SOIL

SOIL

• Soil can be defined as the solid material on the Earth’s surface that results from the interaction of weathering and biological activity on the parent material or underlying hard rock.
• The naturally occurring soil is influenced by parent material, climate, relief, and the physical, chemical and biological agents (micro-organisms) in it.
• A soil is made up of four elements: inorganic fraction (derived from the parent material), organic material, air and water. The abundance of each component and its importance in the functioning of the soil system vary from horizon to horizon and from one soil to another.

Soil Characteristics

Soil Texture

• Soil texture is a term used to describe the distribution of the different sizes of mineral particles in a soil.
• Textures range from clay, sand, and silt at the extremes, to a loam which has all three sized fractions present.
• The main influence of texture is on permeability which generally decreases with decreasing particle size.
• A clayey soil may thus be described as fine, a sandy soil as course, while a silty soil is intermediate.

Soil Air

• The air content of a soil is vital, both to itself and to organic life within it. A certain amount of air is contained between the individual particles except for the waterlogged soils.
• The air in the soil helps in the process of oxidation which converts part of the organic material into nitrogen in a form readily available to the plants.

Soil water

• Depending on the texture of the soil, water moves downward by percolation. The amount of water in the soil varies from almost nil in arid climates which makes life virtually impossible for organisms, to a state of complete water logging which excludes all air, causes a reduction of bacteriological activity, and limits decomposition.
• In damp climates, especially in high latitudes where the evaporation rate is low, water tends to move predominantly downward, particularly in coarse-grained sandy soils. This dissolves the soluble minerals in the soil, together with soluble humus material and carries both downward, a process called leaching or eluviations.
• A typical leached soil is known as podzol, a Russian word meaning ash because the surface layer is often grayish or ash-coloured. In a hot, arid climate, evaporation exceeds precipitation for greater part of the year, so the water tends to move upward and the soil dries out. Consequently, in some areas, a thin salty layer is formed on the surface. This process of

Soil color

• Generally, soil color is determined by the amount of organic matter and the state of the iron. Soil color is also related to soil drainage, with free draining, well AERATED soils (with pore space dominated by oxygen) having rich brown colors.
• In contrast, poorly draining soils often referred to as gleys, develop under ANAEROBIC conditions (the pore space dominated by water) and have grey or blue-grey colors.
• Such colors are the result of oxidation-reduction; iron is the main substance affected by these processes. If the iron is released in an anaerobic environment, then it stays in the reduced state giving it the grey blue color of waterlogged soils.

Factors Responsible for Soil Formation

Soil formation is the combined effect of physical, chemical, biological, and anthropogenic processes on soil parent material.

