In this topic you will learn how soil is formed.

Soil is made of tiny rock particles, minerals, air, water, and humus. Humus is leftover decomposed plant and animal matter.

It takes hundreds to thousands of years to form one inch of soil. Soil forms in different layers. A layer of soil differing from the layers above it and below it is called a horizon. Topsoil is the top layer of soil. Topsoil is usually dark in color. It is rich in humus and minerals. Most plants grow in this part of the soil.

The layer of soil below the topsoil is known as the subsoil. Subsoil is a fairly hard layer. It is made of clay and minerals. Subsoil is usually light in color. Some plant roots may grow down into the subsoil. Pieces of broken bedrock make up the layer below the subsoil. The different soil horizons make up a soil profile. A soil profile is a vertical section of soil from the surface down to bedrock.

There are many different kinds of soils. Each one has its own set of properties. Different soils have different-sized particles. The particles may have been formed from different kinds of bedrock. The amount of humus in soil depends on the organisms living in the area where the soil was formed. Soils may contain different minerals. Plants use up minerals as nutrients to make their own food. Areas with many plants may have few minerals in the soil.

Soils also differ in permeability, the rate at which water can pass through a material. The spaces between soil particles are called pore spaces. Water and air fill these spaces. Sandy soils have many spaces in them, so they have a high permeability. These kinds of soils hold little water. Soil permeability is important to plants. If the water moves quickly through the soil, it goes beyond the reach of the roots. The water also dissolves minerals in the soil and carries them away. Fine soils are not very permeable. Water soaks in slowly and may remain for a long period of time.

Soil is an important resource. It supplies us with food. Soil is a living system because it is the home for many organisms. People protect soil because it is sensitive to changes in water, temperature, and human activity.