Issues with nitrogen fertilizer: Fall 2013
One practice that some have adopted is to apply only part of the N in the fall, with the rest applied the next spring. This approach should reduce the amount of N loss if soil conditions become conducive to loss of fall-applied N. And it provides one of the underappreciated benefits of fall-applied NH3, which is having N dispersed through the soil and easily accessible to the plant in the spring. The drawback is that NH3 application is rather slow and costly compared to most other methods of N application, and so applying lower rates increases the cost per lb of N applied.
I mentioned a few days ago the use of subsurface banding of P and K, accompanied in some cases by placement of NH3 beneath this band in the same operation. Making one trip to apply fertilizer increases efficiency, and as long as soil temperatures are low enough, this can work well. As is the case with banded P and K, there is little evidence that applying NH3 under the P-K band provides a yield advantage over applying dry and N fertilizers separately. There has been some tendency in the past for such “dual placement” operations to start before soils are cool enough for fall N application, which increases N loss potential.
One issue with fall NH3 application in recent years has been whether or if to combine application with tillage. Some have applied N and then tilled, while others have tilled first and then applied (and sometimes tilled again after that). Ammonia is extremely soluble in water, and once dissolved in soil water it converts to ammonium. So if there is a moderate amount of soil moisture present, losses of NH3 should be relatively small, even if soil is tilled. Tillage does form air pockets in the soil, however, and if a marginally dry soil is tilled before NH3 is applied, some NH3 could be released before it can dissolv. This would be noticeable as puffs of “smoke” (actually, water droplets attracted by ammonia) after the applicator, and as the smell of ammonia.
Tilling after application does turn soil up where the sun can warm it, and the warmer temperatures might increase conversion to nitrate. If the soil is dry, tilling after application could also release some NH3 that was not dissolved, especially if the soil dries even more as a result of tillage. It also is probably worth asking as well if soil following NH3 application really needs tillage.
One of the advantages of applying less than full rates of N in the fall is that it delays the final rate decision until the spring, allowing us to note loss conditions, planting dates, and other factors that might affect the rate we apply. At the same time, as we have seen this past year, the amount of N loss can be difficult to determine. This means that the tendency to apply some “insurance” N to make sure there’s enough operates in the spring as well as in the (previous) fall. In fact, despite a lot of evidence to the contrary, there’s an abiding thought that bringing out full yield potential for a crop that gets off to a good start in the spring will require more than the usual amount of N. We may have had a little less of this than usual this past spring – with the crop planted as late as it was it didn’t seem to be off to a great start. The fact that we are hearing of some yields above 250 bushels per acre, often coming with “only” normal amounts of N, should help a few more of us to start to question whether high yields only come when we pour on the N.
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