years and sites, not to give the “perfect” rate in a specific year.
For more information related to using the LSNT, please see ISU publication CROP 3073 Nitrogen Fertilizer Recommendations for Corn in Iowa. More information on best management practices for reducing nutrient loss from agriculture can be found in SP 0435A Reducing Nutrient Loss: Science Shows What Works.