Illinois
Fertilizer Conference Proceedings
January 24-26, 1994
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Ted R. Peck and David W. Franzent1
Two Illinois 40-acre (16 hectare) fields were sampled by three different sampling schemes. The dense 16 by 16 grid (5 rod or 25 meter grid spacing) was compared to both a 4 by 4 grid (20 rod or 100 meter) and a 4 by 4 grid (13.2 rod or 66 meter). Correlations between soil test levels predicted from mapping prepared from the 4 by 4 grid were equal to and often superior to the 6 by 6 grid when compared to the 16 by 16 grid sampling. The 4 by 4 grid and 6 by 6 grid estimates often fell into the same range of fertilizer rate determined ranges, however, this was because many of the pH, P, and K tests were at a high level. Despite wide differences between predicted and actual levels, especially in the 4 by 4 grid, the predicted values were still in the maintenance or zero fertilizer ranges and therefore were classified in the same range.
The 6 by 6 grid best represented the 16 by 16 grid mapping by displaying most of the important soil test level features of the two fields. The boundaries shown by the 6 by 6 grid also approximated those of the 16 by 16 grid sampling. The 6 by 6 grid offers improvements in soil test level prediction over the 4 by 4 grid or less intensive grids now being used to direct variable rate fertilization.
1Ted R. Peck is Professor of Agronomy and David W. Franzen is former Graduate Research Assistant, Dept. of Agronomy, Univ. of IL.
