A CLASSIFICATION-SYSTEM FOR FACTORS AFFECTING CROP RESPONSE TO NITROGEN-FERTILIZATION

Crop response to N fertilizer (N-f) is influenced by factors such as N-f management, soil type, crop sequence, and supply of residual and mineralized N, but there is no framework to define the best strategy to account for a given factor in an N-f recommendation. This paper describes a three-component classification system for evaluating the effect of any factor on yield response to N-f. This system provides (i) a vocabulary to describe clearly the effect of a factor on N-f recommendations, and (ii) insight on how to adjust N-f recommendations for the effect. Factors that affect yield response to N-f but not to N supply (N-s) were classified as shift effects (i.e., movement of a response curve in the x and/or y direction, with no change in coefficients of curvature). Factors that interact with N-f and N-s response were classified as interaction effects. Nitrogen supply was defined as the sum of aboveground plant N content of the control (0 N applied) plot, postharvest fall NO3- in the surface 1.5 m of the control plot, and N-f applied. Two 2-yr experiments were conducted at Rosemount and Waseca, MN, to compare N-f response of continuous corn (Zea mays L.) with that of first- and then second-year corn following alfalfa (Medicago sativa L.). We used the classification system to evaluate effects of crop sequence, year, and location on corn yield response to N-f. Year and crop sequence effects at Rosemount were primarily shift effects, implying that quantifying the effect on Ns would be sufficient to account for these effects on N-s recommendations. In contrast, the interaction model predominated at Waseca. Consequently, at this location simple adjustments of Ns were not sufficient to account for the complexity of crop sequence effects on N-f recommendations. This classification system facilitates the organization, evaluation, and communication of the many factors that influence crop yield.