Variations in Field Level P and K Fertilizer Additions Based on Grid Sampling.

Grid sample size has been and item of debate for many years and what is the proper method of sampling to measure spatial variability in a production field. Today’s technology offers many mechanisms based on GPS coordinates to locate soil samples. One easily surmises that the more samples from an area the better for accurately accounting for the variability. However, the downside is the cost of taking and analyzing the samples. In most areas, the agreed upon size is 2.5 acres but there is no real justification for that size. This exercise started with a 100 x 100 ft grid (0.23 acres) and progressively increased to a 1000 x 1000 ft grid (22.95 acres) to determine total phosphorus and potassium needs for a 46-acre area. The soil test level from each 100-ft grid was averaged for each larger grid size. As an example, nine 100-ft cells were averaged to determine the value for a 300-ft grid. With respect to phosphorus, total phosphorus fertilizer (0-46-0) needs ranged from 3833 to 5490 lb/field depending upon grid size which translates to a 43% range with the lowest level for the 400-ft grid size (3.67 acres) and the highest level with the 100-ft grid. With respect to potash, the range was 4385 to 4752 lb 0-0-60/field, a much narrower range (8%) with the lowest needs from a 300-ft grid (2.07 acres). When using the techniques outlined, one obtains a better measure of the grid but this technique also leads to errors. Detailed sampling allowed for the best coverage of the area but when composited for larger cell size led to an averaged that did not reflect any cell within the grid. Spatial variability can be detected with detailed sampling but may be cost prohibitive.