The best soil core quality and the best quality soil samples come from an undisturbed soil profile. Tillage can have quite an effect on soil test levels. Sampling an undisturbed soil can be done in the early summer or after harvest before any tillage. Soil core quality is very important for getting the most accurate and repeatable soil test results.
In 2013, AGVISE did a project to show the effect tillage can have on the variability of soil test values. We grid sampled (topsoil only) an 80-acre field after harvest and prior to tillage (stubble). Then we waited until the field had been chisel plowed and sampled the same 16 GPS points again. The results in the table show tillage can have quite an effect on soil test results. If you look at the field average for each soil test, they are fairly comparable. But you can see that some points had quite different soil test levels at each point for samples taken before or after tillage. For example, the phosphorus test results were quite different on a few points. This can be caused by the lumpy tilled top soil not flowing into the soil probe evenly, resulting in a different soil core than if the soil had not been tilled. You can see the same variability with the other tests, but to a smaller degree.
Soil sampling in the early summer (topsoil only) or after harvest but before tillage will give you the most accurate and repeatable test results. If you have to sample tilled fields, remember to collect the soil cores from a compressed wheel track to help overcome the effects of tillage.