Dirt, or more accurately, soil, is the surface layer of Earth in which plants can grow. It can be a few inches deep or extend down for several feet. It is made up of tiny pieces of rock of various sizes and shapes, along with air, water, and humus, which is the decomposed remains of plants. (The more humus soil contains, the better it can hold the nutrients and water that plants need to grow.) The different types of rocks that make up soil are usually fine particles of parent rocks that lie farther below Earth’s surface; these have been made fine by erosion worn by wind and water or by geological occurrences like volcanic eruptions and the movement of glaciers. There are many kinds of rocks composed of different minerals and formed in different ways that make up Earth’s crust.