Horizon (Soil)
Horizon (Soil) refers to distinct layers within the soil profile, which run parallel to the Earth's surface. Each horizon possesses unique visible, chemical, or physical characteristics that differentiate it from the layers above and below. Soil profiles can vary greatly, with some containing all five main horizons—O, A, E, B, and C—while others may have only one or two. The O horizon is rich in organic material, primarily composed of decomposed plant matter, while the A horizon contains a mix of minerals and organic matter. The E horizon is characterized by leaching, where minerals are washed away, and the B horizon collects these leached materials. The C horizon lies just above bedrock and consists of less-weathered material similar to the parent rock, while the R horizon represents the solid bedrock itself. Various environmental factors, including climate, parent rock, and time, contribute to the formation of these horizons, resulting in diverse soil profiles across different landscapes. Understanding soil horizons is crucial for appreciating the ecological roles of soil, including its functions in water filtration and providing habitat for numerous organisms.
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Subject Terms
Horizon (Soil)
In geological terms, a horizon is a distinct layer of soil. Horizons run roughly parallel to the surface of the land. Each horizon has different visible, chemical, or physical characteristics from the layers located immediately above and below it. A column, or cross-section, of soil featuring all of its horizons is called a soil profile. A soil profile stretches vertically from the soil surface to the bedrock below. Scientists classify the five main horizons using the following letters: O, A, E, B, and C. Sometimes they use the letter R to represent bedrock. Soil profiles vary greatly. Although some may contain all the horizons, others may contain only one or two. Similarly, the horizons in a soil profile from one location may appear in a different order than the horizons in a soil profile from another location.
Background
Soil is a combination of materials—sediment, minerals, air, water, organic matter, and microorganisms—at Earth's surface. It rests in layers atop unbroken, solid rock called bedrock. Soil forms from a natural geological process known as weathering. Weathering occurs when water, ice, plants, animals, and other mediums break down or dissolve parent rock into smaller particles. Other factors that contribute to the formation of soil and account for the variability in soils from different locations are climate, landscape, parent rock, organisms, and time.
Three main categories of soil exist: sand, silt, and clay. Geologists base these categories on particle size, with sand having the largest particle size and clay having the smallest. Most soils contain a mixture of these three main soil categories. The variable percentages of each are responsible for the texture of a particular soil sample.
Soil serves a number of important functions in the environment, affecting everything from the water organisms drink to the air they breathe. For example, it provides a natural filtration system. As water moves through soil, the soil absorbs impurities. Thus, water that trickles through soil on its way to underground aquifers or to lakes, rivers, and streams on Earth's surface has been filtered. Soil affects the air by absorbing and releasing gases such as carbon dioxide and water vapor. Soil provides a home to countless organisms, from microscopic bacteria to burrowing gophers. Grasses, flowers, trees, and other plants are rooted in soil and draw nutrients from it to help them grow. Additionally, soil serves as the foundation upon which humans build homes and other structures. Soil's integral roles in natural processes and human endeavors are as varied as the layers, or horizons, in a soil profile.
Overview
A soil profile includes a cross-section of all soil horizons from Earth's surface to the bedrock below. Each soil horizon has characteristics that distinguish it from the layer directly above or below it. Some common characteristics of each horizon are outlined below.
O horizons are surface horizons composed largely of organic material—for example, the leaves, twigs, and needles of trees—that has recently fallen to Earth's surface and that has not been saturated with water for a significant amount of time. Sometimes O horizons may be subdivided into two layers designated Oi and Oa. Organic materials in the Oi layer often have arrived so recently that they have just barely started to decay. In contrast, materials in the Oa layer have already partially decomposed. O horizons are quite common in forested areas with many trees.
A horizons are also surface horizons, sometimes covered by an O horizon and sometimes situated right at Earth's surface. A horizons contain both mineral matter, such as sand, silt, and clay, and organic matter. Because both O horizons and A horizons contain organic matter, it can sometimes be difficult to differentiate between the two. A horizons often make up the surface layer of soil in areas with few trees, such as grasslands.
E horizons are subsurface horizons. E horizons are mineral horizons in which a significant amount of eluviation, or leaching, occurs. In eluviation, water transports minerals as it trickles from higher layers of soil to lower layers. These leached minerals then accumulate in the next layer, called the B horizon. As a result of eluviation, particles of soil in E horizons are generally a light color.
B horizons are subsurface horizons where minerals leached from higher layers are redeposited. The accumulation of leached materials in B horizons is called illuviation. B horizons are characterized by dark bands of mineral-rich material and typically contain very little organic material.
C horizons are subsurface horizons located just above bedrock, or the R horizon. C horizons are mineral layers. The material in C horizons undergoes the least amount of weathering and shares many similarities with the parent rock.
R horizons contain bedrock. Technically, R horizons are composed of rock, not soil. Examples include granite, basalt, and quartzite. However, material from the R horizon may contribute to the formation of some soils.
The order of horizons varies considerably in soil profiles. Some profiles may contain all the horizons: O, A, E, B, C, and R. Others may contain only A, B, and C. Still others may contain different variations of one particular horizon, leading to a configuration such as A, B, E, and B.
Beyond the five main horizons—O, A, E, B, and C—a few other soil classifications exist. H horizons, for example, share many characteristics with O horizons. They are predominantly organic material and are typically located atop mineral soils. Unlike O horizons, H horizons have been saturated with water for a significant amount of time. Occasionally, soil profiles include additional layers of ice, water, or limnic materials, represented by the letters I, W, and L, respectively. I layers contain a minimum 75 percent ice. Such layers may appear and disappear, depending on temperature. In some cases, however, ice may drive a wedge between two other distinct layers, thereby forming an I layer. Because some organic soils float on water and because some soils are permanently covered by water, a W may be used to designate a water layer. Limnic materials, or sediments containing a mixture of organic and inorganic material that are deposited in a body of water, may be designated as L layers.
Bibliography
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