Optimum Grid Size for Sampling Soil Variability - Science, Mystery, or Magic

The oral presentation will summarize a project to evaluate grid size for soil sampling when trying to determine soil/nutrient variability.  Initial grids were set up om 100-ft intervals in a field to be set up as an experiment station field.  A 46-acre sub-section was first evaluated in 100' x 100' (0.23 acre) cells then were composited to look at 200' (0.92 acre, 4 cells), 300' (2.07 acres, 9 cells), 400' (3.67 acres, 16 cells), 500' (5.74 acres, 25 cells), and 1000' (22.96 acres, 100 cells).  Total phosphorus and potassium was calculated based a given grid size and summarized for the 46-acre tract.  Often a grid size is listed as 2.5 acres yet there is no known justification for selecting a particular size based on field uniformity or field variability.  Common knowledge would determine that the more samples from a given area the better representation one would have of the inherent variability.  When using a variable rate applicator, the equipment is limited to the size of the spreader and the speed of the machine.  As you will see in the presentation, errors are possible and quite likely depending upon grid size.