A COMPARISON OF 2 NITROGEN CREDIT METHODS - TRADITIONAL VS DIFFERENCE

Cereals and other nonlegumes typically require less fertilizer N when grown following a legume, Nitrogen credits for a previous legume crop often are used to reduce fertilizer N recommendations in combination w ith other site-specific information, Researchers continue to use two m ethods of determining N credits, the traditional and difference techni ques, which often produce unequal estimates, Our objective was to clar ify when each method provides accurate N credit estimates. The traditi onal method compares yield of a nonfertilized nonlegume crop grown in rotation to the fertilizer N response curve of the continuously croppe d nonlegume. This approach assumes that fertilizer N compensates for a ll benefits of rotation. The difference method compares the economic N rate of the nonlegume crop grown in rotation with that of the continu ously cropped nonlegume, We use examples from the literature to demons trate that when non-N rotation effects are present, N credit estimates from the two methods will differ. The difference method is more accur ate and should be used unless it has been demonstrated that non-N rota tion effects are not present.