Soil

If you look at soil closely, you will find many different things. You will find small pieces of rocks or minerals. You will also find humus. Humus is nonliving (dead) plant or animal matter. What else is in soil? Some things you may not see are water, air, and living things.

Soil can take hundreds or thousands of years to form. Through weathering, rock becomes sediment. The sediment gets deeper the longer the rock is weathered. Plants begin to grow in the sediment, and animals move and mix the sediment.

When plants and animals die, bacteria and fungi decompose them. Humus forms. Humus has nutrients for new plants to grow. In this way, living things renew the soil year after year.

Soil forms in layers called horizons. Each horizon has a different amount of sediment, rock, and humus. A soil profile shows these horizons.

The top layer of soil at the surface is called the A horizon, or topsoil. It is home to many living things.

The next layer down is the B horizon, or subsoil. It is often lighter and harder than topsoil. The roots of strong plants grow down into the subsoil.

At the bottom of most soil profiles lies bedrock. The C horizon is above the bedrock and below the subsoil. It is made up of weathered bedrock.

There are many kinds of soils. Each has its own set of properties (color, texture, etc.)

The spaces between small pieces of soil are called pore spaces. The pore spaces in soil act like filters. They remove certain substances from the water as it moves through. This keeps water clean. Materials with these spaces are porous.

The sizes and numbers of pore spaces affect a soil's permeability. Permeability describes how fast water passes through a porous material. For example, sandy soil has a high permeability. The size and shape of sand lets water move freely between the pore spaces.

Soil permeability is important to plants that live on land. That means the type of soil in which plants grow is also important.

Topsoil is home to a large variety of living things. All living things need at least a little water. They need air, too. Plants and animals can survive in soil if only the soil is porous enough.