Checking for Residual Nitrate This Spring - UNL CropWatch, March 6, 2013

Checking for Residual Nitrate This Spring - UNL CropWatch, March 6, 2013

March 6, 2013

The 2012 growing season was unique in many ways. Without irrigation, crop yields were disastrously low, or non-existent. In many cases fields were declared a total loss and the crop was shredded instead of harvested, except for yield check strips for insurance purposes.

Soil tests are indicating highly variable levels of nitrate.

  • In dryland crops where there was little or no yield, residual N is quite high.
  • For irrigated crops, if 2012 yields were quite high, residual N may be low. If yields were good but not exceptionally high and mineralization was above average, nitrate levels may be high.

With irrigation, crop yields in parts of the state were at record high levels. Several factors contributed to high yield, including many cloud-free days and high solar radiation, and no nitrogen (N) loss due to leaching, denitrification or runoff, given there was no rain. Also, frequent irrigation resulted in good moisture conditions for N mineralization throughout the growing season. In several UNL long-term research studies we observed the highest yields ever with unfertilized check treatments, primarily due to high N mineralization rates. Mineralization also continued in the fall after harvest until soils approached freezing, as long as soil moisture was adequate.

Conditions from 2012 have resulted in residual nitrate-N levels this winter and spring which are wide ranging. Reports from crop consultants and fertilizer dealers who conducted deep sampling for nitrate-N last fall indicate that values are often quite high. This is particularly the case where dryland crops failed or produced low yields in 2012. Fertilizer N applied for the crop in 2012 was not used by the crop, and there has not be enough rain to cause N to be lost through leaching or denitrification. For irrigated crops, we are finding both high and low residual N values. If 2012 yields were quite high, residual N may be low. If yields were good but not exceptionally high, and mineralization was above average, nitrate levels this winter may be high.

Sampling done this spring at UNL's South Central Agricultural Laboratory near Clay Center from fields which were in both irrigated corn and soybean last year found a wide range of residual nitrate — mostly resulting from high mineralization rates last summer and fall. At the Lab, most of the residual N is located close to the surface, ranging from 20 to 50 lb N/acre in the surface eight inches. Subsoil nitrate concentrations, from 8 to 36 inches, are generally low, from 20 to 30 lb N/acre. The main issue is that nitrate levels are quite variable, depending on the previous crop, how it was fertilized, and irrigation and weather conditions.

These indicators suggest that it is very important to check residual nitrate-N levels in fields this spring (following both corn and soybean in 2012), because they may be higher than previous years in many cases, and likely will be highly variable from field to field. If fields were soil-tested last fall, those results should still be fairly accurate. This winter has not been very wet or warm, so nitrate concentrations are not likely to have changed significantly from last fall to early spring 2013.

Related Extension Resources

Richard Ferguson, Charles Shapiro, Charles Wortmann, Tim Shaver, Gary Hergert
Extension Soils Specialists