Issues with nitrogen fertilizer: Fall 2013
Except when fall weather is warmer than usual, soil temperatures in Illinois reach the 50-degree mark by about November 1; if they don’t, they are usually on the way down and will reach that level soon. You can see soil temperatures in Illinois at a number of websites; the Illinois State Water Survey site at http://www.isws.illinois.edu/warm/soiltemp.asp gives daily minimum and maximum temperatures at the 4-inch depth under bare soil. Minimum values were below 50 in the northern part of the state on October 30, and maximum values were in the mid- to upper 50s through northern and central Illinois. Keep in mind that there will be some biological activity even at 50 degrees, and any warm spell after application will mean more conversion to nitrate. The goal should be to have soils approach freezing temperatures with as much of the N still in the ammonium form as possible.
It is certainly the case that nitrapyrin or other nitrification inhibitors may not be necessary when soil temperatures are low at the time of application and stay low into the spring. Inhibitors also begin to break down as soils warm in the spring (or get or stay unusually warm in the fall), so that by the time the plants are ready to take up N rapidly, often in early June, much of the N will be in the nitrate form. So what we really hope to get from an investment in an inhibitor is a delay in the conversion to nitrate, so that more of the N is still in the ammonium form as soils warm and water starts to move through the soil, taking nitrate with it. On the other hand, when NH3 is applied late enough, the winter is cold, and the spring is dry, there’s little N loss regardless of N form, and an inhibitor will provide little benefit. This means that using an inhibitor is an approach to managing risk.
Another approach that some take to managing risk of N loss is to apply high N rates in the fall, with or without inhibitor, with the idea that some N loss can happen but that there will still be enough N available the next spring. This can certainly work in terms of having enough N, but it comes at a high environmental cost. Not only do we know that high yields of corn grown in productive soils often do not need the high N rates that some producers apply, we also know that too-high N rates will, sooner or later, mean more loss of N to the environment. While loos to the environment may not seem to be a “real” cost (though the additional N is a real cost), it is a real cost, in terms of things like water treatment to remove nitrate, and in terms of image.