A composite soil sample should represent a uniform field area. Each area should have a similar crop and fertilizer history for at least the last two years. Exclude small areas within a field that are obviously different. These can be sampled separately if they are large enough to warrant special treatment. The field area represented by a single composite sample should represent no more than 40 irrigated acres or 100 dryland acres.
Fewer acres is better.
Sampling Procedure Use a systematic sampling scheme. Obtain an accurate nutrient evaluation of a field site with 15 to 20 surface subsamples per 40 acres and six to eight subsurface cores. Mix these subsamples thoroughly and save one pint for analysis. This pint mixture is the composite soil sample. In some cases, the number of subsamples may be limited by time constraints or availability of labor.