First, lets come up with a better definition of soil. Soil is the surface layer of the earth containing organic matter and capable of supporting vegetation. A few inches to several feet thick, soil consists of fine rock material, organic material, water and air. There are five basic components of rock material , which are clay, silt, sand, gravel, and cobbles. Clay is the smallest particle that makes up soil.
Clay tends to hold a lot of moisture.
Clay can be very expansive when wet. There are parts of Texas where clay can expand a foot or more between the wet and dry season. Next comes silt, silt is a sedimentary material consisting of very fine particles of rock. Most silt comes from the process of erosion of the top layer of earth. Erosion is the general term given to the process by which the surface of the earth is constantly worn away by wind and water.