Avoid fall anhydrous ammonia - expert advice

By Blake Jackson

University of Missouri Extension's John Lory, a state nutrient management specialist, advises against applying anhydrous ammonia in the fall anywhere in Missouri. Fall application increases the risk of nitrogen loss between application and when corn needs it (June-July).

Lory recommends preplant and side-dress applications to minimize nitrogen loss. However, some northern Missouri farmers utilize cooler fall soil temperatures for application, aiming to reduce workload and benefit from lower nitrogen prices.

Minimizing Nitrogen Loss from Fall Applications:

• Apply to cold soil: Warmer soil promotes rapid conversion of ammonia to nitrate, a form prone to leaching. Apply when soil temperature reaches or falls below freezing.

The "50-Degree Rule" Reconsidered:

Many farmers follow the "50-degree rule," delaying application until the 6-inch soil temperature dips below 50°F. Lory's analysis suggests this rule doesn't hold true for northern Missouri. He recommends delaying application until the soil temperature falls below 40°F.

The 50-degree rule originated in regions with colder winters where soil temperatures rarely rise above freezing. In contrast, Missouri experiences winter warm spells that can raise soil temperatures, making fall-applied nitrogen less stable.

Alternatives and Considerations:

• Nitrification Inhibitors: Adding an inhibitor like N-Serve with your nitrogen fertilizer can help.
• Delaying Application Risks: Delaying application until soil temperatures reach 40°F can increase the likelihood of successful overwintering, but also increases the risk of unfavorable weather conditions hindering application.

Weighing the Trade-offs:

While fall application offers good field conditions, it often leads to conversion to nitrate in the fall or early spring, increasing vulnerability to leaching, especially with excessive spring rain. Preplant applications also pose a risk of spring losses. Split or side-dress applications during the growing season are a recommended practice, but wet springs can hinder timely application.

Northern Missouri Considerations:

Fall application doesn't always lead to losses, but farmers should be aware of the risks and take steps to minimize them. Lory suggests:

• Limit fall application to less than half of planned corn and milo acres.
• Monitor spring soil conditions and be prepared to supplement nitrogen on fall-fertilized fields in wet years.

By understanding the risks and implementing these recommendations, Missouri farmers can make informed decisions about fall anhydrous ammonia application.

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