Formation of Soil

After reading this article you will learn about the formation of soil.

Rocks were slowly broken down to smaller and smaller pieces by physical weathering, and then decomposed by chemical weathering to form the parent material, which may be defined as the weathered material from which the soil was synthesized.

The parent material was further decomposed by the action of atmospheric gases like water vapour, carbon-dioxide, sulphur dioxide, nitric oxide etc. at different temperatures. Some of the primary minerals present in rocks were altered to form some secondary minerals.

Some were completely decomposed and the products of decomposition recombined with each other to form some secondary minerals, which also included clay minerals. These clay minerals or simply the clay remained intimately mixed with primary minerals, like quartz or sand to form some soil.

Some lower plants like algae, muss, lichens (association of algae and fungi is called lichens) etc. began to grow on the bare rocks and on this thin layer of soil, respiring to produce carbon dioxide, which reacted with water to form carbonic acid that decomposed the primary minerals to form clay.

Later on, higher plants began to grow and continued this process. These plants continued to add organic matter to the soil in the form of leaves, roots etc. which decomposed in the soil to form humus that combined with the clay to form the clay-humus complex.

Hence the colour of the surface soil gradually darkened. This process continued till a darker layer of soil of about one foot in thickness developed. This is known as the A horizon. The parent material, which at this stage, was at a considerable distance from the surface, could not be subjected to the direct action of atmospheric agents like heat, moisture and gases etc., so it continued to decomposed at a slower rate forming a relatively compact layer called the B horizon.

Some clay, and sesquioxide (Sum of the oxides of iron and aluminum is called sesquioxide, R₂O₃) were gradually washed down or eluviated from, the lower portion of the A horizon and were washed in or illuviated in the B horizon. So the lower eluviated portion of the A horizon became the E horizon. The E horizon is usually lighter in colour and texture may be and a little acidic in reaction.

In the earlier stages of soil development, soils were dominated by characteristics which they inherited from the parent material. They are dominated by the acquired characters at the later stages of soil development.

For example, soils which were developed from the basic parent material were rich in basic elements and alkaline in reaction during the earlier stages of Soil development. Later on these basic elements were gradually washed down by high rainfall and the soils ultimately become acidic in reaction at the later stages of their development.

We have been that as the parent material was gradually converted to soil, definite layers developed. These layers are called horizon. A soil profile (Fig. 3.1) is a vertical section of the earth’s crust that includes different layers called horizons, formed due to the action of soil forming factors and also the deeper layers that influences soil formation.

A theoretical soil profile consists of the following horizon:

O Horizon ― Organic Horizon develops from dead plant materials on forest lands. This surface horizon occurs just above the mineral or inorganic horizon. It has been subdivided as follows:

Oi Horizon ― Consists of under-composed plant material, the original form of which can be recognised.

Oe Horizon ― Consists of incompletely decomposed plant material.

Oa Horizon ― Consists of completely decomposed plant material, the original form of which cannot be recognised. It is black in colour.

A Horizon ― is the top most mineral horizon where humus have been thoroughly mixed and chemically combined with clay to form clay-humus complex. So it is black in colour.

E Horizon ― is the horizon of eluviation from which some clay and sesquioxide and humus have been washed down or eluviated. So it is lighter in texture (i.e. respectively richer in sand) and colour.

EB Horizon ― Transitional layer between E and B. It is more like E than B.

BE Horizon ― Transitional layer between E and B. It is more like B than E.

B Horizon ― is the horizon of illuviation. Some clay, sesquioxide humus etc. have been washed in or illuviated in the B horizon, which is relatively heavier in texture and darker in colour. Lime may accumulate in the B horizon in arid region.

C Horizon ― is the parent material which is the partially or full weathered material from which A, E and B horizon have been developed.

R Horizon ― is the Bed rock.

Solum is the true soil developed by the action of soil forming factors. It includes A, E and B horizon.

Regolith is the unconsolidated material lying about the bed rock. It includes A, E, B and C horizon.

Usually all the above mentioned horizons are not found in every soil profile because they may be immature or over influenced by poor-drainage, topography etc.

Well drained mineral soils usually possess O and Oa if the land is under forest, A or E horizon, B and C horizons depending on soil development.

