The planter has been stored for another year, and grain bin padlocks have rusted shut for the time being, so you think about side dressing some anhydrous ammonia because the warm winter undoubtedly allowed the survival of all the nitrifying bacteria and your corn has no nitrogen left. If that is your plan, University of Illinois soil fertility specialist Fabian Fernandez wants you re-think that plan, and maybe save a considerable amount of money.

Yes, there is no question that soil temperatures were much warmer than usual over the winter, and the bacteria that typically convert ammonium to nitrates were active later into the fall and earlier in the spring. Under normal conditions they would have unlocked your nitrogen from the soil and helped it leach out through field tiles this spring. But how many of you have had field tiles running, and if not, that means that you still have nitrogen in the field to feed your corn crop. 

That is the contention of Fernandez who says dry soils have not been friendly to nitrifying bacteria, and “there is still a considerable amount of ammonium in the soil, probably because not all of it has been nitrified and also because organic nitrogen in the soil is being mineralized to ammonium.” After all there has to be some benefit to the weather. Fernandez says a field with a November application of anhydrous ammonia plus N-Serve registered 80 parts per million (ppm) of ammonium by the end of February. But at mid-May there were still 27 ppm of ammonium still in the soil.  He says nitrate concentrations increased as expected, but there is still ammonium in the soil that will be available to the corn crop.

His initial samples of ammonium were at the 12-inch soil level, but he has also taken samples 12 to 24 inches deep. As you might expect, there has been insufficient rainfall to move the nitrate down in the soil profile and concentrations were low at that level.  Nitrates were only 9 ppm and ammonium was only 3 ppm.

Fernandez says corn is up and growing and starting to take up nitrogen. “If all the nitrogen has already been applied, I do not anticipate a need to apply additional nitrogen for this crop,” Fernández said. “If no nitrogen or only a portion of the nitrogen was applied, now is the time to start applying the balance of the application.” The N demand by corn does not accelerate until the V-5 or 5-leaf stage and most of it is absorbed between V-8 and tasseling. The N uptake process stops at pollination time. He says research has shown that yields are not hurt by nitrogen stress if the anhydrous ammonia application is delayed for some reason and is not applied until the V-6 stage. However, he says timely application is important, because of the opportunity for rainfall (when it happens) to move the N through the soil profile and into the root zone.

Fernandez says the topsoil may be dry, but there is sufficient moisture in the subsoil to retain the ammonia, and he says there should not be a problem with volatilization if it is injected 8 inches down in coarse soil or 6 inches down in finely-texture soil. If there is little chance of rain, Fernandez suggests the use of urea with a urease inhibitor (such as Agrotain) to minimize volatilization, and loss of the mineral 3 to 4 days following application.

Fall application of anhydrous ammonia would typically dissipate in warm fall, winter, and spring soil temperatures, but that may not have happened as expected because soils have also been unusually dry, hampering the nitrifying bacteria that convert ammonium to nitrates. Subsequently, a greater amount of ammonium may be available for corn use this spring than would be expected.

Source: FarmGate blog