Parent material

• This is the material from which the soil has developed and can vary from solid rock to deposits like alluvium and boulder clay. It has been defined as ‘the initial state of the soil system’.
• The parent material cans influence the soil in a number of ways: color; texture; structure; mineral composition and permeability/drainage.
• Soil may form directly by the weathering of consolidated rock insitu (a residual soil), saprolite (weathered rock), or it may develop on superficial deposits, which may have been transported by ice, water, wind or gravity. These deposits originated ultimately from the denudation and geologic erosion of consolidated rock. Consolidated material is not strictly parent material, but serves as a source of parent material after some physical and /or chemical weathering has taken place.
• Soils may form also on organic sediments (peat, muck) or salts (evaporates). The chemical and mineralogical compositions of parent material determine the effectiveness of the weathering forces.
• Climate
  • Temperature varies with latitude and altitude, and the extent of absorption and reflection of solar radiation by the atmosphere. Solar radiation (direct radiation and diff use radiation) increases with elevation, differs seasonally, and is influenced by cloud cover or other atmospheric disturbance (e.g. air) pollution). The absorption of the solar radiation at the soil surface is affected by many variables such as soil color, vegetation cover, and aspect. In general, the darker the soil color, the more radiation is absorbed and the lower the albedo. The effect of vegetative cover on absorption varies with density, height, and color of the vegetation. Hence the absorption differs in areas with deciduous trees (soil surface is shaded by trees most of the year) and arable land (soil surface is not shaded throughout the year). Light, or whitish-colored, soil surfaces tend to reflect more radiation. When incoming solar radiation is reflected, there is less net radiation to be absorbed and heat the soil. Snow is especially effective in reflecting the incoming solar radiation.
  • Temperature affects the rate of mineral weathering and synthesis, and the biological processes of growth and decomposition. Weathering is intensified by high temperatures, hence weathering is stronger in the tropics than in humid regions.
  • Biological processes are intensified by rising temperatures. Reaction rates are roughly doubled for each 10°C rise in temperature, although enzyme-catalyzed reactions are sensitive to high temperatures and usually attain a maximum between 30° and 35°C.
  Biological Factors
  The soil and the organisms living on and in it comprise an ecosystem. The active components of the soil ecosystem are the vegetation, fauna, including microorganisms, and man.
          a. Vegetation
  • The primary succession of plants that colonize a weathering rock culminates in the development of a climax community, the species composition of which depends on the climate and parent material, but which, in turn, has a profound influence on the soil that is formed.
  • Deciduous forest seems to accelerate soil formation compared to grassland on the same parent material under similar climatic conditions.
    b. Meso-/Macrofauna
  • Earthworms are the most important of the soil forming fauna in temperate regions, being supported to a variable extent by the small arthropods and the larger burrowing animals (rabbits, moles).
  • Earthworms are also important in tropical soils, but in general the activities of termites, ants, and beetles are of greater significance, particularly in the sub humid to semiarid savanna of Africa and Asia.
    c. Micro-organisms
  • The organic matter of the soil is colonized by a variety of soil organisms, most importantly the micro-organisms, which derive energy for growth from the oxidative decomposition of complex organic molecules.
  • During decomposition, essential elements are converted form organic combination to simple inorganic forms (mineralization).
  • Types of micro-organisms comprise bacteria, actinomycetes, fungi, algae, protozoa, and soil enzymes.
    d. Man
  • Man is perhaps now the most influential of all organisms. He affects the soil by such activities as: ploughing, irrigating, mining, clearing, disposing and leveling.
    e. Time
  • Time is a factor in the interactions of all the above factors as they develop soil. Over time, soils evolve features dependent on the other forming factors, and soil formation is a time-responsive process dependent on how the other factors interplay with each other.
    f. Relief
  • Relief is not static; it is a dynamic system (its study is called geomorphology). Relief influences soil ? formation in several ways:
    • It influences soil profile thickness i.e. as angle of slope increases so does the erosion hazard. Gradient affects run-off, percolation and mass movement.
    • It influences aspect which creates microclimatic conditions
  Stages of Soil Formation
  Soil formation is a long slow process. It’s estimated that an inch of soil takes 500 to 1000 years to form. Soil is constantly being formed.
  • Stage One
    • This is the rock pulverizing stage. Here the forces of wind, rain, freezing and thawing water, earthquakes, volcanoes all work to slowly pulverize rocks into smaller particles that can make up a soil. At the end of this stage a combination of sand, silt and clay sized particles forms. These form a mineral soil like substance but are unable to support life.
