Soil Groups: Zonal, Intra-Zonal and Azonal

Soil Groups: Zonal, Intra-Zonal and Azonal!

Zonal Soils (Order):

1. Desert Soils (Group):

In arid regions (sub-order), where it is usually dry and hot with high temperature and an extremely low rainfall (less than 5 inch) and there is an almost complete absence of vegetation. Due to very low rainfall, leaching is almost absent. Owing to high evaporation, salts usually ‘travel upwards and accumulate in the surface layer.

2. Grey Soils:

In arid regions, the rainfall is low (5-15 inch) it is enough to cause percolation and movement of salts. Vegetation being scanty, a layer is usually absent. As leaching is poor alkaline salts accumulate in the B-horizon. Horizon-B acquires a grey colour. Clay formation is very meagre. Water table is usually very deep.

3. Chestnut Soils:

In semi-arid regions, the rainfall is slightly higher than in arid regions (15-30 inch), but the temperature is almost equally high. Vegetation is slightly more profuse. Leaching is more marked than in arid regions. Soluble alkaline salts accumulate and form the B-horizon. The decomposition of organic matter is sufficient to cause a slight accumulation of humus in the A-horizon.

4. Chernozems (Black) Soils:

There are belts of land in the semi-arid and sub- humid regions where the rainfall is fairly high (30-50 inch) and where there are alternate wet and dry periods. There is more leaching and the eluviation of soluble salts of both alkali and alkaline earth bases is more pronounced. They are washed out from the upper horizon and are accumulated in the lower horizon.

The soil formation takes place under conditions of slight alkalinity or neutrality. These conditions are favourable to the accumulation of humus and humic substances. Hence, the organic matter content of these sods is high. They are usually black in colour due to formation of calcium humate.

5. Laterite Soils:

In humid tropical and sub-tropical regions, both temperature and rainfall are high (80-100 inch or more). The soluble products of weathering are completely washed out due to heavy leaching. Considerable eluviation takes place there is no horizon differentiation as the eluviated substances are not re-deposited in the lower layers. Hence, no true B-horizon is formed.

As the alkali and alkaline earth bases are removed from the seat of formation, the residual soil is acid in reaction. Vegetation is very profuse. The decomposition of organic matter is rapid due to the great microbial activity brought about by heavy precipitation and high temperature.

The organic matter content of these soils is, therefore, low. The process of lateritic soils formation is called laterisation. These soils are characterised by their red, yellow and orange colours due to presence of iron oxides in various degrees of hydration.

6. Podsols:

In temperate regions, both rainfall and temperature are low. Vegetation is very profuse in this region. Due to low temperature, humus begins to accumulate giving rise to humic acids. Due to the liberation of humic acids, soil formation takes place under acid conditions.

The podsol is characterised by having three distinct horizons, horizon-A or the horizon of eluviation, horizon-B or the horizon of illuviation and horizon-C, the parent material. The whole soil profile is acidic in reaction. The colour is ash-grey.

7. Brown Earths:

In some parts of the temperate regions, especially in warmer climates where both temperature and rainfall fluctuate considerably and where there are deciduous forest. Then it is a slight accumulation of humus, but it is not of the highly acid type as in the case of the podsol.

Leaching of the soluble products of weathering is continuous but not excessive. Soil formation takes place under conditions of slight acidity. They develop a brown or reddish-brown colour due to the presence of hydrated iron oxides. Horizon differentiation is not so well-marked as in the podsol.

8. Tundra soils:

In regions of extremely low temperature, as in the arctic zone, the surface is almost perpetually covered with snow. The rainfall is about 8-10 inch or even less. Very often there is a permanent frozen layer of soil at some depth not far below the surface. The movement of water that penentrates into the soils is usually arrested when it meets the frozen layer.

The water moves up when the surface gets dry on evaporation and moves down again where there is rain. There is alternate downward and upward movement of water for a very small period (summer and rainy season). Vegetation is very scanty and mainly consists of mosses and lichens. Whatever organic matter accumulates decomposes very slowly. There is very little chemical and microbial activity and the process of eluviation is very sluggish. Soil formation is very similar to weak podsolization.

