Testing nitrogen (N) levels this fall can save producers time and money next spring, a University of Nebraska-Lincoln soils specialist says.
Fields that will be planted into corn or sorghum next year may show higher than normal nitrates. Soil sampling for nitrogen levels following harvest will determine exact N levels. This gives producers a chance to plan for spring fertilizer applications, says Charles Shapiro, soils specialist at UNL's Haskell Agricultural Laboratory near Concord.
"Fall is a convenient time for farmers to test nitrogen levels because they have the winter to develop a fertilizer plan before spring planting. In some years they also can save money by taking advantage of lower fertilizer prices," Shapiro says.
N generally is available for plant growth. Therefore, the purpose of N testing isn't to determine whether or not nitrogen exists in the soil, but to determine how much N is available for plant use, Shapiro says.
"Nitrogen carry-over should be relatively high this year due to the drought," Shapiro says. "In situations where yields were low, a lot of the nitrogen didn't get used."
One benefit of high carry-over is the decreased need to apply N fertilizer in the spring. However, producers need to remember that N levels can decrease over the winter months. Wet winters cause N to leach below the root zone, thus resulting in lower levels than indicated by fall measurements, he says.
"There are some environmental conditions that would reduce soil nitrogen in the spring compared to the fall, but if conditions do not get excessively wet, then the fall soil tests should be valid," Shapiro says. "It is possible that where fields received rain this fall, which increased soil microbial activity, released a flush of nitrogen."
N tests are taken using a soil probe to collect samples from different locations within the field. It’s important to take soil samples correctly, Shapiro says. Soil probes generally are inexpensive or can be borrowed from fertilizer consultants or local extension offices.
The samples are then sent to a lab for testing. Analysis cost depends on whether a complete soil analysis is done or one only for soil nitrates, Shapiro says. The cost for soil organic matter and nitrates is around $10-15. A complete analysis is about $25. Each laboratory has its own combinations, Shapiro says.
For more information about fertilization for corn, consult UNL Extension NebGuide G174, Fertilizing Suggestions for Corn, available online at http://www.ianrpubs.unl.edu/epublic/pages/publicationD.jsp?publicationId=142  or at local extension offices.