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Any fall nitrogen application carries the risk that fertilizer will be lost from the soil before the corn crop can use it in May and June. Applying nitrogen too early in the fall increases the risk that the nitrogen will be lost from the soil.

Most fall nitrogen will be applied as anhydrous ammonia. When applied to frozen or nearly frozen soils, the nitrogen tends to stay in the ammonium form that resists over-winter losses. If you apply anhydrous ammonia too early in fall to a warm soil, the nitrogen will convert to nitrate, a highly mobile form of nitrogen. Nitrate nitrogen can be lost from the soil if we have saturated soils in winter or spring before corn has a chance to use the fertilizer.

The rule of thumb in Northern Missouri (north of I-70) is that farmers can minimize the potential for fall nitrogen loss if they wait to apply anhydrous ammonia until the average daily soil temperature at six inches is below 50 degrees. However this rule was developed in more northern conditions like Minnesota where soils cool more quickly. We are currently investigating if we should delay fall anhydrous applications until soil temperatures are 40 degrees or less.

Average soil temperature at six inches does not consistently stay below 50 degrees until mid November and 40 degrees until early December at Novelty MO (see Figure 1). October is too early to apply fall anhydrous ammonia in Northern Missouri if you are concerned about over-winter losses.

In southern Missouri, fall and winter soil temperatures rarely remain cold enough to delay nitrate conversion. Soil temperature at six inches at Glennonville MO typically stay above freezing so some conversion to nitrate can occur through most of the winter. Fall application of anhydrous ammonia is not recommended in southern Missouri.

Products can be added to nitrogen fertilizers to delay the conversion of nitrogen to nitrate. These products reduce the risk of nitrogen losses from nitrogen applications. N-serve (a nitrification inhibitor) can be added to anhydrous ammonia. High fertilizer prices make the economics of inhibitors more favorable.

The Missouri Agricultural Weather Network monitors soil temperature at six inches at nine locations in Northern Missouri. Average daily soil temperature was still in the mid 50's on October 16, 2006 despite recent cool temperatures. Monitor soil temperatures in Northern Missouri at the website: http://agebb.missouri.edu/weather/reports/soysoil6.asp.

Horizon Point custom weather analysis system for farmers provides farmers with the opportunity to have site specific weather reports sent directly to their e-mail address. This system has an option to have six-inch soil temperature included in your email report. Learn more and sign up for Horizon Point at http://agebb.missouri.edu/horizonpoint/.

John Lory
Plant Science Division and Commercial Agriculture Program