Soil Nitrogen Loss

Here in the central Midwest, every time we think spring weather has arrived, another wintry blast hits us. While the mix of cold and warm fronts has brought some needed moisture (by way of snow) to parts of the Midwest, some of us are beginning to cry foul. It's been a good three or four months since we have had any decent tractor time and some of us are getting antsy. But wait we must.

Soils are beginning to warm, except for those recently covered by a new snow fall, and the frost is disappearing. And this means that tile lines are starting to run and many growers are beginning to wonder if they are losing much of that nitrogen they put on last fall. We can make some guesses as to what is happening to nitrogen by reviewing the processes involved.

Of course it’s nitrogen in the nitrate form (NO3-) that is soil mobile. The process of nitrification, mediated by soil microbes, converts ammonia forms of N to the mobile nitrate form.  Whether we apply anhydrous ammonia or ammonium, factors that slow down or stop the nitrification process are the same things that slow down the bacteria: cold or frozen soils or nitrification inhibitors. So if last fall you applied your nitrogen in an ammonium form and the soils quickly dropped below 50 F, you can expect that nitrogen will still be in the ammonium form. If, on the other hand, you got to the fields early last fall and the soils were warm and moist for a period of time after application, then some of the nitrogen was likely converted to the nitrate form. This does not mean that it will be lost, only that it becomes susceptible to leaching under the right conditions.

As soon as the frost disappears, precipitation over the top of a saturated soil can result in leaching and you can expect some nitrates to move out of the soil. As the soils warm up to 50 to 55 degrees this spring, the microbes will get down to business and restart the process of converting ammonium to nitrate. In warm moist soils, this process can move along quickly and convert much of the ammonium to nitrates before the first of June. In soils which remain saturated for more than a few days, the process of denitrification is a possibility (converting existing nitrates in the soil to a gaseous form). This process is moderated by anaerobic bacteria and can result in some significant N loss to the atmosphere. Denitrification tends to occur in low-lying areas of fields where water remains ponded, provided that the soils are warm and have nitrate present.

Once spring is officially here and you are ready to go out into the field, make sure to check Tractor Time and all of the other products in Morning Farm Report to get the most out of this growing season.