Intra-Zonal Soils:

The intra-zonal soils possess, in general, the characteristics which mark them out as distinct and different from zonal soils. In other words, the effect of the local factors, topography, drainage etc., is superimposed on that of the active soil forming agents.

The important intra-zonal soils are:

(1) Saline and alkaline soils

(2) Rendzina soils

(3) Bog or marsh soils.

(i) Saline and Alkaline Soils or Solod soils:

Generally, these soils develop in arid and semi-arid regions. These also develop in low-lying areas where the water table is high. Soluble sodium salts are accumulated on the soils surface due to capillary rise. In saline soils, the salts usually present are the chloride, sulphate, carbonates etc. The soil reaction is alkaline (pH is more than 7.0).

(Note: For detail see saline and alkaline soils).

In saline soils, generally, no horizon differentiation and the whole profile looks more or less uniform. The soil is poor in humus and is devoid of structure. If rainfall is sufficient to wash down the soluble salts of saline soil, it gives rise to alkali or black alkali soil. Organic matter dissolves in sodium carbonate and moves down along with clay particles into lower layer. Horizon-B develops a well-defined columnar structure, becomes very compact and almost impervious to water and acquires a dark colour (black).

(ii) Rendzina Soils:

In temperate regions, rendzina soils are very often formed from calcareous limestone. The clay remains saturated with calcium and the soil is neutral in reaction. Horizon-A (upper layer), is dark grey in colour while the lower layer, horizon-C, is light grey or even white in colour. Horizon-B is usually absent.

(iii) Bog or Marsh Soils:

In tropical and sub-tropical regions, these soils developed in the presence of a high water table in podsol zone, these soils are formed under swampy conditions in the presence of large quantities of organic matter. The profile usually consists of two horizons i.e., A and C. The presence of underground water impedes the process of eluviation and illuviation with the result that horizon-B is absent or very weak-development.

Horizon-A is dark coloured layer (or black), mixture of humus and mineral matter; often contains un-decomposed plant remains; fairly deep; acidic reaction. Horizon-C, is bluish or greenish-grey; very often mottled; rusty streaks very prominent, almost continually saturated with ground water.

Azonal Soils:

Development of soils has been influenced more by the nature of its parent material are known as azonal soils.

These soils are divided into three groups:

(a) Lithosols,

(b) Regosols, and

(c) Alluvium.

(a) Lithosols:

These soils have a very thin layer and are- full of stones. They are usually situated on slopy terrains where no soil development is possible.

(b) Regosols:

These are also undeveloped soils and consist mainly of loose or unconsolidated sands such as sand dunes. Lithosols and regosols have no agricultural significance.

(c) Alluvium:

Alluvial soils are derived from water- deposited sediments. Most alluviums are highly fertile soils. These are deposits of silt and other fine material brought down by rivers sometimes over long distances. As the materials are deposited every year they have had no time for soil development. These soils are deep and do not show prominent horizon development.

These soils vary greatly in their physical and chemical properties. Drainage conditions also create many variations. Mostly these soils are poorly drained and have a greyish colour with mottling’s. Most of these soils are acidic, ecepting those in drier area. Soils developed on older alluvium generally show development of profile characteristics.

An alluvial soil profile from Indo-Gangetic plain is described as follows:

0-4″ Light yellowish-brown, sandy loam, weakly granular, pH 7.7.

4-14″ Dark yellowish-brown clay loam, crumby to sub-angular blocky, pH 8.5.

14-30″ Dark yellowish-brown clay loam, sub-angular blocky, pH 8.9.

30-50″ Yellowish-brown clay, pH 9.2

50-74″ Yellowish-brown clay loam, blocky, loam blocky, pH 9.5.

Alluvial soils are agriculturally very important. They are usually planted to rice in most of the tropical Asian countries. In India, wheat and other crops are also grown in the central and western parts of the Indo-Gangetic plains where a colder climate persists for a longer time.