Time To Pull Samples For Late Spring Soil Nitrate Test

Soil test can show whether there's adequate nitrogen available for optimum corn yields or if the corn will likely benefit from side-dressing additional N. It's important to collect the soil samples properly.

Published on: May 29, 2011

Heavy rains in recent days have stopped field work for awhile in many Iowa areas and have also resulted in some soil erosion. Corn planting is essentially finished (at least for the first time), but many farmers have a field or two of soybeans yet to be planted.

"Much of the early-planted corn has reached the V3 growth stage now as seen in the above picture, which is the time to start pulling soil samples for checking for soil nitrate levels," says Jim Fawcett, Iowa State University Extension field agronomist in eastern Iowa. "Soil samples should be taken for this test when the corn is between 6 to 12 inches tall."

Fawcett says there is some increase in interest among farmers in using the soil nitrate test this year, perhaps partly due to the extensive nitrogen losses in the past few years as well as the high cost of nitrogen fertilizer. Another reason for more interest is that the soil test information is needed for participants in some of the options that are offered by the USDA Conservation Stewardship Program.

The soil test can show whether there is adequate nitrogen available for optimum corn yields or if the corn will likely benefit from side-dressing some additional nitrogen. "It's important to collect the soil samples properly," says Fawcett.

How to Take Soil Samples

  1. Soil samples should be pulled to a one foot depth when the corn is V3 to V6 growth stage (about 6-12 inches tall). It is recommended to pull at least 16 soil cores in 2 sets of 8 and pull them in a systematic way across corn rows.
  2. Pull the first core in the row, the next 1/8th of the distance between rows, the next 1/4th of the distance, and so on. This is to try to reduce the likelihood of your sample happening to hit a fertilizer band on each core.
  3. If anhydrous has been applied, 24 cores per sample should be taken (3 sets of 8). A subsample of the soil can then be sent in a soil sample bag to the lab for nitrate testing. Don't allow the bags to sit for a day or two in a hot pickup before sending them in to the lab.

What  will you get back from the lab? The lab test will show how many parts per million of nitrate are in the soil, says Fawcett. Usually 25 parts per million (ppm) is considered to be the magic number, although in springs with excess rainfall (20% above normal from April 1 to the sampling date), a lower critical value of 20 to 22 ppm should be used. This is because with the excess rain there will be more of the nitrate that has leached below the top foot but hopefully will still be in the corn root zone.

"I've used 20 ppm as the critical value for the last few springs," says Fawcett. For more details on nitrogen management and using the late spring soil nitrate test, see http://www.extension.iastate.edu/Publications/PM1714.pdf.