Soil develops in layers. The top layer consists mostly of humus. It is called topsoil. The next layer down is called subsoil. Subsoil contains more rock particles than topsoil and less humus. It is not as good for growing plants. The layer below the subsoil has many pieces of rocks. The pieces get bigger when they are deeper in the ground. Most plants cannot grow in this rocky layer. The soil layers all rest on solid rock. It is called bedrock. The pieces of rock in soil come from bedrock.