Remember 50 Degrees for Fall Nitrogen

Wait until soil cools to safe level before you apply anhydrous. Rod Swoboda

Published on: Oct 11, 2005

As the crop continues to be harvested, some farmers are already starting to apply nitrogen for next year's crop. Barb Stewart, state agronomist with USDA's Natural Resources Conservation Service, says you might want to wait until the ground gets colder before you knife in the anhydrous ammonia.

"In October, it's usually too warm," she says. "Anhydrous ammonia, if it is applied when soil temperatures are above 50 degrees, increases the risk of nitrates leaving farm fields, which is a waste of your money spent for fertilizer. Those nitrates that slip away will pollute the groundwater and streams too. It's a waste of fertilizer as well as increasing health risks for others."

"Farmers need to wait until the soil temperature has cooled to 50 degrees and is continuing to cool before applying this fertilizer," says John Sawyer, Iowa State University Extension soil fertility specialist.

He says when anhydrous ammonia is applied to soils warmer than 50 degrees, a chemical process can quickly convert it into nitrate. The nitrate can then be leached from the soil, resulting in the movement from the field into water resources, or it can be lost as nitrogen gas if soils become water-saturated.

Even waiting for 50-degrees is no guarantee

"Waiting for cold soils does not guarantee that fall-applied N will be a successful application practice," says Sawyer. "Warm fall conditions might occur after you apply it, or warm and wet conditions may occur the next spring and early summer. However, if you decide to make applications in the fall, then waiting until soils are cold is better than applying early."

The same goes for inclusion of a nitrification inhibitor. Products like N-Serve will work better after soils have cooled, he notes.

Soil temperatures can be found at several Web sites. One that shows the three-day, 4-inch depth soil temperature estimates for each Iowa county is "Soil Temperatures for Agriculture." The site also can be accessed through the Agronomy Extension Soil Fertility Web site, either from the weather page or nitrogen topic page.

You can check local soil temperature

You can access the average daily soil temperatures from yesterday, two days ago, and three days ago, and you can get the six- to ten-day weather forecast. Four-inch soil temperatures for the previous day recorded at specific sites in Iowa can be found at the Iowa Environmental Mesonet Web site.

"The 4-inch soil temperatures are estimated for each county based on interpolation of observed soil temperatures at 14 locations," says Elwynn Taylor, ISU Extension climatologist . "The estimates are for soil temperatures on level, bare soils. The reliability of the estimates is within 3 degrees F with normal conditions in Iowa."

The variation in soil temperature within a locality is generally more than 8 degrees F depending on slope of the land and condition of soil. Also listed are dates during the past five years when soils cooled to below 50 degrees F.

Don't be fooled by temporary cold spell

During the past five years the dates when soils cooled below 50 degrees F varied considerably, from late October to late November. "Don't get fooled by a temporary cold spell, especially if it occurs early in the fall. Also watch the six- to 10-day weather forecast, noting that a forecast for 'above' average temperature may signal soil warming," says Taylor.

ISU Extension offers a number of resources for farmers wanting to know more about this soil fertility process.

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