To Credit or Not to Credit Nitrogen Following Fallow Fields?

By Dan Kaiser & Fabian Fernandez

A previous blog post mentioned a potential nitrogen (N) credit for a crop following a fallow situation. The current U of M guidelines suggest that up to a 75-pound N credit can be taken when a crop follows fallow conditions. To be clear, fallow includes only situations where a crop was not grown during the previous cropping season and does not include situations where a crop that may induce fallow syndrome is grown. Going into the 2020 growing season, growers may be wondering: Should I take any additional N credits? Under what circumstances should I credit N? Where does this N credit come from?

Under normal N crediting circumstances we assume that a plant, such as a legume like soybean or alfalfa, is biologically fixing nitrogen which may in turn be available to the following year’s crop. Under fallow conditions when no crop is growing, where would a potential N credit come from? Mineralization of N in the soil will occur whether a crop is growing in the field or not, as soil microorganisms are still active in the absence of a crop. If nitrogen is mineralized and plants are not present to take up the mineralized N, and if nitrate is not leached from the soil profile, then a credit could be expected due to N carried over from the previous year.

Should you expect a nitrogen credit the year following fallow conditions?

There are a few things to consider:

1.  If your soils are sandy or do not retain N well, it is unlikely there would be a credit from a fallow situation.
2. If your soils were saturated for an extended period of 2019, denitrification may have reduced the potential for nitrate carryover. In addition, saturated soils may reduce the mineralization of N from organic matter.
3. If you had periodic flushes of weeds in your field, remember that weeds do take up nitrogen. This could reduce or increase the amount of carryover N depending on whether N is released from the plant material this spring.

So should I credit nitrogen or not?
This decision should be made on a site-by-site basis. The best way to assess potential N credits is to take a two-foot soil sample in the spring before fertilizer application to see how much nitrate is in the soil. Elevated N in the soil following fallow conditions could then be credited. Soils in central and western Minnesota commonly carry over some residual nitrate from one year to the next. As with any crediting situation, be sure to base your starting N application rate on the corn-following-corn recommendations, then subtract your credit. Or, if you are using a two-foot soil N test, make sure you know how much N to credit from the two-foot sample using university guidelines.