Most progressive precision soil sampling programs are sampling fields on a 2- or 3-year cycle. Often in the course of 2 to 3 years, there have been changes in the personnel or equipment used to collect those samples. There are a few clues in your soil test results that can be examined to validate if those samples were collected consistently.
The first clue to check is the organic matter level. The organic matter has the least potential to change significantly over the course of a few years. Even under intensive management to increase organic matter, such as no till, cover crops, and residue management, it is unlikely to see the soil test level increase by more than 0.1% per year. Any drastic change in organic matter levels likely indicate inconsistent sample depth, contamination of the sample with crop residue or manure, or an inadequate number of soil cores being collected to make up the sample.
The cation exchange capacity (CEC) should remain relatively consistent from sampling event to sampling event. The CEC is a measurement of the negative charge in a soil which comes from the clay mineralogy and organic matter that make up the soil. These 2 factors do not noticeably change in just a few years. On a routine soil analysis, the CEC is calculated from the extractable levels of calcium, magnesium, potassium, and hydrogen. Calcium and magnesium are generally the greatest contributors to the CEC. Unless extremely high rates of lime or gypsum have been applied, these 2 nutrient levels generally stay consistent resulting in a consistent CEC calculation.
Surprisingly, one of the numbers on your soil test that should not drastically change from sampling one sampling event to the next is phosphorus (P). Assuming your soil test P is at an agronomically desirable level, a high yielding corn or soybean crop are not likely to lower your soil test level more than 4 or 5 ppm in a single growing season. If the soil test P level changes more than 10—15 ppm between routine sampling events, it may be the result of inconsistent soil sampling procedure.
To truly compare soil test results from one sampling to the next, it is critical to minimize the variability. To do so soil needs to be sampled to the same depth, following the same crop, at the same time of year, and consist of at least 8 cores.
If you have any questions regarding irregular soil test results, please contact your ALGL agronomist.