The Main Ingredients in Soil

Ingredients

• Soil consists of four main ingredients: air, water, rock particles and organic matter. Weathered rock particles are the main ingredient of soil, but soil also contains 20 to 30 percent each of air and water, and about 5 percent organic matter. Organic matter includes decomposed plants and animals, along with millions of organisms that live in the soil. The proportions of these ingredients determines what you can do with the soil. For instance, black humus consists of a lot of decayed organic matter that's useful for growing plants, while clay soils are often used in construction.

Types

• Soil scientists describe soil by the amount of sand, silt and clay that it contains. Sand is the largest particle and feels rough when you rub it between your fingers. Silt feels smooth when it's wet or dry. Clay feels smooth when it's dry, but sticky when it's wet. Clay is the smallest particle and silt is in between sand and clay. The size of the particles in the soil determine how well the soil drains and how well it can hold nutrients. The type of soil that forms depends on the types of rocks that can be found in the area, the types of plants and animals that live in the area and the overall climate and geography.

Organisms

• A spoonful of soil can contain between 100 million and 1 billion microorganisms like fungi and bacteria. In addition to breaking down dead plants and animals, these organisms change air and nitrogen in the soil into compounds usable by plants. In addition, some bacteria make acids that break down rocks, turning them into more soil. Other organisms like earthworms, ants and termites add air to the soil, improve drainage and redistribute the soil when they make tunnels.

Formation

• Soil begins forming when exposed rocks start eroding and are chemically changed, or when certain materials, like volcanic ash, are deposited on the ground. These rocks, ash and other deposits are called "parent material." Snow, wind, sunshine, water and heat break down the parent material over a period of hundreds or thousands of years. The remains of decomposing animals and plants are added into the soil as it forms. Topography also affects the type of soil that forms because soils at the bottom of a hill are wetter than ones found on a slope and soil formed on sunny slopes will be drier than soil formed on shady ones.