The term CLAY LAYER UNDER SOIL can be summarized as a hardpan, which is a 7-letter word. Hardpan is a layer of soil that is composed of dense, compacted material that is often made up of clay, silt, and other particles. It is usually found beneath the surface of the soil and can make it difficult for water and air to penetrate. Hardpan can also impede the growth of roots and limit the amount of nutrients available to plants.