Soil temperatures and nitrogen retention

By Greg LaBarge, CCA, Ohio State University Extension agronomist

With current fertilizer nitrogen prices and concerns about availability, we want to take advantage of every unit of N we apply for the 2022 corn crop. One place to look for N is from fall manure applications. The conditions at and after application will affect the amount of N available. We can make predictions of loss, but doing a pre-sidedress nitrogen test or another in-season soil test will increase the confidence in how much N is there for the crop.

One good guide to conditions that reduce N loss is soil temperature. The soil temperature guideline used for anhydrous ammonia is to make applications once soil temperatures are less than 50 degrees F at 4-inches deep and continue to go down. 

Retaining the ammonium in fall-applied manure follows the same principle. If we apply when the soil temperature is less than 50 degrees, the bacteria that convert ammonium (soil stable N) to nitrate (leachable N lost through tile) are less active. Soil temperatures at the 4-inch depth were below 50 degrees by mid-November across the state. Therefore, when manure was applied after mid-November 2021, we expect to retain more N than earlier applications.

The other important factor in retaining manure N is placement. Surface applied versus incorporated manure will result in higher N losses. Data from Minnesota show the amount of applied N available in year one from different incorporation timings with different manure types. For example, dairy manure, with broadcast/incorporated more than 8 days has 25% N available, broadcast/incorporated in 0.5 to 8 days has 45% N available, broadcast/incorporated in 0 to 12 hours has 55% N available. Shorter periods between application and incorporation result in more N retained.

A soil test will be the best way to evaluate N for the 2022 corn crop from manure applications. We have used the Pre-Sidedress Nitrogen Test (PSNT) with success. In 2021, Glen Arnold and I used the PSNT on incorporated fall applied manure plots at different rates replicated four times at Northwest Ag Research Station. Nitrogen applications were reduced by 45 to 90 pounds per acre with 240 bushels per acre yield, equal to the 186-pound nitrogen (MRTN) rate applied as 28% UAN.  Consider the available N from fall manure application when determining your sidedress nitrogen needs.

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