Geography Notes - for Competitive Exams

Part - 5

Soil is a mixture of broken, loose rock fragments and organic matter which can support plant growth. Soil is considered the end product of weathering.

One of the popular classifications of soils divides them into three broad orders, each of which is further divisible into sub-orders. This classification popularly called Marbutt's classification is based on the role of various environmental factors in the development of soils.

According to Marbutt's classification, all soils are grouped into three broad soil orders as zonal order, intrazonal order and azonal order each of which is further divided into sub-orders and soil groups.

The Zonal order includes soils the characteristics of which are dominated by the influence of climate and natural vegetation. Zonal soils are considered fully mature soils and they have a perfectly developed profile. Podzols are an example of such soils.

The Intrazonal order includes the soils the characteristics of which show influence of local features of parent material, terrain or age and the influence of zonal factors like climate and vegetation on the soil characteristics is relatively less significant. Calcimorphic and hydro-morphic soils are examples of this order.

The Azonal order includes the soils that are immature and they generally lack the B horizon in their profile. Alluvial soil is an example of Azonal order.

The fertility of a soil depends upon its mineral and humus content and climate.

Most mature soils have a layered arrangement of strata called horizons. Most mature soils have three horizons, A, B and C. This arrangement of soil layers is called soil profile. Occasionally a thick layer of humus may accumulate over the A horizon of a soil. Such a thick layer of humus is often called the O horizon.

Leaching is the process of removal of minerals in solution from the upper layers to the lower layers of soil.

A set of processes operating under a particular type of environmental conditions leading to evolution of typical characteristics of soils is called a pedogenic regime.

The most important pedogenic regimes are podzolisation, laterisation, calcification, salinisation and gleization.

Podzolisation regime commonly occurs in areas of humid temperate climate and is a typical regime of the coniferous forest regions. Leaching occurs in Podzolisation regime, and is thus low in fertility. These soils generally have accumulation of humus in thick layers. Podzols are typical soils formed through this regime.

Laterisation regime is associated with warm and humid tropical areas where wet and dry season alternate. This is a typical regime of the tropical and equatorial rain forests. Lack of humus is characteristic of soils formed under this regime. These soils are highly porous and display red colour due to the presence of iron oxide and they are low in fertility. Laterites are typical soils formed under this regime.

Calcification is the pedogenic regime associated with areas where evaporation on an average exceeds precipitation (e.g., in regions of mid latitude steppe climate). They usually have high fertility. Calcimorphic soils are typically formed under this regime.

Salinisation regime is associated with poorly drained areas with a desert climate. Lack of drainage channels in the desert and semi desert regions leads to flow and accumulation of surface run-off in local depressions. Due to high concentration of salts in the upper layers, these soils are not suitable for cultivation.

Gleization regime occurs in poorly drained, but not saline, environments in areas of moist and cool or cold climate. These soils are not suitable for cultivation.

The major soil types of the world are podzol soils, laterite soils, prairie soils, chernozem soils, desert soils, calcimorphic soils, tundra soils, hydromorphic soils and alluvial soils.