Multiple regression models for extractable soil Zn, yield, and wheat grain Zn.

Models were determined by a step-wise selection process that maximizes Akaike’s Information Criterion. For each response variable (extractable Zn, yield, and grain Zn), one model was fit considering only soil and plant explanatory variables (models 1, 2, and 5). A second model additionally considered appropriate management variables (models 3, 4, 6, and 7). None of the considered management variables improved the extractable Zn model. See Table 1 for a list of explanatory variables included in the maximum scope for each model selection process.