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Soil horizons and erosion

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Review your understanding of soil formation, soil horizons, and erosion in this free article aligned to AP standards.

Key points

  • Soil is the loose surface material that covers most land. Soil is a mixture of minerals, organic matter, living organisms, gases, and water.
  • Soil is produced from rocks, or parent material, as a result of weathering. Weathering describes the breakdown of rocks by physical, chemical, or biological processes. Particles that break away during weathering are transported and deposited as layers of soil on land, or layers of sediment underwater.
  • Soils have layers called horizons. Soil horizons are distinguished by various properties, including color, texture, mineral content, and organic content.
  • The vertical arrangement of horizons is known as a soil profile. Soil profiles can help distinguish soil types, and can also be used to predict soil fertility. The generalized soil profile below includes four major soil horizons: O, A, B, and C:
A diagram shows a cutaway profile of soil with four horizons, which are horizontal layers with distinct features. The O horizon is the thinnest horizon and is just beneath the vegetated surface. Below that is the A horizon, which is thicker. Below the A horizon is the even thicker B horizon. Roots extend from the O horizon to the bottom of the B horizon. At the bottom of the diagram is the C horizon, which contains large rock pieces.
  • The O horizon, or organic horizon, is made up mostly of organic matter such as leaf litter and decomposed plant material. This layer can be thin, thick, or not present at all, depending on how a soil forms.
  • The A horizon, or topsoil, is the upper layer of soil in which plants have most of their roots. It has a high concentration of organic matter and microorganisms. So, this layer and the O horizon are often the most nutrient-rich and productive layers in a soil profile.
  • The B horizon, or subsoil, is made up mostly of minerals from weathered parent material. It is usually lighter in color, ranging from yellow to reddish brown. The B horizon is less fertile than the A and O horizons, and is not capable of producing abundant plant growth.
  • The C horizon is a layer of poorly weathered or unweathered rock. It contains a high concentration of parent material and is generally infertile.
  • Soil erosion is the removal of the fertile top layers of soil. Soils can be eroded naturally by wind and flowing water. Erosion can be slowed by plants, whose roots help anchor the top layers of soil.
  • Soils can also be eroded as a result of human activities. Deforestation, agriculture, and urbanization have greatly increased the rate of soil erosion in many places around the globe.
  • Intact soils filter and clean the water that moves through them. In this way, soils provide humans with an ecosystem service—they help provide clean water for drinking and other purposes. So, protecting soils from erosion can benefit humans and society.
An image shows a pit dug away to reveal soil horizons. At the bottom is a layer of light-colored rocky soil. Above that is a darker layer of rocky soil. Above that is an even darker layer of fine soil with plants growing from it.
Soil horizons. Image credit: “Jornada G-IBPE soil pit horizons.jpg" by Dr. H. Curtis Monger, CC BY-SA 3.0 .

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