The importance of collecting a good sample cannot be overemphasized. No matter how accurate the analytical results or how knowledgeable the person who interprets the results, the developed fertility programs cannot be better than the initial sample collected. If the sample is not representative of the field or area of the field in question, the analytical results will be of little value.
While there can be large variations in soil test values within a field, equally large variations exist for samples collected only inches apart. As a result of this variability, it is necessary to collect and consolidate 15 to 20 individual subsamples from each field or portion of a field regardless of the acreage represented by the sample. At a minimum, it is best to collect a separate composite sample for every 40 acres in a field. Regardless of whether the field is to be managed uniformly across or if inputs will be variably managed within a field, it is best to delineate and individually sample portions of the fields that are similar (e.g. top, side-slope, bottom of hills; high, medium, low yielding portions of a field; etc.). The greater the number of samples collected from a field, the better the information base will be on which to develop an overall fertility program.
Soil sampling depth is extremely important and should be consistent from person-to-person and year-to-year. Proper sampling depth for soil pH, organic matter (OM), phosphorus (P), potassium (K), and zinc (Zn) is the surface six to eight inches, since this is the depth that the soil tests were calibrated for in university research. Sampling deeper or shallower than this will provide misleading results. An exception is made for no-till and very reduced-till systems where soil pH should be monitored and managed at a depth of two to three inches since that is the limit of the depth to which soil acidity accumulates in these systems. For available nitrogen (N), chloride (Cl), and sulfur (S), samples should be collected to a minimum depth of 24 inches since these nutrients are mobile in soils. The importance of consistency of sampling depth cannot be overemphasized. Remember, consistency of sampling depth from person-to-person and year-to-year is extremely important for developing the longer term value of a soil test history.