The sand stays on the top because it is large. The silt goes down a little with the water and makes a layer because it is small. The silt is smaller than some of the spaces in the soil. The clay can go down even lower with the water because it is the smallest. The sand will make one layer, the silt will make another layer and the clay will make another layer. The humus can move down with the water and make a layer too. The silt and clay and humus can move down because of the spaces in the soil. But, the silt, clay, and humus will fill the spaces in the soil. When the spaces in the soil are closed, it makes it hard for air to go into the soil. Plant roots do not go where there is no air. When we dig down we find layers in the soil.