Which type of soil has the lowest permeability?

Clay has the lowest permeability among the options given. Permeability refers to the ability of a soil to transmit water through its pore spaces. Clay consists of very fine particles that are densely packed, which greatly reduces the size of the pore spaces and results in minimal movement of water. This characteristic makes clay significantly less permeable compared to coarser materials like gravel and sand, which have larger particles and more substantial spaces between them, facilitating better water flow. Silt, while also finer than sand, still allows more water to pass through than clay due to its slightly larger particle size and better soil structure. Thus, clay’s composition and particle arrangement lead to its classification as the type of soil with the lowest permeability.