A Classification System for Factors Affecting Crop Response to Nitrogen Fertilization
- John A. Lory ,
- Michael P. Russelle and
- Gyles W. Randall
Crop response to N fertilizer (Nf) is influenced by factors such as Nf 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 Nf recommendation. This paper describes a three-component classification system for evaluating the effect of any factor on yield response to Nf. This system provides (i) a vocabulary to describe clearly the effect of a factor on Nf recommendations, and (ii) insight on how to adjust Nf recommendations for the effect. Factors that affect yield response to Nf but not to N supply (Ns) 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 Nf and Ns 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 NO-3 in the surface 1.5 m of the control plot, and Nf applied. Two 2-yr experiments were conducted at Rosemount and Waseca, MN, to compare Nf 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 Nf. 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 Nf 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 Nf recommendations. This classification system facilitates the organization, evaluation, and communication of the many factors that influence crop yield.
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