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Classification and distribution of soils UPSC | World soils types | Biogeography | Physical Geography

The World Soil Classification of Curtis Fletcher Marbutt (1863–1935) is one of the best-known soil classifications. He emphasized the strong relationship between climate, vegetation, and soil in his soil classification.

As per Marbut, world soil can be classified into three major classes:

  • Zonal Soil Group
  • Intrazonal Group
  • Azonal Soil Group

Zonal Soil:

Zonal soils are well-developed soils that are formed by parent rocks through normal soil-forming processes (climate, vegetation, parent material, weathering, etc.).

There are 6 continuous regions of zonal soils in each hemisphere from the equator to the poles. Most extensively zonal soils are found in the northern hemisphere as there are extensive lands in the Northern Hemisphere. Zonal soils have a well-developed soil profile and have four distinct soil horizons ( humus, fine soils particle, weathered materials, and parent rock).

The following are six zonal soils that are distributed from the equator to poles in the both hemisphere.

  • Laterite Soil
  • Desert Soil
  • Chestnut Soil
  • Chernozem Soil
  • Taiga Soil or Podzol Soil
  • Tundra Soils

Laterite Soil:

  • Laterite soils are found in low latitudes( near the equator) where temperature and rainfall are intense and sufficient.
  • Rain forests are found on laterite soils.
  • In the lower latitudes, there is a higher temperature and high rainfall which leads to higher weathering. Due to excessive rainfall, minerals present in this weathered material such as calcium and potassium dissolve in water and seep or leach into the lower horizons of the soil profile.
  • Due to the high temperature and rainfall, there is a high bacterial growth which eats up all the humus from the soil.
  • The upper layer of laterite soil is composed of coarse particles, which contain fewer nutrients and less humus.
  • The second and third horizons of the soil are rich in minerals and are fertile; Thus it is suitable for large plants and not suitable for crops.

Global distribution of laterite soils are:

  • Amazon basin
  • Congo basin in Africa
  • Indonesia and Malaysia and other southeast Asian countries.

Desert Soil:

  • It is found along with the tropics of cancer and Capricorn in both hemispheres or the west coast of the tropical and subtropical desert.
  • It is coarse grain and sandy soil
  • Rich in mineral with no humus
  • Not suitable for vegetation

The following are regions where desert soils are found:

  • Shahara Desert in Africa
  • South West Asia or the Arabian peninsula
  • Thar desert in India
  • The Sonoran and Mojave Deserts in North America
  • The Atacama Desert in South America
  • The Kalahari Desert in South Africa
  • West Australian Deserts

Chestnut Soil:

  • It is found in the following regions where Mediterranean types of climate are found.
    • Around the Mediterranean sea. 
    • It is the California region of North America.
    •  In southwest Chile.
    • The southernmost part of Australia
  • It is famous for the production of citrus fruits and supports a wide range of vegetation.
  • Very fertile soil for the plantation crops.

Chernozem Soil:

  • It is dark in color Soil.
  • It is found in a temperate grassland in both hemispheres. Climatic conditions are semi-arid. Found in:
    • Pampas grassland in Argentina.
    • Veld Temperate grassland in South Africa.
    • Prairies grassland in Central North America.
    • Steppe in Central Eurasia
    • Downs temperate grassland in Australia.
  • It is a mineral and humus-rich soil.
  • Upper soils are more fertile than lower horizon soil hence it is suitable for grasses and a wide range of agriculture.

Taiga Soil:

  • It is light-colored Podzole soil. It is also called forest soil because coniferous vegetation is found here. It is mineral-rich but lacks humus. Lack of humus because of two reasons;
  • The coniferous forest does not shed their leaf.
  • There is a low decomposition rate because of low bacterial growth in the low-temperature environment.
  • Taiga soils are not suitable for agriculture.

Tundra Soil:

  • Tundra soils are found in higher latitudes [ 70-80 degrees latutude]. It is absent in the southern hemisphere because of the absence of land. It is found in Alaska, Canada, green land, Finland, and Northern Siberia. During the winter season, it is covered with ice cover. In the Summer season, it supports very small grasses.

Intrazonal soils;

Intrazonal soils are half-mature soil and are not found in the continuous zone. These soils are semi-mature soils. 

Examples of Intrazonal soils are:

  • Calcimorphic Soil is found in the Savana and temperate forests. Due to capillary action, calcium came up to the upper layer.
  • Hydromorphic soil or peaty soil in swamp areas. Examples are the Terai region of India, the Prepit swamp in northern Europe, the Lagoon areas, etc.
  • Holomorphic or saline soil are found on the coastline of some region. For example, it is found in the Kachchh region of Gujarat, South Africa, etc.

Azonal Soil:

Azonal soils are not formed from locally present parent rock; This type of soil comes from transportation and deposition by various agents (water, wind, glaciers, etc.). The form factor of azonal soil is not dependent on the local landform conditions and climatic factors do not have any major influence on soil formation.
The profile of azonal soil is not well demarcated.

The following are Azonal soils

  • Alluvial soils
  • Lithosol soil

Alluvial Soil;

It is developed by river deposits.

Distribution of Alluvial Soil is in:

  • Northern plain of India
  • Eastern North America
  • Eastern China
  • Southeast Asia except for Indonesia and Malaysia

Lithosol soil; 

  • It is made of hard rock or newly formed rock.
  • For example, the soils found in the Tibetan plateau and the Himalayan region are lithosol soils.

Try to solve the following questions:

  • The present classification of the soils of the world gives their economic significance. (UPSC 1992)
  • Classify soils based on their zonal distribution and describe the characteristics of pedals. (UPSC 2015)
  • Distinguish between intrazonal and azonal soils. Describe in brief the characteristics and importance of azonal soils. (UPSC 2018, 150 words, 10 marks)
  • Present a general classification of world soils and mention, in brief, their characteristics. ( 65th BPSC geography)

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