    • They are missing nitrogen. It may seem nitrogen should be the least of a being’s worries. After all the air we breathe is made up of about 78% nitrogen gas. The problem is that plants cannot use nitrogen in this form. For them it needs to be converted to either ammonia which is a combination of nitrogen and hydrogen or nitrates - a combination of nitrogen and oxygen.
  • Stage Two
    • This is the early stage of soil formation. Here life is added specifically by lichens.
    • Lichens are a symbiotic relationship of algae and fungus. The algae have the very important role of fixing the nitrogen, changing it from nitrogen gas to a form, the plant can use. It also captures the sunlight and creates sugars and oxygen. The fungus provides a place for the algae to live, along with water and the mineral nutrients it needs.
  • Stage Three
    • At this time the little pockets of soil have formed to the extent that some larger plants, plants with  roots can have a go at growing.
    • The first pioneers will be short lived but as their bodies are added to the layers of soil forming, the soil becomes more capable of supporting life. Humus builds and soil horizons begin to form.
  Stage Four
  • • The soils are developed enough to support thick vegetation.
  Soil Forming Processes
  The four major processes that change parent material into soil are additions, losses, translocations, and transformations.
  • Additions
    • The most obvious addition is organic matter. As soon as plant life begins to grow in fresh parent material, organic matter begins to accumulate. Organic matter gives a black or dark brown color to surface layer.
    • Other additions may come with rainfall or deposition by wind, such as the wind-blown or eolian material. By causing rivers to food, rainfall is indirectly responsible for the addition of new sediment to the soil on a food plain.
  • Losses
    • Most losses occur by leaching. Water moving through the soil dissolves certain minerals and transports them into deeper layers. Some materials, especially sodium salts, gypsum, and calcium carbonate, are relatively soluble. They are removed early in the soil’s formation. As a result, soil in humid regions generally does not have carbonates in the upper horizons.
    • Fertilizers are relatively soluble, and many, such as nitrogen and potassium, are readily lost by leaching, either by natural rainfall or by irrigation water.
    • Solid mineral and organic particles are lost by erosion. Such losses can be serious because the material lost is usually the most productive part of the soil profile.
  • Translocations
    • Translocation means movement from one place to another. In low rainfall areas, leaching often is incomplete. Water starts moving down through the soil, dissolving soluble minerals as it goes. There isn’t enough water, however, to move all the way through the soil. When the water stops moving, then evaporates, salts are left behind. Soil layers with calcium carbonate or other salt accumulations form this way. If this cycle occurs enough times, a calcareous hardpan can form.
    • Translocation upward and lateral movement is also possible. Even in dry areas, low-lying soils can have a high water table. Evaporation at the surface causes water to move upward. Salts that are dissolved in solution will move upward with the water and deposit on the surface as the water evaporates.
  • Transformations
    • Transformations are changes that take place in the soil. Microorganisms that live in the soil feed on fresh organic matter and change it into humus. Chemical weathering changes parent material. Some minerals are destroyed completely. Others are changed into new minerals. Many of the clay-sized particles in soil are actually new minerals that form during soil development.
    • Other transformations can change the form of certain materials. Iron oxides (ferric form) usually give soils a yellowish or reddish color. In waterlogged soils, however, iron oxides loose some of their oxygen and are referred to as being reduced. The reduced form of iron (ferrous) is quite easily removed from the soil by leaching. After the iron is gone, generally the leached area has a greyish or whitish color.
  Soil Classification 
  • Soil Classification concerns the grouping of soils with a similar range of properties (chemical, physical and biological) into units that can be geo-referenced and mapped.
  • Soils are divided into: (i) zonal, (ii) intrazonal, (iii) azonal categories.
   Zonal
  • A soil whose characteristics are dominated by the influence of climate and vegetation is known as a zonal soil. These soils occur on gently undulating land where drainage is free and where the parent material is of neither extreme texture nor chemical composition. They occur in latitudinal zones.
  There are seven main types of zonal soils:
  • Tundra Soils:
    • These soils extend over the tundra region, covering northern parts of North America, southern fringes of Greenland and northern Eurasia. The exact character of these soils depends on the ground ice position, slope and the vegetation. If the slope is stable, peaty soils are formed due to slow organic and chemical action. In case of steep slopes, thin soils result.
  • Podzols:
