Now is the time to plan your nitrogen (N) management for 2008. The first important question is how much N should I apply? University of Minnesota N guidelines should be a starting point.

The guidelines are available in an Extension publication titled, “Fertilizing Corn in Minnesota,” available online at www.shopextension.umn.edu; search for item number 03790.

The amount of N to apply depends on four factors: 1) productivity of the growing environment; 2) previous crop; 3) price of fertilizer N to value of corn crop ratio; and 4) the amount of risk a grower wants to assume.

A large part of Minnesota suffered from drought this summer. Because of this, there is a good chance that larger-than-normal amounts of residual soil nitrate-N may be available for use by the 2008 corn crop. In those cases where increased residual nitrate-N is possible, a soil test for nitrate-N will be needed to adjust the N application rate.

Expect increased soil nitrate-N in situations where you are going to plant corn after corn, and in corn planted after soybeans where the soybean crop died or was extremely drought-stressed during the cropping season. In a corn/corn situation, the corn did not use as much N because of drought stress.

In July, there was little N mineralized from the organic matter because of drought conditions. Some areas experienced good August rains. In those areas, there probably was an increased release of inorganic N from the organic matter because of the more favorable soil moisture condition. This nitrate-N probably was not entirely used by the corn plant, as corn takes up most of the N it needs for growth by pollination.

In the soybean/corn situation, if the soybean plants did not die during the growing season, they would have utilized soil nitrate-N up to mid-August, and thus have left little excess nitrate in the soil. If the plants died prematurely, then there could be excess nitrate-N.

The best time to obtain a soil test for soil nitrate-N will depend on several factors. First, if the field is in western Minnesota (approximately west of Highway 71), a soil sample from the surface 2 ft. can be taken in the fall or spring. For the fall soil sample for nitrate-N to be accurate, it must be taken after the soil temperatures in the surface 6 in. have stabilized at 50° F or lower. A soil sample obtained before that time will, in most cases, underestimate the amount of nitrate-N in the soil.

The N application for corn should be adjusted based on the amount of residual nitrate-N as indicated in the University of Minnesota corn N guidelines.

If the field is in eastern Minnesota, a soil sample to a depth of 2 feet should be obtained in the spring, after the frost is out of the soil and before the corn is planted. This soil sample should be analyzed for nitrate-N. The amount of credit for the nitrate-N concentration in the sample can then be determined as listed in the publication, “Fertilizing Corn in Minnesota,” based on the use of best management practices.

In southeastern Minnesota, fall applications of N are not recommended. In south-central Minnesota, late-fall applications with a nitrification inhibitor are acceptable but with greater risk, while in southwestern, west-central and northwestern Minnesota, late-fall applications without a nitrification inhibitor are acceptable with greater risk. In late-fall applications, the N fertilizer must be an ammonium type.

What is meant by late fall? Late-fall application should occur only after soil temperatures in the surface 6 in. are at or less than 50° F. Applications made at temperatures greater than this (earlier in the fall) are suspect to conversion from ammonium to nitrate, and thus possible losses in the spring to movement in the soil and/or denitrification.

Fall-applied N should be planned so that it stays in the ammonium form throughout the fall, winter and early spring. Otherwise, when present in the nitrate form, there is a substantial risk for loss of N.

John A. Lamb is a University of Minnesota soil scientist and David Nicolai is a crops educator with University of Minnesota Extension.