Soil scientists restrict the word ‘soil’ to the surface material which, over a long period of time, has come to have distinct layers or horizons.
Soil has certain distinctive physical, chemical and biological qualities, which permit it to support plant growth
and which differentiate it from the infertile substratum lying below.
While soil is composed of both mineral and organic particles, the underlying layers may usually be composed of mineral material.
Soil is a dynamic layer-a changing and developing body-as many complex chemical, physical and biological activities continue constantly in it.
Soils become adjusted to conditions of climate, landform and vegetation and change internally when the controlling conditions change.
The scientific study of the characteristics, development and distribution of soils is called pedology; the process of soil formation is known as pedogenesis.
Many types of processes and influences, together known as Soil formers, act to develop a soil.
Some of these are passive conditions; others are active agents.
Many years ago, Russian pedologist Dukuchaiev firmly established five main soil formers:
(i) parent material;
(ii) landform;
(iii) time;
(iv) climate; and
(v) biological activity.
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 boge 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.
(a Podzol soils :
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 U.S.A.
, 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.
(b) Grey-brown podzolic :
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 U.S.A.
(c Red-yellow podzolic soils :
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.
(d) Latosols :
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
bricks called laterites.Areas of occurrence are India, south-east Asia, Amazon Basin and Congo Basin.
(e) Tundra soils :
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, i.e., tundra regions of Siberia and North America.
(f) Chernozem soils or black earth :
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. Natural 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, U.S.A. and in the Deccan in India.
(g) Prairie soils (Brunizem soils) :
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 com. These are found in the Mississippi valley and the great plain states of the United States.
(h) Chestnut soils :
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.
(i) Brown soils :
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.
(j) Grey-desert soils and red-desert soils :
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.
(k) Hydromorphic soils :
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 bfcarbonates of sodium,
calcium, magnesium and potassium); and alkali soils (predominantly sodium salts, especially sodium carbonate NaCOS)
The most famous salt area is the salt flats of Great Salt Lake in Utah, on which numerous automobile speed records have been set.
(I) Cateimorphic soils :
Calcknorphic 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 subhumid tropics are productive agriculturally, yielding cotton, com and alfalfa.
THE SOILS OF INDIA :
Soils can be divided into two broad types: (i) sedentary or residual soils which are formed directly from the rocks under them or in situ and transported;
(ii) azonal soils, the soils from in situ that are transported to valleys and deltas by water and wind
. To the former type belong black soil, red, laterite, podzolic soils found in forests, saline and alkaline soils and peaty soils
. The azonal type includes the soils of the forests in the northern mountains, soils constituting the coastal lowland and plains of India and desert soils.
(a) Alluvial soil : The most widely spread and important soil. The entire northern plains are made of alluvial soil.
These soils also extend in Rajasthan and Gujarat also found in the eastern coastal plains particularly in the deltas of the Mahapadi,
the Godavari, the Krishna and the Kaveri rivers. Alluvial soils are deposited by three important Himalayan river systems the Indus,
the Ganga and the Brahmaputra. According to their age alluvial soils can be classified as old alluvial (Bangar) and new alluvial (Khadar).
The bangar soils has higher concentration of kanker nodules than the Khadar. It is more find in Maharashtra, Saurashtra, Malwa,
Madhya Pradesh, Chhattisgarh and along the Godavari and the Krishna valleys.
(b) Red and Yellow Soil :
Develops on crystalline igneous rocks in areas of low rainfall in the eastern and southern parts of the Deccan plateau.
Found in parts of Orissa, Chhattisgarh, southern parts of the middle Ganga plain and along the piedmont zone of the Western Ghats.
These soils develop a reddish colour due to diffusion of iron in crystalline and metamorphic rocks, looks yellow when it occurs in a hydrated form.
(c) Laterite Soil :
Develops in areas with high temperature and heavy rainfall, humus content of the
soil is low.
Laterite soils are suitable for cultivation with adequate doses of manures and fertilizers. Found in Karnataka, Kerala,
Tamil Nadu, Madhya Pradesh, and the hilly areas of Orissa and Assam.
After adopting appropriate soil conservation techniques this soil is very useful for growing tea and coffee.
Red laterite soils in Tamil Nadu, Andhra Pradesh and Kerala are more suitable for crops like cashew nut.
(d) Arid Soil :
Ranges from red to brown in colour, are generally sandy in texture and saline in nature, lacks humus and moisture.
The lower horizons of the soil are occupied by Kankar because of the increasing calcium content downwards,
restricts the infiltration of water. After proper irrigation these soils become cultivable.
(e) Forest Soil :
Found in the hilly and mountainous areas. Soil texture varies according to the mountain environment where they are formed.
They are loamy and silty in valley sides and coarse grained in the upper slopes. In the snow covered areas of Himalayas,
these soils experience denudation and are acidic with low humus content. The soils found in the lower parts of the valleys particularly on the river terraces and alluvial fans are fertile.
Indian Soils with Percentage of Coverage
Soil Types Percentage of
Total Area
Alluvial soils 22.16
Black soils 29.69
Red and Yellow soils 28.00
Laterite soils 2.62
Arid soils 6.13
Saline soils 1.29
Peaty and Organic soils 2.17
Forest soils 7.94
SOIL EROSION :
Soil erosion is the wearing away and redistribution of the earth’s soil layer. It is caused by the action of water, wind and ice,
and also by improper methods of agriculture. If unchecked, soil erosion results in the formation of deserts.
It has been estimated that 20 per cent of the world’s cultivated topsoil was lost between 1950 and 1990.
If the rate of erosion exceeds the rate of soil formation (from rock), then the land will decline and eventually become infertile.
SOIL CONSERVATION :
These include various methods of crop cultivation to ensure protection of the top soil. Contour Farming is practised when the soil cannot absorb all the rain that falls on it
A simple practice of farming across the slope keeping the same level as far as possible has many beneficial effects.
The ridges and rows of plants placed across the slope form a continual series of miniature barriers to the water moving over the soil surface.
Their effect is great in reducing runoff, soil erosion and loss of plant nutrients. Mulching prevents soil from blowing off and being washed away,
reduce evaporation, increase infiltration to keep down weeds, improve soil structure and increase crop yields.
Strip Cropping is another form of controlling the runoff erosion and thereby maintaining the fertility of soil.
Strip cropping employs several good farming practices such as crop rotation, contour cultivation, stubble mulching and cover cropping.
Strip cropping includes contour! strip cropping, field strip cropping, wind strip cropping and permanent or temporary buffer cropping.
Contour strip cropping involves growing a soil exposing and erosion-permitting crop in strips of suitable widths across the slopes on contour,
alternating with strips of soil protecting and erosion-resisting crops. It shortens the length of the slope checks
the movement of runoff water and helps to delist it, and increases the absorption of rainwater by the soil.
Wind strip cropping consists of planting tall growing crops and low growing crops in alternately arranged
straight and long but relatively narrow parallel strips laid out right across the direction of the prevailing wind regardless of the contour.
In permanent buffer strip cropping, the strips are established to take care of the steep or highly
eroded slopes in fields under contour strip cropping. The strips are generally legumes, grasses or shrubs.