CLASSIFICATION OF WORLD SOILS

CLASSIFICATION OF WORLD SOILS
Soil scientists recognise that soils can be subdivided into three orders: the zonal soils formed under conditions of good soil drainage through prolonged action of climate and vegetation; intrazonal soils formed under conditions of very poor drainage (like in bogs or flood-plain meadows) or upon limestones whose influence is dominant; and azonal soils with no well-developed profile characteristics either because they are on steep slopes or for lack of sufficient time to develop. While zonal and intrazonal soils can be classified and have distinctive profile characteristics due to long development, the azonal soils have poorly developed profiles and cannot be classified.

Podzol soils are zonal soils of cool humid climates and are most widely distributed. Podzol soils require cold winter and adequate precipitation throughout the year. Found in the northern Great Lakes states of USA, Asia, and mountain parts of New England, these soils are low in fertility and cannot produce crops to feed large populations. Coniferous forests are associated with podzol soils.

Grey-brown podzolic soils contain important bases but are somewhat acid. Deciduous forests (oak, beech, maple) are associated with this soil. They are found over western Europe, north China and northern Japan and eastern-central USA.
Red-yellow podzolic soils lie in the zone of increas­ingly warmer climate but abundant precipitation, occupying the southern United States, southern Brazil and south­eastern Paraguay; smaller coastal zones in south Africa, Australia and New Zealand. Natural vegetation of this soil type is dominantly rain forest of both tropical and tem­perate classes.

Latosols are soils of humid tropical and equatorial zones, corresponding closely with wet equatorial climate and the tropical wet-dry climate, favourable for luxuriant growth of broad leaf evergreen rain forest and woodland. The local accumulations of iron and aluminium sesquioxides develop into layers that can be cut out as building bricks­called laterites. Areas of occurrence are India, south-east Asia, Amazon Basin and Congo Basin.

Tundra soils are sometimes classified as intrazonal because of being poorly drained. Tundra climates of the northern continental fringes provide favourable conditions for tundra soils, Le., tundra regions of Siberia and North America.

Chernozem soils or black earth is the most widely distributed of the zonal soils in semi-arid climate. They are associated with humid continental climates, hot summers and cold winters and drought periods. Naturai vegetation of these soils are steppe grasslands and prairies. Rich in calcium, the, outstanding feature of these soils is their productivity for small grain crops-wheat, oats, barley and rye. They occur in Ukraine, USA and in the Deccan in India.

Prairie soils (Brunizem soils) are similar to the cher­nozems in the general profile and appearance but do not have an excess of calcium carbonate. Prairie soils are extremely productive with somewhat moist climate; most important crop is corn. These are found in the Mississippi valley and the great plain states of the United States.

Chestnut soils are similar to chernozems but contain less humus and so are lighter in colour. These soils are fertile under conditions of adequate rainfall or irrigation. But they lie in the hazardous' belt in which years of drought and adequate rainfall alternate; the semi-arid middle lati­tude steppe lands, in North America and Asia.

Brown soils replace the chestnut soils in the still more arid regions, with still less humus and thus have a lighter colour. These are tYpical of the middle-latitude steppes and support a light growth of grasses suitable for livestock grazing.

Grey-desert soils and red-desert soils are soils of middle latitude deserts and tropical deserts. Grey desert soils contain little humus due to sparse vegetation growth. Red desert soils are found in more arid, hotter tropical deserts. Humus is minimum. The activity of plants and animals reaches the minimum in red desert soils. The colour is derived from small amounts of oxides of iron.


Hydromorphic soils (intrazonal) are associated with marshes, swamps, bogs, or poorly drained flat uplands. Holomorphic soils (intrazonal) are formed by the pedogenic process of salinisation. The areas where silts and clays make up a large proportion of the soil-body are called holomorphic soil. These are classified into saline soils (containing chlo­rides, sulphates, carbonates and bicarbonates of sodium, calcium, IJ1<1gnesium and potassium); and alkali soils (pre­dominantly sodium salts, especially sodium carbonate NaC03). The most famous salt area is the salt flats of Great Salt Lake in Utah, on which numerous automobile speed records have been set.

Calcimorphic soils are another class of intrazonal soils whose characteristics are strongly related to the presence of lime-rich parent material. The process of calcification (introduction of calcium) is dominant in the formation of calcimorphic soils, The soils of sub humid tropics are pro­ductive agriculturally, yielding cotton, corn and alfalfa.