|The way to avoid nitrogen immobilization, which can reduce corn yield, is to apply extra nitrogen on the surface or shallowly incorporate it. The question: How much do you apply and when?
"From test plots, we have learned that stopping immobilization in corn following soybeans, with incorporated crop residue, usually requires 60 lb. per acre of surface-applied nitrogen,” says Farm Journal Field Agronomist Ken Ferrie. "In continuous corn, it usually requires 100 lb. per acre.”
That's the total amount, including fall surface applications of products like diammonium phosphate and spring-applied applications, such as weed and feed.
"The 60-lb. and 100-lb. figures are general rules,” Ferrie continues. "Some fields may be exceptions. It's possible that a field with high Illinois Soil Nitrate Test values could mineralize so much nitrate at the right time that immobilization is not a problem.
"When I take spring nitrate tests, I expect to find about 10 to 15 ppm [parts per million] nitrate per acre. But, occasionally I find 25 ppm or 30 ppm. Those fields can mineralize enough nitrogen to feed the microbes and the crop.
"On the other hand, you can have a field like the one in the adjacent story where 100 lb. per acre would likely not be enough,” Ferrie adds.
Another exception to the 60-lb. and 100-lb. rule is strip-tilled and no-tilled fields. "Because you don't bury residue, microbe populations don't increase as dramatically as with tillage,” Ferrie says. "But, there still are massive amounts of roots below ground for the microbes to decompose. So, you still need to broadcast additional nitrogen on the surface.”
One way to learn how much nitrogen your soils immobilize is to apply strips of 0, 60, 90 and 120 lb. per acre and see which rates green up the corn.
You can also pull a spring soil nitrate test or fall stalk nitrate test. Remember, those results may change from year to year because nitrate can quickly be lost if you get heavy rainfall.
If you apply part of your 60 lb. or 100 lb. of nitrogen in the fall, it should be in the ammonium form, Ferrie points out. Nitrate can be lost if the weather turns wet.
With spring applications, you want partly ammonium, which is used by microbes, and partly nitrate, which is used by plants.