Don't Be Confused By Late Spring N-Test Reports

Here's how to use and interpret the results, to make the decision whether or not to apply more nitrogen. Rod Swoboda

Published on: Jun 21, 2004

A number of farmers, crop consultants and fertilizer dealers have used the Late Spring Soil Nitrate Test during the past few weeks, and are asking questions about the results. While every situation is different, many tests have come in around the 10 parts per million (ppm) range for non-manured fields, and in the 15 ppm range for manured fields. So what does this mean?

The test uses 25 ppm as the upper end of the range signifying that no additional fertilizer N is required. "Thus, you don't have to apply any additional nitrogen fertilizer if your soil tests 25 ppm or more for nitrate," explains Brian Lang, an Iowa State University extension crop specialist in northeast Iowa.

This number is lowered to about 21 ppm if spring rainfall exceeds 20% of normal (which ours has), notes Lang. The number is further lowered to about 15 ppm if the field was manured and the heavy rainfall occurred.

An example of how to interpret the N test

He offers the following example of how you should interpret the test results.

"Say your soil test is 10 ppm. You subtract that from the 21 to give you 11," says Lang. "This number is then multiplied by 8 to give a fertilizer N recommendation for sidedressing. That figures out to be 88 pounds per acre in this case."

What about manured fields? In many of these fields this spring, the soil test results are showing 15 ppm of N present in the soil. "In this case the 15 ppm number is subtracted from your 15 ppm soil test level to give you zero," says Lang. "Thus, no additional fertilizer nitrogen is required."

The publication explaining soil nitrate testing can be found on the Web at www.extension.iastate.edu/Publications/PM1714.pdf (This link requires your computer to have Adobe Acrobat Reader. For a free download, click HERE.)

Farmers who have used the test a number of times over the years are more familiar with the yield responses in their particular fields. "They have their own results to indicate whether adding zero or 30, or 60 or even 90 pounds of fertilizer N as a sidedress application would pay in their situation," he says.

Some farmers want to use the test too late

Some farmers who did not use the test were asking about using it in mid-June. "By that point, we were quickly getting to the end of the window where this test is useful," says Lang. "You need to use the test when the corn is between 6 to 12 inches tall."

The test is not perfect, but if it errs, the error tends to be on the low side. In other words it might recommend a little more N then what might be needed. In fairness to this test, it is still the most actuate means we have available to measure N conditions in this wet environment, says Lang.

"Otherwise, we use color of the corn plant as the guideline to help decide whether or not you should add additional N as a sidedress,'" he notes. "Green corn suggests we are in good shape. Pale corn suggests the need for 30 to 60 lbs of N. Visibly N deficient corn suggests the need for 60+ pounds of N assuming the plants are still healthy and not suffering from excessive root rots from which they may not recover."

If plants are "green" now but turn more pale color later, research has shown that corn will respond to N applications all the way to tassel stage. The following is a link to a 1999 article on this issue. Lang refers to it because it does a nice job of summarizing most everything we've already discussed about N in a wet spring. Go to www.ipm.iastate.edu/ipm/icm/1999/6-14-1999/estnloss.html