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 with other site-specific information, Researchers continue to use two methods of determining N credits, the traditional and difference techniques, which often produce unequal estimates, Our objective was to clarify when each method provides accurate N credit estimates. The traditional method compares yield of a nonfertilized nonlegume crop grown in rotation to the fertilizer N response curve of the continuously cropped nonlegume. This approach assumes that fertilizer N compensates for all benefits of rotation. The difference method compares the economic N rate of the nonlegume crop grown in rotation with that of the continuously cropped nonlegume, We use examples from the literature to demonstrate that when non-N rotation effects are present, N credit estimates from the two methods will differ. The difference method is more accurate and should be used unless it has been demonstrated that non-N rotation effects are not present.