Many factors can influence the accuracy of soil test results, ranging from field sampling technique, sample preparation, and quality control in the laboratory. Many people expect that if a field is sampled more than once, the soil test results should be identical. When identical results are not obtained from successive sampling, much concern about soil test reliability is of ten expressed.
We have analyzed soil test results from some controlled field experimental sites which help provide an understanding of variability which can occur naturally in the field, how various field sampling techniques influence soil test readings obtained, and how laboratories duplicate readings from the same samples tested on different dates. The examples discussed here represent only a few of the many scenarios which can affect soil test results.
Wells, Kenneth L. and Case, Vern, "Variability In Soil Testing" (1995). Agronomy Notes. 21.