Soil Profile and Horizons – UPSC World Geography Notes

The Soil is the topmost layer of the Earth’s crust, mainly composed of organic minerals and rock particles that support life. A Soil Profile is a vertical cross-section of the soil, made of layers running parallel to the surface, known as soil horizons.

Soil Profile

A vertical section through various layers of the soil is termed the soil profile. The soil comprises three horizontal layers: true soil at the top, subsoil, and bedrock.

Each layer varies in feel (texture), color, depth, and chemical composition. These layers, termed horizons, are shaped by internal processes such as leaching or capillary movements/upward movements of materials and water.

The study of the soil profile involves examining a hexagonal column of soil taken as a sample.

A soil horizon is a layer typically parallel to the soil surface, exhibiting physical characteristics distinct from the layers above and below.

Horizons are primarily defined by noticeable physical features, especially color and texture.

The uppermost horizon, rich in humus and minerals, is generally dark in color, fostering soil fertility and providing nutrients to plants. This layer, known as the topsoil or the A-horizon, is soft, porous, and has a higher water retention capacity.

The next layer, the B-horizon or the middle layer, contains less humus but more minerals. It is generally harder and more compact.

The third layer, the C-horizon, is composed of small lumps of rocks with cracks.

Components of the Soil Profile

A soil horizon constitutes a distinct layer within the soil. Running approximately parallel to the soil surface, this horizon possesses unique properties and characteristics different from the adjacent layers above and below. The soil profile, a vertical section depicting all horizons, extends from the soil surface to the parent rock material.

Within the regolith, encompassing all weathered material in the profile, two components stand out: the solum and the saprolite. The solum includes the upper horizons, representing the most weathered portion of the profile. The saprolite, situated directly above the solid, consolidated bedrock but beneath the regolith, is the least weathered portion.

Soil Horizons

They evolve through interactions involving climate, living organisms, and the land surface, unfolding over time. Horizons typically form through either the selective removal or accumulation of specific ions, colloids, and chemical compounds. This process is often driven by water seeping through the soil profile from the surface to deeper layers, resulting in diverse soil textures and colors within the horizons.

There are two primary types of soil horizons: organic and mineral.

Organic horizons, denoted with the capital letter O, rest above mineral horizons and originate from plant and animal matter. The upper Oi horizon consists of decomposing organic matter easily recognizable by sight, such as leaves or twigs. The lower Oa horizon contains humus, which has broken down beyond recognition.

Mineral horizons: There are four main mineral horizons: A, E, B, C.

O Horizon: Layers characterized by organic material. Some O layers consist of undecomposed or partially decomposed litter (such as leaves, needles, twigs, moss, and lichens). They may be on top of either mineral or organic soils.

A Horizon or Surface soil: Part of the topsoil where organic matter is mixed with mineral matter. This layer has the most accumulation of organic matter and soil life. Nutrients like iron, aluminum, clay, and organic matter are sometimes dissolved and carried out in this layer. Depleted of (eluviated of) iron, clay, aluminum, organic compounds, and other soluble constituents. When depletion is pronounced, a lighter-colored “E” subsurface soil horizon is apparent at the base of the “A” horizon.

E Horizon: “E” stands for eluviated layer. Light-colored eluviated layer, eroded of nutrients. Significantly leached of clay, iron, and aluminum oxides, leaving a concentration of resistant minerals like quartz in the sand and silt sizes. Present only in older, well-developed soils, generally occurring between the A and B horizons.

B Horizon or Subsoil: Subsurface layer reflecting chemical or physical alteration of the parent material. Accumulates all leached minerals from the A and E horizons. Iron, clay, aluminum, and organic compounds accumulate in this horizon (illuviation, opposite of eluviation).

C Horizon or Parent rock: Partially weathered parent material accumulates in this layer, especially in sedimentary deposits. Least weathered horizon, also known as saprolite; unconsolidated, loose parent material. May accumulate more soluble compounds (inorganic material).

R Horizon or Bedrock: Denotes the layer of unweathered bedrock at the base of the soil profile. Unlike the above layers, R horizons largely comprise continuous masses of hard rock. Soils formed in situ will exhibit strong similarities to this bedrock layer. These areas of bedrock are under 50 feet of the other profiles.

Significance

The examination of the soil profile holds importance in agricultural sciences as it allows for the determination of land use patterns. Land capability classification relies on the study of the soil profile and horizon.

FAQs on Soil Profile and Horizons

Q: What is a soil profile?

A: The soil profile is the vertical section of soil that comprises three main layers – A-horizon, B-horizon, and C-horizon.

Q: How does the soil profile impact plant growth?

A: The composition of each soil horizon directly influences nutrient availability and structural support for plant growth.

Q: What defines a soil horizon?

A: A soil horizon is a distinct layer in the soil profile with unique physical and chemical properties.

Q: How are soil horizons named?

A: Soil horizons are named using letters (A, B, C) based on their position in the profile, indicating specific characteristics.

Q: What factors influence the thickness of the soil profile?

A: Climate, parent material, vegetation, topography, and time collectively shape the thickness of the soil profile.

Q: What causes the shimmering effect above farmland during hot days?

A: The heat haze or mirage is a result of rapid temperature variations between the warm ground and the cooler air, causing light to refract.

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