A Comparison of Two 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.