    • These soils occur south of the tundra region in North America, northern Europe and Siberia and are associated with conifers and heath plants. In these soils, the horizon-A is colloidal and humus rich, horizon-E is bleached and ash- grey, horizon-B is brown clayey. Depending on the composition of horizon-B, the soils could be humus- podzol, iron-podzol or gleypodzol. These soils are generally infertile and require lime and fertilizers if put to agricultural use.
  • Brown Forest Soils:
    • These soils occur south of the podzol region in milder climates of eastern to USA, northern Europe and England. These soils are associated with deciduous forests and derive their brown appearance from the equitable distribution of humus and sesquioxides.
    • There is less leaching, because there is no downward movement of sesquioxides. The brown forest soils are generally less acidic.
  • Lateritic Soils/Latosols/Ferralsols:
    • These soils cover large areas of Asia, Africa, South and Central America and Australia. These soils are generally associated with tropical and sub-tropical climates with a short wet and long dry season and thick vegetation.
    • During the dry season, in these areas, there is intense physical and chemical weathering and organic activity. During the wet season, an intense leaching causes washing down of humus, organic and mineral colloids, clay and other soluble material.
    • The upper horizons are, as a result, acidic with minimum organic content. The insoluble oxides of iron and aluminum give the upper layers a characteristic red color. The lower horizons are clayey. The lateritic soils are generally poorly differentiated but have deep horizons and are suitable for mining. These soils are generally infertile due to low base status.
  • Chernozem/Prairie/Steppe:
    • These soils are associated with grasslands receiving moderate rainfall in northern USA, the Commonwealth of Independent States (former USSR), Argentina, Manchuria and Australia.
    • The chernozems are characterised by high mineral content and low organic content. Calcium carbonate is quite high in the profile. The upper horizons are dark, mineral-matrix-base rich. The humus content is around 10%. The parent material of chernozems may be ‘loess’ (wind eroded sediments). The soft, crumb structure imparts fertility to these soils.
    • The chestnut soils occur on the arid side of chernozems, and are associated with low-grass steppe. The lime content is still higher in these soils compared to the chernozems.
    • The prairies represent the transitional soils between chernozems and the brown forest soils and reflect the element of increasing wetness. These soils are characterized by less leaching, no calcium content and taller, coarser grasses. In the corn regions of the USA, prairie soils are quite fertile.
  • Grumusols/Reddish Brown Soils:
    • These are dark clayey soils of savanna grasslands which occur on the drier margins of the laterites. These regions experience warm climate with wet-dry seasons.
    • There are no eluviated and alluvial horizons but the wholesolum is base-rich which gives these soils a dark appearance. These soils support scattered trees, low scrubs and grasses. During the dry season, these soils show cracks.
  • Desert (Seirozems and Red Desert) Soils:
    • Seirozems or grey desert soils occur in mid-latitude deserts of Colorado and Utah states of USA, in Turkmenistan, Mongolia and Sinkiang. These soils occur on the extreme sides of chestnut soils and have a low organic content. Lime and gypsum are closer to the surface. Being rich in bases, the seirozems are good for irrigation.
    • The red desert soils occur in the tropical deserts of the Sahara, West Asia, Pakistan, South Africa and Australia. These soils are characterized by lack of vegetation and lack of leaching. The insoluble oxides of iron and aluminum give these soils a red color. The red desert soils are generally base rich, sandy and gravelly.
  Intrazonal Soils
  • Intrazonal soils is a soil which has been influenced in its development less by climate and vegetation than by other local factors, such as defective drainage, excessive evaporation or an unusual parent material (such as lime stone), terrain or age.
  • They can be sub-divided into:
    • Hydromorphic: Bog soils are formed under cool, temperate, continental climates. In these soils the upper layer is peaty while the lower layer is gleyey
    • Calcimorphic: Wherever the limestone is exposed, rendzinas are formed which are dark, organic rich and good for cultivation in humid regions.
    • Halomorphic: These soils occur mostly in deserts.
  Azonal Soils
  • A soil which has not been sufficiently subjected to soil –forming processes for the development of a mature profile and so is little changed from the parent rock material.
  • Azonal soils do not have B horizon because it is too immature. Thus, the A horizon lies immediately above the C horizon.
  • Examples are soil forming on scress, recently deposited alluvium, sand dunes, and newly deposited glacial draft, wind-blown sand, marine mud fats and volcanic soils.
  Soils of India
  • Since Independence, scientific surveys of soils have been conducted by various agencies.
  • Soil Survey of India, established in 1956, made comprehensive studies of soils in selected areas like in the Damodar Valley.
