Is Your Nitrogen Still in the Field?

Late spring soil nitrate test can give you a status report in case you need to apply some N as a sidedress.

Published on: May 22, 2007

A number of farmers were able to get their nitrogen applied for this year's corn crop, and now they are wondering with all the rain this spring, what has been the impact of the soggy soil on their nitrogen? Is it still in the field? Or has it disappeared?

"It's certainly been a difficult spring, but one message we've been trying to get across to folks is to stay the course and try to follow the plan you've laid out and stick to it as much as you can," says John Sawyer, Iowa State University Extension soil fertility specialist.

Obviously the excess moisture is causing some concerns about nitrogen supply and what may have happened to that nitrogen fertilizer or the N in manure that was applied. Nitrogen losses are related to nitrate, he explains.

Use the late spring soil nitrate test

Nitrogen in the ammonium form, like you have with anhydrous ammonia, is more stable than N in the nitrate form. When soil gets excessively wet, the ammonium sticks to the soil particles and can't be lost as easily as it can with nitrate leaching or gaseous losses of N from denitrification.

The applications of nitrogen that were made later in the spring have less risk of loss than those that were made in the fall or in the early spring.

There are some tools to help evaluate nitrogen status. One of those you can use in late May or early June is the late spring soil nitrate test. Soil samples for that test are usually collected in late May or early June to help you evaluate the nitrogen status in your fields.

Aerial sensing can provide clues, too

Another newer approach is to use the corn crop as an indicator of nitrogen status. "Using crop sensing or aerial photography and use of sensors, we can look at that. You have to use those in June, when the corn is knee-high to waist high," says Sawyer.

"If you've already applied the N, you can wait and use the soil nitrate test," he adds. "Then if you need to apply more N you can apply it as a side-dress treatment if you need to. Fertilizer dealers don't necessarily have the applicator toolbars available for everyone to sidedress, but fortunately not everyone in the state is going to have to make touch-up applications."

Those farmers with greater loss conditions may be facing the need to sidedress. You have some time to get that application made.

Applying liquid N on emerged corn

One of the questions Sawyer has had this year is that if you were not able to get preemergence herbicides applied, and you still need to apply nitrogen for corn, can you apply liquid N and the herbicide together as a mixture after the crop has emerged? Farmers are asking: Will it hurt the crop?

Herbicide labels restrict that application. So you need to check the label and be careful in trying to make those types of applications. To avoid crop injury, applying the liquid nitrogen and the herbicide separately would be much better than trying to apply them at the same time.

What if you see yellow corn, after emergence? That might be an indication of a nitrogen deficiency, if soils are cold and wet, says Sawyer. "The corn plant doesn't like that," he says. "So we will see a fair amount of yellowish-looking corn as it emerges this spring - until the soils warm up and dry out. You need to give the corn some time to grow and get past some of those wet conditions and see what the crop is looking like."

Sidedress strip applications in fields

Another way to check, if you are nervous about your nitrogen supply, is you can get into the field and make some side-dressing strip applications across the field, of some additional N and watch the results. "If it looks better where you apply the supplemental N, then maybe you have some losses you need to take care of. On the otherhand, if things look fine, you'll probably be in good shape. So just some visual observation along that line will help also," says Sawyer.

With corn following soybeans, some farmers are concerned that in a wet year they wonder how much potential there is to lose the nitrogen that was fixed in the ground by the soybean? How much of a factor is that?

What's interesting is you harvest the soybean crop. The leaves drop early and that residue can be degraded fairly early in the fall of the year. So we do have a fair amount of nitrate formation after that soybean crop. That nitrate from the crop residue, in conjunction with the mineralization of the nitrogen from the soil organic matter, maybe residual nitrate from other sources, that is all potentially subject to losses when you have wet conditions.

When we look at nitrate in surface water or coming out of tile lines, we have to remember that a lot of that N is from that soil resource, it's not just from that year's fertilizer application. So those losses could be taking place.

"What we tend to see in our research data between different years is when we have years with wetter springs, those years tend to be more responsive to fertilizer N, meaning you tend to get a larger yield increase from fertilizer application, and you tend to need a slightly higher rate of N application," says Sawyer. "We suspect some of that is going to happen this year, so it's beyond what happens with the fertilizer N you applied. It also relates to what is happening to that soil nitrate or soil nitrogen."