Categories: There are six categories of classification in

Soil Taxonomy

(i) order

(ii) sub-order

(iii) great group

(iv) sub-group

(v) family

(vi) series.

Order: The order is based on the soil-forming process. In a given order, soil properties are similar in their genesis. There are the following twelve soil orders in soil taxonomy

2. Sub-order:

It is composed of a Formative element of order + any differentiating character like temperature, moisture, drainage, diagnostic horizon, etc. e.g. Aquolls Wet soil-Mollisols (lol). Aquents Wet soil – Entisols (ent) Argid Aridisol (id)

3. Great group:

It is composed of order + sub-order + one or more specific characters which modify the sub-order. e.g. Argiaqoll Mollisol - order all - sub-order wetness Arg - argillic horizon

4. Sub-group:

The sub-groups are sub-divisions of the great groups. There are more than 1200 sub-groups.

5. Family:

The family is differentiated on the basis of texture, mineralogy, temperature, and soil depth. Some 6600 families are recognized.

6. Series:

The series is a sub-division of the family and is the most specific unit of classification. Differentiating characteristics are primarily based on the kind arrangement of horizons. About 16,800 soil series are recognized. Diagnostic horizons are found in the surface or sub-surface. The diagnostic surface horizons are called epipedon, whereas the diagnostic horizon found in the sub-surface is called subsurface diagnostic horizon or endopedons. Epipedons are six in number. They include the upper part of soil darkened by some organic matter, the upper eluvial horizons, or both They are described below

Surface horizon

• Molic: Thick dark-colored, high base saturation, strong structure

• Umbric: Same as Mollic but Low base saturation

• Orchric: Light-colored, low organic matter, may be hard when dry

• Histic: Very high in organic content, wet during some part of the year

• Anthropic: Man modified Mollic Horizon, high in available P

• Plaggan: Man-Made sod like horizon created by years of maturing Subsurface Horizon

• Argillic: Silicate clay accumulation

• Natric: high in sodium, columnar, or prismatic structure

• Spodic: Organic matter, Fe and Al oxides accumulation

• Cambic: Changed or altered by physical movement or by chemical reactions

• Agric: Organic and clay accumulation, accumulation just below the plow layer

• Oxic: Highly wethered Fe and Al oxides and nonsticky silicate clay

• Calcic: Accumulation of calcium carbonate and magnesium carbonate

• Gypsic: Accumulation of gypsum

• Salic: Accumulation of salts

• Albic: Light colored clay, Fe and Al removed

The major classification of Indian soils

1.Alluvial soil [43%]

2.Red soil [18.5%]

3.Black / regur soil [15%]

4. Arid/desert soil

5.Laterite soil

6.Saline soil

7. Peaty/marshy soil

8.Forest soils

1.Red soils:

➢ Red color in red soils is due to the presence of various oxides of iron. They are either formed from the products of decomposition of rocks washed to a lower level. They include soils locally known as red sandy soil and red alluvium.

➢ They are mostly formed under sub-humid climates from a number of rock formations like granite, shales, etc.

➢ Their main features are a light texture, porous structure, absence of lime, and low soluble salts.

➢ They are generally poor in fertility constituents such as N, P, K, and lime and are highly deficient in organic matter.

➢ They have a low base exchange capacity.

➢ They are slightly acidic to neutral in reaction, the pH ranging from 6.0 to 7.5. The characteristic clay mineral is kaolinite.

➢ Area of red soils is whole of Chennai and Mysore, part of A.P., M.P., Orissa, and Bihar (Chhotanagpur), Birbhum (W. Bengal), Santhal Pargana (Bihar), Mirzapur, Jhansi, and Hamirpur districts of U.P. and eastern half of Rajasthan.

➢ Most of the red soils have been classed in the order Alfisols

(2) Lateritic soils:

✓ They are found mostly in areas of high rainfall. They are light in texture. There is no retention of water.

✓ They are deficient in lime and are slight to moderately acid in reaction.

