With the wettest spring on record in the state of Iowa, fields have undoubtedly lost some nitrogen. How much N is left in the soil? A good way to check the N status of the your fields is to take 1-foot depth soil samples when the corn is 6 to 12 inches tall, says John Sawyer, Iowa State University Extension soil fertility specialist.
At least 16 soil cores (24 is better) should go into each sample and about a cup of this (soil bag full) sent to a lab for analysis. Cores should be pulled in a systematic way going across corn rows (i.e. first core pulled in the row, next one is pulled one-eighth the distance between rows, next one-fourth the distance between rows, etc.). Either take the samples to the nearest soil testing lab or send them via overnight delivery. If the test shows you need to apply more nitrogen, do so as a sidedress treatment.
Iowa State University Extension has a publication that describes the process of sampling soil for the late spring nitrate test. There's also an information sheet with instructions for sending the samples to the ISU testing lab in Ames. The cost for analysis is $5 per sample.
Don't just assume you need to add more nitrogen for your corn this spring
How much nitrogen is still in the soil? There were various reports in the news this spring about higher levels of nitrate showing up in water supplies used by cities and towns in Iowa to supply water to residents. There's no denying that tile lines are running again, after last year's drought. After more than a year of drought conditions there's bound to be nitrate in the water, leaching from soil following all the rain this spring. And some of the nitrogen that was in fields last fall has undoubtedly been moved down deeper in the soil profile, due to the moisture recharge of both top soil and subsoil throughout Iowa.~~~PAGE_BREAK_HERE~~~
However, keep in mind that neither of these factors automatically equate to nitrogen deficiency conditions for the 2013 corn crop, points out Mick Lane, research communications coordinator for the Iowa Soybean Association's On-Farm Network. Understanding the nitrogen cycle is also important in properly managing nitrogen for crop production, he adds.
Here's what to do to help you decide whether more nitrogen is needed this year There are a couple of things growers can do before assuming that more nitrogen is needed this year. First, scout your fields early and often, looking for nitrogen deficiency symptoms. If you see symptoms or have reason to believe corn may be running short of N (maybe you didn't get N applied before planting, or maybe you only applied a portion of what you anticipated the crop might need), then consider making use of the late-spring soil nitrate test to determine current soil nitrate content. Most commercial soil and plant tissue testing labs will run this test.
Details for collecting samples can be found on pages 2-3 of ISU Extension publication PM 1714 "Nitrogen Fertilizer Recommendations for Corn in Iowa". Also read the article "Predicting the Reliability of the Late Spring Soil Nitrate Test", by Peter Kyveryga, ISA On-Farm Network senior research associate.
Corn does best if the soil nitrate level is between 21 and 25 parts per million, says Kyveryga. If soil nitrate content within the top 12 inches of soil is less than 21 parts per million, there's a higher probability that the crop would respond to additional nitrogen. If soils have received a recent application of manure, however, the critical nitrate content is only 11 to 15 ppm, since historically, microbial action in the soil continues to mineralize nitrate from manure organic matter throughout the growing season. For more information on managing nitrogen in a wet spring, see the ISA On-Farm Network publication referred to previously in this article and the ISU Extension publication "Nitrogen Fertilizer Recommendations for Corn in Iowa", PM 1714. Information on general nitrogen management for corn can be found in a number of presentations from past On-Farm Network conferences, including Basics of Nitrogen, Nitrogen Management: Forms, Timing and Placement and the On-Farm Network's July 2008 Update newsletter.