  • The National Bureau of Soil Survey and the Land Use Planning an Institute under the control of the Indian Council of Agricultural Research (ICAR) did a lot of studies on Indian soils. In their effort to study soil and to make it comparable at the international level, the ICAR has classified the Indian soils on the basis of their nature and character as per the United States Department of Agriculture (USDA) Soil Taxonomy.
  • On the basis of genesis, colour,composition and location, the soils of India have been classified into:
  1. Alluvial soils
  2. Black soils
  3. Red and Yellow soils
  4. Laterite soils
  5. Arid soils
  6. Saline soils
  7. Peaty soils
  8. Forest soils.
  Alluvial Soils
  • Alluvial soils are widespread in the northern plains and the river valleys.
  • These soils cover about 40 per cent of the total area of the country.
  • They are depositional soils, transported and deposited by rivers and streams.
  • Through a narrow corridor in Rajasthan, they extend into the plains of Gujarat.
  • In the Peninsular region, they are found in deltas of the east coast and in the river valleys.
  • In the Upper and Middle Ganga plain, two different types of alluvial soils have developed, viz. Khadar and Bhangar.
  • Khadar is the new alluvium and is deposited by floods annually, which enriches the soil by depositing fine silts.
  • Bhangar represents a system of older alluvium, deposited away from the flood plains. Both the Khadar and Bhangar soils contain calcareous concretions (Kankars).
  • These soils are more loamy and clayey in the lower and middle Ganga plain and the Brahamaputra valley.
  • The sand content decreases from the west to east.
  Black Soil
  • Black soil covers most of the Deccan Plateau which includes parts of Maharashtra, Madhya Pradesh, Gujarat, Andhra Pradesh and some parts of Tamil Nadu.
  • In the upper reaches of the Godavari and the Krishna, and the north western part of the Deccan Plateau, the black soil is very deep.
  • These soils are also known as the ‘Regur Soil’ or the ‘Black Cotton Soil’.
  • The black soils are generally clayey, deep and impermeable.
  • They swell and become sticky when wet and shrink when dried.
  • Chemically, the black soils are rich in lime, iron, magnesia and alumina.
  • They also contain potash. But they lack in phosphorous, nitrogen and organic matter.
  • The colour of the soil ranges from deep black to grey.
  Red and Yellow Soil
  • Red soil develops on crystalline igneous rocks in areas of low rainfall in the eastern and southern part of the Deccan Plateau.
  • The soil develops a reddish colour due to a wide diffusion of iron in crystalline and metamorphic rocks. It looks yellow when it occurs in a hydrated form.
  • The fine-grained red and yellow soils are normally fertile, whereas coarse-grained soils found in dry upland areas are poor in fertility.
  • They are generally poor in nitrogen, phosphorous and humus.
  Laterite Soil
  • Laterite has been derived from the Latin word ‘Later’ which means brick.
  • The laterite soils develop in areas with high temperature and high rainfall.
  • These are the result of intense leaching due to tropical rains.
  • With rain, lime and silica are leached away, and soils rich in iron oxide and aluminum compound are left behind.
  • Humus content of the soil is removed fast by bacteria that thrive well in high temperature.
  • These soils are poor in organic matter, nitrogen, phosphate and calcium, while iron oxide and potash are in excess.
  • Hence, laterites are not suitable for cultivation; however, application of manures and fertilizers are required for making the soils fertile for cultivation.
  • Laterite soils are widely cut as bricks for use in house construction.
  Arid Soils
  • Arid soils range from red to brown in colour.
  • They are generally sandy in structure and saline in nature.
  • In some areas, the salt content is so high that common salt is obtained by evaporating the saline water.
  • Due to the dry climate, high temperature and accelerated evaporation, they lack moisture and humus.
  • Nitrogen is insufficient and the phosphate content is normal.
  • Lower horizons of the soil are occupied by ‘kankar’ layers because of the increasing calcium content downwards.
  Saline Soils
  • They are also known as Usara soils.
  • Saline soils contain a larger proportion of sodium, potassium and magnesium, and thus, they are infertile, and do not support any vegetative growth.
  • They have more salts, largely because of dry climate and poor drainage.
  • They occur in arid and semi-arid regions, and in waterlogged and swampy areas.
  • Their structure ranges from sandy to loamy.
  • They lack in nitrogen and calcium.
  Peaty Soils
  • They are found in the areas of heavy rainfall and high humidity, where there is a good growth of vegetation.
  • Thus, large quantity of dead organic matter accumulates in these areas, and this gives a rich humus and organic content to the soil.
  • Organic matter in these soils may go even up to 40-50 per cent.
  Forest Soils
  • As the name suggests, forest soils are formed in the forest areas where sufficient rainfall is available.
  • The soils vary in structure and texture depending on the mountain environment where they are formed.
  • They are loamy and silty on valley sides and coarse-grained in the upper slopes.
  • In the snow-bound areas of the Himalayas, they experience denudation, and are acidic with low humus content.
  • The soils found in the lower valleys are fertile.