Soil is the loose surface material that covers most land. It consists of inorganic particles and organic matter. Soil provides the structural support to plants used in agriculture and is also their source of water and nutrients. Soils vary greatly in their chemical and physical properties. Soil Horizons:  Residual soil forms over many years as mechanical and chemical weathering slowly change solid rock into soil. Soil formation begins when bedrock cracks because of ice wedging or other processes of mechanical weathering. Water, oxygen, and carbon dioxide seep into the cracks and cause... Show more

Soil is the loose surface material that covers most land. It consists of inorganic particles and organic matter. Soil provides the structural support to plants used in agriculture and is also their source of water and nutrients. Soils vary greatly in their chemical and physical properties.

Soil Horizons:  Residual soil forms over many years as mechanical and chemical weathering slowly change solid rock into soil. Soil formation begins when bedrock cracks because of ice wedging or other processes of mechanical weathering. Water, oxygen, and carbon dioxide seep into the cracks and cause chemical weathering. Plants eventually start growing and cause biological weathering. As the weathered material collects, layers called soil horizons develop. The greatest degree of weathering is in the top layer, because this is where water and air first come into contact with rock. Each successive lower layer is less altered by weathering.

Most soils have at least three distinct layers, called A, B, and C horizons. A cross-section of all the horizons is called a soil profile.    
The A horizon is the top layer of soil. It is also called topsoil. This layer is usually darkest in color because it has the highest proportion of organic material. Topsoil is where most soil organisms live, including insects, worms, and other animals in addition to plants. Plant roots help to hold this layer of soil in place. Minerals in topsoil may dissolve in rainwater and soak into the next soil layer. Rainwater also transports tiny mineral particles such as clay deeper into the soil.    
The B horizon is the next layer of soil. It is also called subsoil. This layer is lighter in color than topsoil because it contains less organic material. It is also where soluble minerals and clay particles from topsoil accumulate. Because of the presence of clay and other minerals, subsoil holds more water than topsoil does.    
The C horizon is the lowest layer of soil. It consists of partially altered bedrock. There is some evidence of weathering in this layer, but pieces of the original rock are still visible and identifiable. 

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