Soil taxonomy or classification of soils, is the grouping of soils on the basis of their morphology (texture and structure), appearance, form and fertility. The first complete US taxonomic classification was published in 1938 and modified in 1949. Although the US system has application worldwide, many countries–France, Canada, China, India, Brazil and Russia use their own systems. e present US classification organises all soils into 11 orders, 54 sub-orders, 238 great groups, 1,932 subgroups and then families and series.
In traditional classification, the major divisions of soil are Zonal, Intrazonal and Azonal.
These soils reflect climate and vegetation to a large degree, hence, their name. They are formed on well drained sites on non-extreme parent material. Polar desert soils, Podzols, Brown earths, Prairie soils, Chernozem, Chestnut soils, Brown and grey semi-arid soils, Grumusols, Red earths, and Lateritic soils fall under zonal category.
These are well developed soils formed where some local soil forming factor is dominant. Soils in this category belong to four main groups, namely saline, hydromorphic, calcareous and organic. Each group consists of variety of soil types.
These have poorly developed profiles because some factors of soil formation have inhibited soil formation. Lithosols, alluvial soils and dry sands fall in this category.
Pedalfers and Pedocals
Soils may also be classified as pedalfers and pedocals. Pedalfers are found in humid climates over 63.5 cm. annual rainfall while pedocals are found in areas having less than this rainfall. Pedalfers and pedocals are further sub-divided into several groups. Soils coming under pedalfer group are tundra, podsols, grey, brown, red and yellow, and laterite soils while the soils belonging to pedocal group are chernozem (black), brown, grey and light brown soils. Desert and mountain soils are found in desert and mountain areas respectively. Some important soils are described as under:
Very little vegetation grows in the cold climate of tundra region. But this soil contains high proportion of humus which is added by slow but gradual decay of scant vegetation. The parent rock contents are prominent in this soil. Lichen and moss grow abundantly in this soil.
These are ash-grey coloured soils and are found in high-latitude coniferous forest belt having very long cold winters, short cool summers and a moderate precipitation throughout the year. There is large scale downward movement of salts which This known as leaching. Slow breakdown of needle leaves results in slow formation of humus. e bacterial activity is also limited. The soils are acidic, infertile and of limited value for agriculture. But grey-brown podsolic soils found in mid-latitude deciduous forest belt in Western Europe and north-eastern USA and relatively less leached, have fair humus content and remain fairly fertile by manufacturing, fertilization and crop rotation. They are very useful for dairying and mixed farming.
Red and Yellow Soils
These soils are found in hot and humid climate of the tropics. Th ere is large scale leaching of these soils. Iron content gives them red colour. A greater bacterial activity consumes more and leaves little humus in them in spite of plenty of organic matter. They suffer equally from calcium deficiency. These soils are acidic and are soon exhausted unless fertilizers are used.
This group of soils is found in hot and humid regions of Equatorial Rain Forests and Savanah climates. These soils are heavily leached due to high rainfall resulting in acute shortage of humus. Oxides of iron, aluminium and manganese are deposited in its upper layer which gives red-brown colour to this soil. is soil is not much useful for agriculture but hard wood trees, bushes etc. grown on it. e main areas of this soil are the Zaire Basin, Amazon Basin and south-east Asia.
This is a very important soil of the pedocal group. This is also known as black-soil. This soil is found in semi-arid lands. Chernozem is one of the most fertile soils of the world and is rich in humus and calcium. It is unleached and has a crumby or nut-like structure. It can be used year after year even without adding fertilizers to it. It has great moisture-holding capacity and requires little irrigation. The steppes of Ukraine, Central USA, Central Africa, South America and Australia have become famous for growing big harvests of grain crops like wheat and oats or for livestock grazing on these soils. The chernozem of Ukraine is such a proli producer of wheat that it has earned the distinction of Bread Basket.
The prairie soils of USA are nearer to pedocals although found in moist parts of temperate grassland regions. They are found in east Europe also. They are poorer in lime than the black earths, have good crumby soil structure and are highly fertile. Maize is the main crop of prairie soils.
Chestnut soils have developed towards the drier parts of Chernozem soils in Asia and North America. These are dark brown to look at and their prole is similar to chernozem. But their colour is lighter than chernozem due to deficiency in humus. Agriculture in these soils is possible only with the help of irrigation but they provide good pastures.
Tropical Black Soil
Tropical areas have black soil resulting from volcanic activity. It becomes sticky when it is wet but it develops cracks when it dries up. It has great moisture-holding capacity and contains large variety of minerals. This type of soil is more prominent in India (especially in Maharashtra) where it is known as regur. This is very useful for the cultivation of cotton. s type of soil is found in Kenya, Morocco, north of Argentina and parts of West Indies also.
Desert soils are found in desert areas of temperate and tropical zones having low rainfall. Such soils are unleached and alkaline due to high rate of evaporation. But they badly lack in humus due to thin vegetation cover. Desert soils are grey in temperate region and red in hot deserts. They are t for agriculture only when the texture is e, salt is less and irrigation is available. There are patches of sterile or barren soil in the belt of pedocals under dry steppe-like climatic conditions. It is known as ‘usar’, the term collectively applied to all kinds of saline and alkaline soils in the plains of north India, particularly in Uttar Pradesh. The clay soil with bad or imperfect drainage in low-lying plain areas is found covered with a great amount of reh (white, greyish or ash coloured salt) in dry periods. As the salts are transferred from below, even canal irrigation helps in their accumulation in such areas change is almost invariably positive. Population change can be measured both in terms of absolute numbers as well as in terms of percentages.