Soil Taxonomy: Definition, Sample Questions

Soil Taxonomy is a scientific terminology used to identify the classification and mapping of soil based on various properties. The soil is categorized into different types based on their properties. Soil is home to many organisms. It is basically formed through the process of rock weathering. Soil can also be termed as a porous medium, formed on the topmost layer of Earth’s crust. The classification of soil is mainly done on the basis of its texture, proportions, and different forms of organic and mineral compositionsSoil taxonomy is a hierarchical soil classification system consisting of six categories: Order, Suborder, Great group, Subgroup, Family, Series.

KEYWORDS: climate, humus, porous, soil, organic, soil taxonomy, soil classification, weathering.

Soil Profile

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A vertical section of the soil that is made up of layers running parallel to the surface is defined as soil profile. Each layer has a different colour, depth, texture, and chemical composition. These layers are also called horizons.

• The O-Horizon- It is the upper layer of the topsoil and is primarily composed of small rocks, dried leaves, twigs, surface organisms and other organic matter that can be decomposed. 

• The A-Horizon- This layer is also known as the humus layer and contains organic material. The humus helps in making the soil fertile. 

• The E-Horizon- This is the subsurface to all the layers of soil and is generally found in forested areas. The soil colour is light.

• The B-Horizon- Also known as the subsoil, this layer absorbs the rainwater flowing through the topsoil and stores it for future use. 

• The C-Horizon- This layer is formed of broken bedrock and lacks organic matter. Also known as saprolite, this layer is present below the B-horizon.

• The R-Horizon- The base layer of the soil, this layer contains partially weathered and unweathered rocks. Various kinds of limestone, basalt and granite are found in this layer. 

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Soil Types

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There are generally four types of soil classified on the basis of their texture, proportions, and various forms of mineral and organic compositions.

• Sandy soil: the soil contains a greater quantity of big particles. This type of soil is best used for drainage systems. 
• Silt soil: This type of soil has smaller particles as compared to sandy soil. It also has good ability to hold the water and is also used for agricultural purposes.
• Clayey soil: These types of soil have more amounts of fine articles. The particles are very packed within and have no space for air or other matter to flow. It has a sticky consistency when it’s wet, but is very smooth in its dried form.
• Loamy soil: Loamy soils are considered as the agricultural soil, because it maintains the balance of all the three soil types i.e. sandy, silt clay in equal proportions. 

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Soil Taxonomy in Brief

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Soil taxonomy is termed as the study of soil classification.This kind of classification of soil is similar to biological classification. There are twelve classes of soils in the order level. They are as follows.

Gelisols

They are also known as the permanent frozen soils, which are mostly found at extremely high elevated areas like the Arctic regions and Antarctic. In plants, this soil follows the downward movement of water and it consists about 10 to 12 percent of the Earth’s glacier-free land area.

Histosols

This soil is mainly composed of organic matter in their upper section which is formed by the decomposition  of twigs, grass dried leaves, and mosses with the help of microbes present in the soil. This type of soil is highly fertile for agricultural purposes.

Andisols

Weathering of volcanic materials gives birth to this kind of soil and possesses a huge quantity of both nutrients and water, which leads in making these soils very fertile and productive in nature.

Oxisols

These types of soil are great deposits of quartz and iron oxides. They are mostly found in the tropical as well as subtropical region. As they are very poor in nutrients and are less fertile hence, they are not suitable for gardens or other plantations.

Vertisols

This is a clay type of soil. It also has the capacity to swell and shrink.

Aridisols

It is formed from the combination of gypsum, salt, or carbonates. They are found in large quantities in both hot and cold deserts, including the dry valleys of Antarctica. They are very less fertile.

Mollisols

They are very fertile as they have the deposition of magnesium and calcium. These types of soil are also referred to as grassland soil since they are found in the grasslands.

Alfisols

This is a type of extended soil that is found under forest vegetation having a similar climatic region.15 to 18 percent of the Earth’s glacier is covered with this soil.

Inceptisols

Are soils that are widely found in various ecological settings. Further divided into seven suborders, they are more developed than Entisols but lack the characteristic of other soil orders.

Spodosols

They are acid soils that have a light coloured E-horizon covering a reddish-brown spodic horizon. Naturally infertile in nature, the soil needs lime in order to be fit for agriculture.

Ultisols

Found in tropical areas of the world and humid temperate, Ultisols are acidic forest soils which have relatively low fertility.

Entisols

It is the last in soil taxonomy. There is no soil development. They are available on the A-horizon also known as topsoil. It develops in areas where deposition rate is faster than the soil development rate.

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Things to Remember

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• The study of soil classification is termed as Soil taxonomy.
• The National Cooperative Soil Survey of the United States is responsible for determining this system of soil classification.
• The classification system consists of six categories: Order, Suborder, Great group, Subgroup, Family, Series.
• The vertical section through various layers of the soil is identified as soil profile. Each layer is known as horizon.
• Soil profile is divided into six horizons- O-horizon, A-horizon, E-horizon, B-horizon, C-horizon, R-horizon.
• There are twelve classes of soils in the order level. They are- Gelisols, Histosols, Andisols, Oxisols, Vertisols, Aridisols, Mollisols, Alfisols, Inceptisols, Spodosols, Ultisols and Entisols.

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