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In Minnesota, each year seems to have a different weather pattern. This year (2002) is no exception. Rainfall has been more than adequate for most of the state. In contrast to the past two years, the soil profile throughout most of Minnesota contains a substantial amount of moisture. Although the root zone is not fully saturated, the high moisture content does place added stress and emphasis on management of nitrogen fertilizer this fall.

There is general recognition that fall applied nitrogen 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 nitrogen. As stated many times in the past, this loss can be the consequence of leaching or denitrification. The key to preserving nitrogen in the ammonium form is application of nitrogen fertilizer after the soil temperature drops below 50^oF.

Long term records show that average date of the first 50^oF soil temperature is not in early October for the large majority of southern Minnesota. The table that follows shows the average date of the first 50^oF soil temperture at a depth of 4 inches. These dates are for a bare soil situation. A residue cover would insulate the soil and 50^oF would probably occur at a date later than the one listed.

Table 1. Date for the first occurrence of 50^oF at a depth of 4 inches for several locations in Minnesota

The soil temperature data show that there can be a substantial amount of time between the first measurement of 50^oF and the date when temperatures above this benchmark are usually not measured. At Lamberton for example, this average time interval is more than 9 days, (compare dates in Table 1 and the Minnesota Map).

For most of central Minnesota, a constant soil temperature of 50^oF or less is not measured until after October 14. This date extends until after October 28 for a large part of southern Minnesota.

Therefore, in order to keep loss of nitrogen from fall applications to a minimum, it's absolutely essential to delay fall applications to mid to late October for much of Minnesota. This delay is especially important in years when the root zone is nearly recharged (fall, 2002).

It's important this year, for crop producers to be patient. The risk of N loss from early application of fertilizer N this fall is substantial. Published Best Management Practices (BMP's ) have served as the guide for management of nitrogen fertilizers. For example, fall application of fertilizer N is not a suggested practice for all of southeastern Minnesota as well as where soils are sandy. The risk of loss of fall applied nitrogen is very high in these production situations.

In any discussion of fall applied nitrogen, there is always a suggestion from someone to recognize that there will be some loss from early applications and compensate for this by increasing the rate applied. This argument is flawed. From an economic perspective, there is no justification for applying more than needed. This adds to the cost of production while not producing higher yields. This practice would also increase the potential for loss of nitrogen due to leaching thereby having a negative effect on environmental quality. There is no substitute for patience this fall.