Sample Distribution Sample distribution usually depends on the degree of variability in a given area. In relatively uniform areas smaller than 25 acres, a composite sample of 20 to 30 cores taken in a random or zigzag manner is usually sufficient. Larger areas are usually subdivided into smaller ones. Non-uniform areas should be subdivided on the basis of obvious differences such as slope position or soil type. Banding fertilizer creates zones of very high fertility in soils because the fertilizer is mixed with only a small portion of the soil.
Samples taken in the band can greatly overestimate the overall fertility of a site.
Because the position of fertilizer bands is rarely known with certainty, one should take more random samples than usual in fields with fertilizer bands and vary sampling position with respect to row location to ensure that the bands do not bias test results. For non-uniform sites, a systematic sampling approach is best. Sampling in a grid pattern can give an idea of variability in a field and fertilizer application can be adjusted according to the distribution of soil test results within the grid. The grid spacing can vary from as little as 30 feet to several hundred feet. Often the grid spacing is some multiple of fertilizer applicator width.