Our purpose was to determine the influence of the textural fraction and its mineralogical characteristics on the contents of the exchangeable (Ki) and nonexchangeable (Knoi) forms of soil potassium (K). Contributions of each textural fraction on Ki were as follows: 73–80% for clay, 8–18% for silt, and 2–10% for sand. The clay fraction relative to the contribution to Ki was not the same for all the soils studied. Thus, not only the quantity of clay must be taken into account, but also the cation exchange capacity (CEC) of the involved minerals. For Knoi, the total contribution showed a greater dispersion, the corresponding figures being as follows: clay 35–68%, silt 25–52%, and sand, 4–25%. Therefore, clay was not always the main contributor. In some soils, silt was the most important fraction due to the presence of muscovite.