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 mi neralized N, but there is no framework to define the best strategy to account for a given factor in an N-f recommendation. This paper descri bes a three-component classification system for evaluating the effect of any factor on yield response to N-f. This system provides (i) a voc abulary to describe clearly the effect of a factor on N-f recommendati ons, and (ii) insight on how to adjust N-f recommendations for the eff ect. 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 curv e in the x and/or y direction, with no change in coefficients of curva ture). Factors that interact with N-f and N-s response were classified as interaction effects. Nitrogen supply was defined as the sum of abo veground plant N content of the control (0 N applied) plot, postharves t 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 c ompare N-f response of continuous corn (Zea mays L.) with that of firs t- and then second-year corn following alfalfa (Medicago sativa L.). W e used the classification system to evaluate effects of crop sequence, year, and location on corn yield response to N-f. Year and crop seque nce effects at Rosemount were primarily shift effects, implying that q uantifying the effect on Ns would be sufficient to account for these e ffects on N-s recommendations. In contrast, the interaction model pred ominated at Waseca. Consequently, at this location simple adjustments of Ns were not sufficient to account for the complexity of crop sequen ce effects on N-f recommendations. This classification system facilita tes the organization, evaluation, and communication of the many factor s that influence crop yield.