Soil Tests for Nitrogen Recommendations for Corn.

A significant portion of corn (Zea mays L.) nitrogen (N) needs may be met by soil residual and/or mineralized organic N. Nitrogen management recommendations can be improved by better understanding and estimation of this soil N supply pool (SNSP) on a field by field basis. In the past few decades, many soil testing approaches have been developed and evaluated to better estimate the SNSP. However, only a few Land Grant University recommendation systems include specific SNSP estimates either as soil type specific book values that adjust the total N requirement in mass balance type equations or through the use of estimated N release based on measured soil organic matter.  More typically, states recommend a soil nitrate test, either pre-plant or pre-sidedress, to adjust initial guidance based on the soil nitrate pool and more recently a few states have also included a soil organic N mineralization test. In this presentation, we will briefly discuss the importance of the SNSP for estimating external N requirements of corn and illustrate how some states recognize this SNSP when deriving a recommendation. We will also illustrate how the plant-soil resilience concept (where soil N mineralization is directly related to external crop N requirement) may be used in conjunction with soil testing to improve external N recommendations.