Soils: Classification, Factors & Formation processes

Soils are a complex mixture of minerals, organic matter, water, and air that support plant growth and provide a habitat for many microorganisms and animals. Soils are formed over time through the weathering of rock and the accumulation of organic matter.

Soil, the biologically active, porous medium that has developed in the uppermost layer of Earth’s crust. Soils are made up of four main components: mineral particles, organic matter, water, and air. The mineral particles provide the basic structure of the soil and are composed mainly of clay, silt, and sand. The organic matter is composed of dead plant and animal material, and is important for soil fertility. The water and air found in soils are essential for the survival of soil organisms and plants.

Soils can be classified into different types based on their physical and chemical properties, such as texture, structure, pH, and nutrient content. Some common soil types include:

  1. Sand: Soils with a high proportion of sand particles are well-drained but can be dry and low in nutrients.
  2. Clay: Soils with a high proportion of clay particles are heavy and poorly drained but can be rich in nutrients.
  3. Silt: Soils with a high proportion of silt particles are intermediate between sand and clay soils in terms of drainage and nutrient content.
  4. Loam: Soils that are a mixture of sand, silt, and clay are considered ideal for plant growth, as they have good drainage and a balance of nutrients.

Soils play an important role in many ecosystem services such as water regulation, carbon sequestration, nutrient cycling, and biodiversity conservation. Human activities such as deforestation, urbanization and intensive agriculture can lead to soil degradation and erosion, which can negatively impact the ecosystem services provided by the soil.


  • Parent material is a passive control factor. Soils can be any in-situ (residual soils) or transported deposits (transported soils). But the weathered material from the parent rocks directly influences the chemistry of the Soil. For e.g.- Black soils from Basaltic lava rocks.
  • Climate: Temperature and moisture amounts cause different patterns of weathering and leaching. Wind redistributes sand and other particles especially in arid regions. The amount, intensity, timing, and kind of precipitation influence soil formation. Seasonal and daily changes in temperature affect moisture effectiveness, biological activity, rates of chemical reactions, and kinds of vegetation.
  • Topography: Steep slopes facing the sun are warmer. Steep soils (Mountainous soil) may be eroded and lose their topsoil as they form. Thus, they may be thinner than the more nearly level soils that receive deposits from areas upslope (Alluvial soil). Deeper, darker colored soils may be expected on the bottom land.
  • Biological factors: Animals and micro-organisms mix soils and form burrows and pores. Organisms decompose the leaves and mix them with the upper part of the soil. Plant roots open channels in the soils. Grass roots easily decompose and add organic matter. Humans can mix the soil so extensively that the soil material is again considered parent material. 
  • Time: The length of time the soil forming processes operate, determines maturation of soils and profile development. A soil becomes mature when all soil-forming processes act for a sufficiently long time developing a profile. Soils developing from recently deposited alluvium or glacial till are considered young and they exhibit no horizons or only poorly developed horizons.


  • Soil enrichment/addition refers to the process of adding nutrients or organic matter to the soil to improve its fertility and productivity. Materials are deposited on the soil from above as well as materials moving in from below with rising groundwater. Also obvious are additions of mineral material due to flooding, landslides, and other geologic events. Perhaps not so obvious is the nearly constant additions of atmospheric dust to the soil surface. Some of this dust can travel long distances and is important to the overall fertility of a region. Rainfall is also an addition.
  • Soil removal/losses refers to the process of removing soil from one location and transporting it to another. Erosion is a major form of soil loss. Loss can also occur as nutrients are taken up by plants that are then harvested and removed. Transpiration is the movement of water into a plant through its roots and subsequent removal by evaporation from its leaves. Water also evaporates from the soil directly. As minerals and nutrients move through the soil into the groundwater or out of the rooting zone of the plants, this too is considered a loss.
  • Soil translocations are like losses in that they involve the movement of materials. Translocation differs in that the material is not removed from the soil; instead, it moves from one location to another. This internal movement can be divided into illuviation (movement into) and eluviation (movement out of). Eluviation is the process by which a material is removed from a zone. Illuviation is the process by which material moves into a zone. Salts and highly soluble minerals like gypsum and carbonates (lime) can dissolve in the soil water and then move to wherever the water moves. As the soil water evaporates, the dissolved materials (salts etc.) will precipitate (form solids) out of the water. This is common in arid areas, where salts are moved to and concentrated at the soil surface as water evaporates. Burrowing animals from ants to tortoises dig holes and physically translocate soil throughout the soil profile. Clay and organic matter can also be translocated as water physically moves them deeper into the profile as it percolates downward.
  • Soil transformation refers to the physical, chemical, and biological processes that change the properties of soil over time. As an example, leaf litter that is added to the soil is eventually decomposed. This decomposition is a transformation process. Likewise, rocks weathering to soil is also a transformation process. The initial minerals in the rock are transformed to clays and other minerals in the soils over time. If the soil is compacted, the amount of pores or void space is reduced. This too transforms the soil by decreasing the ability of gases and water ability to move through the soil. Compaction also makes it harder for roots to penetrate the soil. When soil water freezes, it is transformed from a liquid to a solid (ice); this, too, is a transformation.


A vertical section (or cutting) through the soil showing the different layers of soil is called soil profile. A soil horizon is a layer generally parallel to the soil surface, whose physical characteristics differ from the layers above and beneath. Each layer differs in feel (texture), colour, depth and chemical composition.

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