What Is Soil?
Soil is a mixture of very small pieces of rock and rotted materials from plants and animals. This rotting plant and animal material, called humus, gives soil its dark color. Humus makes the soil soft and spongy enough to hold water, and also contains minerals that plants need in order to grow. Soil is all around us. It covers almost all of the Earth’s surface and is important and useful to us in many ways.
We eat fruits and vegetables that grow in the soil. Houses are built and paper is made from the lumber which comes from trees grown in the soil. Soil impacts our atmosphere releasing gasses such as carbon dioxide into the air. The soil plants an important role in cycling nutrients including the carbon and nitrogen cycles.
Soil is often described using several characteristics including texture, structure, density, temperature, color, consistency, and porosity. One of the most important properties of soil is the texture. Texture is a measure of whether the soil is more like sand, silt, or clay. The more like sand a soil is the less water it can hold. On the other hand, the more like clay a soil is, the more water it can hold.
Soil is made up of many layers. These layers are often called horizons. Depending on the type of soil there may be several layers. There are three main horizons (called A, B, and C) which are present in all soil.
Organic – The organic layer (also called the humus layer) is a thick layer of plant remains such as leaves and twigs.
Topsoil – Topsoil is considered the “A” horizon. It is a fairly thin layer (5 to 10 inches thick) composed of organic matter and minerals. This layer is the primary layer where plants and organisms live.
Subsoil – Subsoil is considered the “B” horizon. This layer is made primarily of clay, iron, and organic matter which accumulated through a process called illuviation.
Parent material – The parent material layer is considered the “C” horizon. This layer is called the parent material because the upper layers developed from this layer. It is made up mostly of large rocks.
Bedrock – The bottom layer is several feet below the surface. The bedrock is made up of a large solid mass of rock.