Many Corn Fields in Western Iowa Showing N Deficiency

Soil nitrate test results show a number of farmers need to sidedress some nitrogen for 2007 corn.

Especially in the western half of Iowa, a lot of cornfields are now showing signs of nitrogen deficiency. Tracy Blackmer, head of the On-Farm Network for the Iowa Soybean Association, has been looking at fields closely due to the potential loss of nitrogen caused by heavy rains this spring." In the past several days we've been doing some intensive testing of soil using the late-spring soil nitrate test near Jefferson in west-central Iowa," he reports. "Of the 20-something fields we've sampled soil from in the last three days, the test results have shown nitrogen deficiency in every one of those fields." It's been a challenging year in the western part of the state, no doubt about it. "We focused on the area near Jefferson for this soil nitrate testing this spring because of the rainfall patterns they've had in 2007 and because we have data on previous years when those farmers have experienced nitrogen losses."

Some fields are very deficient of N

Is that deficient or "way deficient"? "I'd say we are seeing some fields that are somewhat deficient and some fields I would call very deficient," says Blackmer. "We are using the fast, rapid test kits. We are double-testing by also sending these soil samples to the lab at ISU. However, these, in-the-field, rapid test kits are usually pretty accurate, so I think we're going to see the results coming from the testing lab at ISU also showing nitrogen deficiency in these fields." He adds, "I've never before seen this many soil samples test this low, using the late spring soil N test." In the area around Jefferson farmers have had 6 inches of rain in the last few weeks. "A lot of areas, especially in the western half of Iowa, I would call quite wet this year," says Blackmer. "When we've looked at the data historically in that particular area - for example if we look at the results for 2004 - over half of the samples we stalk tested in the fall showed that the stalk concentrations were low enough that the farmers probably lost yield. Whereas the average across all the other years, the other six years, only 16% of the stalk samples were deficient in N."

What is unusual about this year?

"We know that in wetter years you are more likely to see nitrogen loss. I guess it's not that surprising but it is surprising to see this many fields that are deficient in nitrogen at this time of the year," says Blackmer. ISU is running the lab test on the "quick test" samples and those results aren't back yet. "But I've conducted the quick test enough in past years that I feel these numbers are probably fairly representative, especially when you compare the weather we've had this year to those other wet years when we've had significant N losses. I also know that when we're out there sampling now, a lot of the corn plants in the fields look like they are nitrogen deficient at this point in time." Blackmer says he's seeing some nitrogen stress on corn already? So what's he recommend farmers do? "I think they should be taking soil samples and using the late-spring soil nitrate test to get a handle on just where they are at, at this point in time," he says. "Also, having a reference strip - sidedressing vs. no sidedressing--in their field will help so they can look for a difference - as time goes on. That will be important because a lot of things can change even after today. But you look at the main factor - how much rain and how much nitrogen is lost and what is still available in the soil."

Corn uses most N in the next few weeks

"This story isn't over yet," says Blackmer. "What we're predicting now is: we need to know how much rain we are going to have in the next four to six weeks. That is when the corn plant takes up most of its nitrogen."

It's too bad that a lot of nitrogen has leached from the soil because of the wet weather we've had this spring. The N that's leached away isn't available to the corn plant. That's too bad because people in these wet-test areas, such as western Iowa, want to grow a lot of corn - just like everyone else does this year.

Everyone wants to optimize their profitability and if there is a way you can predict you are going to be short on N, you'd like to correct that deficiency if possible, notes Blackmer. Of course you want to stay in the game and you don't want to violate any rules. Most farmers probably put on right up against the maximum rate per acre they could apply last fall or earlier this spring. Now what can they legally do to be able to add more nitrogen back?

Check your nutrient management plan

In terms of the NRCS plan - or DNR's nutrient management plan for manure - you must have some evidence to justify applying more nitrogen. "I'm assuming everyone is following their nutrient management plan to begin with," he says.

"Some of the confusion we've had in past years is some people interpreted that as a cap - that you could apply no more than a certain amount. Actually, based on ISU recommendations - there are two different sets of suggestions--one for manure and one for fertilizer. Both DNR and NRCS will follow guidelines based on using the late-spring soil nitrate test and what that test reading is. You can apply varying amounts of N based on what the test results are," says Blackmer.

He says you should follow ISU Extension guidelines which are on the ISU Web site. For just applying regular fertilizer for corn, the nonmanure fertilizer, its Extension publication PM 1714 that contains the correct guidelines you want to follow. For manure management, it would be ISU Extension publication PM 1811. Both of those publications give the guidelines very clearly.

How much N per acre can you apply?

The guidelines in PM 1714 are for fertilizer in corn, and Extension publication PM 1811 for manure management. Then based upon the results of your late-spring soil nitrate test of which you need to run, you can determine how much nitrogen per acre you can apply on a particular field of corn, he says.

Based on manure management plans, isn't there a number of "up to 90 pounds per acre" that is specified as the amount of nitrogen you can apply and still be within the regulations? Is that a relevant number? "Yes, but how much you should apply depends on how low or high your soil sample is for nitrate content," answers Blackmer. "You get different degrees of deficiency in different fields. So on the lower range, the publications will recommend zero to 30 if you are going to apply more fertilizer nitrogen. The point is, your field can't be slightly deficient and then you apply 90 pounds of N per acre. So you need to make sure that you are deficient enough to be able to apply the higher rate. "

Some farmers need to be looking at sidedressing nitrogen now before the corn grows too tall. "Also, there are some nitrogen applicators available that will let you go into the field of taller corn and apply N even into mid-season," he says.

What about cost of nitrogen?

Another consideration is the high cost of nitrogen fertilizer. "When someone talks about the fact that you can apply up to 90 pounds of additional N as a sidedress application, you need to consider that nitrogen is running about 50-cents a pound today for liquid N. That's pretty pricey. If you apply 50 pounds, it's $25 per acre plus application cost. So I wouldn't get all excited to go out and put on 90 pounds unless you really know you have a good reason to," he adds.

Some people have had to replant their corn due to flooding, and now they also have to apply some extra nitrogen as a sidedress. Those flooded-out areas of fields are where you need the extra nitrogen the most, he notes. In some of those areas they can't even get in to take the soil sample for the late-spring soil nitrate test yet. Some of those guys replanting are getting stuck in the wet fields."

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