UNDERSTANDING THE MAGDOFF PRE-SIDEDRESS NITRATE TEST FOR CORN

Problems associated with using excess N for corn (Zea mays L.) production include NO3-N pollution of groundwater as well as economic loss to the farmer. The Magdoff PreSidedress Nitrate Test (PSNT) is a soil test that provides more accurate fertilizer N recommendations for corn than previous N recommendation systems based on anticipated effects of manure and crop management. Using the test can result in significant reductions of fertilizer N use on high-N supplying soils. This article provides the background for understanding how and why this soil test functions for corn in the humid regions of the Midwest and Northeast. The pattern of high-crop N uptake rates relative to mineralization indicates that adequate nutrition requires the presence of a large available N pool in the soil at the start of corn's period of rapid growth. In addition, there is little NO3-N leaching in most soils during normal growing seasons and when NO3-N is moved deeper in the root zone, some enrichment remains in the surface 1 ft. This combination of phenomena allows NO3-N in a 1-ft soil sample taken just prior to the period of rapid growth to reflect fertilizer N requirements.