Soil pH or soil reaction is an indication of the acidity or alkalinity of soil and is measured in pH units. Soil pH is defined as the negative logarithm of the hydrogen ion concentration. The pH scale ...

Acidifying processes rarely affect the pH of calcareous soils, except occasionally in the top few centimetres of undisturbed soil. Failing to correct soil acidity can result in losses to crop yield ...

then comparing the results to a chart. Depending on the type of kit, there may be other steps involved. "Soil pH testing kits ...