What materials make up soil?

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Measuring soil color.

Soil scientists separate soil using a stack of several sieves. The sieves have holes with slightly different sizes. The coarsest sieve is at the top of the stack, and the finest sieve is at the bottom. Most soils contain many kinds of material. All soils consist mainly of two kinds of material: particles of minerals and rocks, and organic matter. Organic matter is any matter that is or once was living.

Soil is likely to have several kinds of rock and mineral particles. A few kinds are very common. The three most common kinds are quartz particles, feldspar particles, and small pieces of rock. A soil sample is very likely to have a lot of at least one of these three kinds of particles. Quartz particles have irregular shapes. They look gray and glassy. Their surfaces are often stained brown or orange, because they are coated with rust. Feldspar particles are usually white or cream-colored. Their surfaces are often flat, at least partly, rather than irregular. There are many kinds of rock particles. You can tell them apart from the mineral particles because rocks are made of many different particles of minerals, all stuck tightly together.

The finest part of a soil sample is probably mostly very small flakes of clay. They are too small for you to see even with a hand lens. Sandy soils are loose and easy to dig. Soils with a lot of clay are harder to dig. Some plants like sandy soils and others like soils with more clay. Most soils have lots of organic matter. Some of the organic matter is in the form of living things, such as earthworms, insects, and microorganisms. Most soils are also rich in decaying plants. If the plant has decayed only slightly, you can usually recognize scraps of leaves, roots, and seeds. When the plant has decayed more, it turns into a soft, fine, dark material called humus. Humus is very important in soils. New plants can easily put their roots into humus. It is also good at holding water for later use by growing plants.