Fall Nitrogen application

Wednesday, November 5, 2014

Harvest is progressing steadily, opening up fields for the application of fertilizer for next year. Soil temperature is critical in the consideration of applying anhydrous ammonia in the fall. Recommendations indicate that soil temperatures should be below 50 degrees fahrenheit with a trend to cooler temperatures. The Agebb website as of Nov. 2 indicated that the soil temperature on bare soybean ground was approaching an average of 50 degrees fahrenheit. Soil temperatures can be checked on the Agebb website from various weather stations in Missouri on a daily basis.

Waiting for colder soil temperatures will help in limiting the amount of nitrogen that can potentially be lost. Anhydrous ammonia (NH3) is a positively charged ion that is attracted by electrostatic forces to the negatively charged soil. Ammonium is not leached or lost by denitrification. Therefore, it will stay in soil even if the soil becomes excessively wet. Nitrate, which is produced by soil microbes from ammonium in a process called nitrification, is a negatively charged ion, is repelled by the negatively charged soil, and is leachable and subject to denitrification.

Soil nitrification is a microbial-mediated process, the rate is influenced by several factors that affect biological activity, such as ammonia in soil water, temperature, soil aeration, soil pH and soil moisture, but soil temperature has the largest influence. The optimum temperature for nitrification is around 90 degrees fahrenheit. Below 50 degrees fahrenheit, the rate slows rapidly, but nitrification continues until 32 degrees fahrenheit.

Nitrification inhibitors slow the conversion of ammonium to nitrates. If more ammonium remains in the soil during wet springtime periods, then less nitrate will be present and subject to loss. Nitrification inhibitors are not fool proof to stop nitrification and formation of nitrates. They degrade in soil, which lessens effectiveness over time.

There are four considerations in deciding how to manage fall applications of Nitrate.

What to apply -- For fall applications, the only recommended sources are anhydrous ammonia and ammonium sulfate. Nitrogen sources containing nitrate should not be used since it will not become adsorbed onto exchange sites and can be easily leached from the soil.

When to apply -- Fall application of nitrogen is recommended when the soil temperature reaches 50 degrees fahrenheit and forecasts are for continued cooling not by date.

Where to apply -- Soils with high potential for nitrate leaching in the fall or early spring (sandy soils or those with excessive drainage) should not receive fall nitrogen applications. Northern Missouri should have adequate cold temperatures to prevent nitrogen loss.

How to apply -- Soils that are too dry or too wet can result in ammonia loss to the atmosphere because the knife tracks may not seal properly. If you apply manure, poultry litter or other animal-derived fertilizer, incorporate then into the soil to avoid volatilization.