Forest gaps have different microclimatic conditions as compared to the surrounding areas, depending on gap size and surrounding forest types and affecting the biological, chemical, physical, and hydrological processes in the forest openings. The objective of this study was to determine the effect of forest gap size and surrounding forest cover type (beech or mixed stands) on the soil of forest opening by analyzing several soil chemical soil properties (pH, electrical conductivity -EC, organic matter -OM, and nutrient content). The study was conducted in the Yuvacik watershed in Izmit (Turkey) and a total of 31 forest gaps of different size (1.44-37.33 ha) and elevations (848-1169 m a. s. l.) were studied. Gaps were divided into three groups with size 0-5 ha, 5-15 ha, and > 15 ha. Results showed that forest gap size significantly affected all the investigated chemical properties of the soil, except for soil pH. As gap size increased, sodium (Na+) concentration in the soil decreased from 22.72 to 19.57 mg L-1 while potassium (K+) and magnesium (Mg+2) concentrations increased from 83.88 to 134.62 mg L-1 and from 59.46 to 123.96 mg L-1, respectively. Medium-sized gap soils had the lowest OM content, as well as the lowest calcium (Ca+2) and nitrogen (N+3) concentrations. Surrounding forest types significantly influenced soil chemical properties in the openings, except for EC, N+3, and phosphorus (P-PO4-3). Soils in the gaps surrounded by mixed forest had significantly lower pH but higher OM content, K+, Na+, Ca+2, and Mg+2 concentrations than soils in beech forest gaps.