The procedures were rather primitive until about the 1920's, at which time significant advances in soil testing technology were realized. Soil testing may be defined as any physical or chemical measurement of soil. Growers in most parts of the world Utilize soil testing to one degree or another, and for various reasons. One surprising fact is that of soil testing laboratories generally don't actually test soil. They test extracts of soils at least when performing chemical tests.
Soil samples are generally mixed with an extracting solution of some kind.
The mixture is then shaken and then filtered resulting in a clear liquid extract which is then subjected to various chemical tests. The extracting solution may be distilled water, although with most laboratories one or more dilute acids are used to extract nutrients from soil samples. The theory is you want your extracting solution to remove from the soil those nutrients that are reasonably available to plants. Nutrients that are in chemical forms that will never be available to a plant are not really of any use to the soil testing lab or the farmer. At the same time, distilled water as an extracting solution only tells you what is available today, and doesn't give you much of an idea of what the reserve fertility levels are.