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A Comparison of Two Nitrogen Credit Methods: Traditional vs. Difference

Lory, John A., Russelle, Michael P., Peterson, Todd A.
Agronomy journal 1995 v.87 no.4 pp. 648
nitrogen fertilizers, estimation, accuracy, crop rotation, continuous cropping, Zea mays, field crops, legumes, cover crops, Trifolium incarnatum, Glycine max, crop yield, fertilizer requirements, application rate, grains
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 and 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.