✓ The pH varies from 5.0 to 6.0. They are low in base exchange capacity.

✓ Laterite soils formed at high levels have a pale red color is highly gravely and are poor in all fertility constituents.

✓ These formed at low levels have a darker color probably due to a great accumulation of humus, a slightly finer texture, and are quite well-drained.

✓ These soils are found all along the west coast of Maharashtra, Mysore, and Kerala, on tops of hills in the Deccan, Madhya Pradesh, and in Orissa along the Eastern Ghat.

(3) Black soil:

➢ The black soil developed from Basaltic rock under semi-arid conditions. The soils are black or dark brown in color.

➢ They include soils locally known as regur or black cotton soil, deep black soil, medium black soil.

➢ Their texture ranges from sandy loam to heavy clay. Some black soils may be porous and others may be compact and impervious.

➢ One of the characteristic features of black soil is that it swells on wetting during the rainy season and shrinks and cracks in summer.

➢ The base exchange capacity of deep black soil is quite high.

➢ The pH varies from 7.5 to 8.5. The soils are on the whole low infertility.

➢ The soils are deficient in nitrogen and phosphorus. They are rich in potash and lime.

➢ The black soils are found in parts of Maharashtra, Gujarat, M.P., Rajasthan, U.P., A.P., Chennai, and Mysore.

(4) Alluvial soils:

✓ They are characterized by extreme depth and grey or grayish-brown color.

✓ Their texture varies from sandy loam to clay loam. The structure is also variable, loose and free-draining in the case of sandy soils, and compact and impervious in clayey soils.

✓ These soils are most fertile; they are deficient in nitrogen, phosphorus, and humus but are well supplied with lime. Their base exchange capacity is comparatively low.

✓ The pH varies from 7.0 to 8.0.

✓ They are very extensive in area and cover large parts of Rajasthan, Punjab, U.P., Bihar, and West Bengal and extend even into west Assam and north Gujarat.

(5) Desert soils:

✓ They have developed in arid regions mostly under the influence of physical weathering.

✓ They are mainly sandy. They contain large amounts of soluble salts and varying proportions of lime.

✓ They have a high pH and are very poor in fertility constituents.

✓ Desert soils are found in large parts of Rajasthan, south Punjab, and in the range of Kutch.

✓ The temperature regime is very high throughout the year. The rainfall ranges from 50 cm to less than 10 cm.

(6) Saline and alkaline soils:

➢ These soils are developed in arid and semi-arid regions. Basin-shaped topography (poor drainage) is also responsible for their development.

➢ They are mainly found in the black soil region in the south and west, in the Indo-Gangetic alluvium in the north, and in the deltaic and coastal regions all along the west and east coast.

(7) Peaty and marshy soils:

➢ They are formed in the depression under submerged conditions and have acquired a blue color due to the presence of ferrous iron.

➢ Peaty soils are found scattered in Kerala, north Bihar, and north Uttar Pradesh and have developed in humid regions as a result of the accumulation of large quantities of organic matter.

(8) Tarai soils:

✓ Tarai soils have a wet regime and high water table conditions for the most part of the year.

✓ Tarai soils are foothill soils and extend in strips of varying widths at the foot of the Himalayas in Jammu and Kashmir, U.P., Bihar, and West Bengal.

✓ The soils were derived from the materials washed down by the erosion of mountains.

✓ The parent materials are alluvial sediments and consist of hard clay.

(9) Brown hill soils:

➢ These soils are formed on the hills under forests. They are mainly found in the Himalayas on sandstones and shales.

➢ The surface soils are dark brown loam to silty clay in texture, acidic to neutral in reaction.

➢ These soils may be classified in order of Alfisols.

(10) Sub-mountain soils:

✓ These soils are formed in high rainfall regions of sub- Himalayan under forests.

✓ The soils are acidic in reaction.

✓ The organic matter accumulation is high and there is the absence of free lime.

(11) Mountain meadow soils:

✓ These soils occur at high elevations in the Himalayas.

✓ The soils are shallow with grass vegetation.