Cate and Nelson (1965) % yield versus soil test level Two Groups: 1. probability of response to added fertilizer is small 2. probability of response to added fertilizer is large A.Percent yield values obtained for a wide range in locations (fertilizer rate studies) Percent yield = yield at 0 level of a nutrient / yield where all factors are adequate B. Soil test values obtained (Check Plot) Will generate a single % yield and one soil test value for each location C. Scatter diagram, % yield (Y axis) versus soil test level (x axis) Range in Y = 0 to 100% D. Overlay -overlay moved to the point where data in the +/+ quadrants are at a maximum -point where vertical line crosses the x = critical soil test level

Critcal level depends on the extraction method used and crop being grown. Cate-Nelson: Maximizes the computed chi-square value representing the test of the null hypothesis that the # of observations in each of the four cells (quadrants is equal).

120 100 80 Percent Yield 60 40 20 20 40 60 80 100 120 Soil test P, ug/g