The current study shows that there is a correlation between the supply of potassium and mineral composition of soil, which take them as samples by drilling tow pedons, one of them at College of Agriculture, and the college of Engineering of Tikrit University after they are described morphologically, and classified to principal horizons. The results of evaluation tests gave reflection in behavior and nature of potassium case, and traditional, thermodynamic and exchange standards show that potassium supply is high level, and the studied soil does not require a potash fertilizer under these conditions. Perhaps the reasons attributed to the occurrence of chloritization phenomenon through results of XRD because of brucite layer precipitation between interlayers of expansible clay minerals(2:1). Specially montmorillonite clay which makes physical blocking exchange or cross-linked between structural units of montmorillonite clay inhibits ionic exchange, and it causes migration of ions to soil solution as nutrient elements are enough for the require of completing plant growth. So other minerals play an important role in potassium supply such as montmorillonite clay which distinguishes with highly layer charge and highly potassium content because it is inherited form mica mineral in the studed soil. As well as the presence of potash feldspar minerals and palygorskite.