Soil is composed of mineral and organic matter, water, and air. The actions of wind, rain, ice, and sunlight break down rock into smaller particles ranging in size and texture from clay and silt to sand and gravel. Air and water ﬁll the gaps between the larger particles. Plant roots bind particles together and raise minerals from deep in the ground. Plants and their remains form food for burrowing insects and earthworms. When bacteria and fungi decompose the dead plants, animal droppings, and dead animals, the end product is dark, fertile humus. This top layer of earth, dark and rich in organic matter, is called topsoil. It varies in depth and is tilled in the cultivation of gardens and farm ﬁelds. Iowa has some of the best topsoil in the United States and in the world. The root systems and organisms of the region’s past vegetation—prairies, marshes, bogs, and fens—transformed the raw mineral deposits into an invaluable natural resource.