Fig. 4. Cate-Nelson models of relative light reflectance measurements and relative clipping yield for turfgrass in relation to soil NO₃–N desorbed from in situ anion exchange membranes. Relative reflectance and yield values are expressed as a fraction of plateau values for reflectance or yield for each sample date. Vertical lines to the x-axes represent critical x values which divide data into two populations, one which has a high probability of response to higher soil NO₃–N values and one which has a low probability of response. Horizontal lines minimize data in quadrants I and II. Error I indicates the percentage of observations in quadrant I and represents an overprediction of the critical x value. Error II indicates the percentage of observations in quadrant II and represents an underprediction of the critical x value. Data are pooled from 23 dates across two growing seasons. Number of observations for each plot is given by n.