When farmers being to plough vigin soils, the original layered condition of the soil from 15-25cms depth is destroyed. This layer has been thoroughly mixed up by ploughing and it includes A and E horizon when they are sufficiently deep. Otherwise it may include a little part of B horizon also. This mixed layer is called the furrow slice designated as Ap horizon.

Factors of Soil Formation:

Rocks are first broken down to smaller and smaller pieces which are further decomposed to form the parent material. Therefore a parent material may be defined as the unconsolidated, fully or partially weathered material from which the solum has been presumed to have developed.

Parent materials have been classified on the basis of their silica content as acidic or Felsic, intermediate, basic or mafic and ultrabasic or ultramafic Acidic parent material contains more than 66 per cent silica. Intermediate parent material contains 52 per cent to 66 per cent silica.

Basic parent material contains 45 to 52 per cent silica. Ultra basic parent material contains less than 45 per cent silica. Acidic parent material is gradually decomposed by weathering agents to form lighter coloured and textured soils of a relatively poor fertility because the acidic parent material is rich in sand and poor in basic nutrient elements. Basic parent materials are relatively richer in basic elements, plant nutrients and poorer in sand than acidic parent materials.

So they are gradually decomposed by weathering agents to form a darker, heavier (clay) soils of relatively higher fertility and an alkaline reaction. Finer textured soils are richer in humus than coarser textured soils.

The influences of the parent material are more clearly seen in the earlier stage of soil development. But as the time passes basic elements and plant nutrients are gradually washed down from soils that had developed from weathering of basic parent material.

Consequently the soils developed from the basic parent material become poor in basic elements and plant nutrients, and acidic in reaction over a long period.

The nature of soils developed from the weathering of a few important parent materials is as follows.

1. Lighter Coloured Igneous Rocks:

The compositions of Rhyolite, and Granite are almost similar, and therefore, they weather to form acidic parent material, which in turn decomposes to form friable, permeable sandy loam to loam soils of light yellow colour due to their low iron content.

The primary minerals decomposed to form the secondary clay minerals like illite, vermiculite, Montmorillonite etc. in drier climates, which may decompose to kaolinite when both rainfall and temperature increase.

Therefore the soils which have developed in a dry climate may contain fair amounts of plant nutrients and are alkaline in reaction, but those which have developed in a warm humid climate are poor in plant nutrients and moderately to strongly acidic in reaction.

Both trachyte and syentite consists almost entirely of orthoclase. But they also contain low amounts of biotite and hornblende. Since orthoclase weathers very slowly to form clay, the soils developed from weathering of trachyte and syenite is medium in texture and fertility and natural in reaction.

2. Dark Coloured Igneous Rocks:

Andesite, diorite, basalt, gabbro, perdotite and dunite consists of ferromagnesian and calcium bearing minerals, which rapidly weather to form dark coloured clayey soils rich in plant nutrients, basic elements and alkaline in reaction. Usually Montmorillonite and allied mineral smectite (group) dominate the clay minerals. If the rainfall and temperature further increases, the Montmorillonite decomposes to kaolinite.

3. Sedimentary Rocks:

Sandstone mainly consists of quartz, but a little feldspar and mica are also present as impurities.

In general, parent material formed from weathering of sandstone decomposes to form light coloured sandy soils, poor in plant nutrients and acidic in reaction.

Shales are made up of clay minerals and smaller quantities of mica, feldspar, hornblende and quartz. Sometimes a little calcite may be present.

Shales usually weather to from darker coloured, relatively impermeable clayey soils of a high base and plant nutrient status and alkaline in reaction in which usually illite and Montmorillonite dominates the clay minerals. Soils are usually very fertile.

Limestone and dolstone contain more than 50 per cent calcite and dolomite respectively. They weather to form darker coloured clayey soils rich in plant nutrients and basic elements and alkaline in reaction in a cool dry climate.

But the nature of soil formed from the weathering of limestone and dolstone in a warm humid climate depends upon the nature of the impurities present in them because calcium carbonate and calcium magnesium carbonates are washed down by high rainfall and the impurities are left behind.

If the impurities are mainly clay, then dark coloured clayey soils fairly rich in plant nutrients and alkaline in reaction are formed. But if the impurities are quartz or iron oxide, then light coloured, highly permeable sandy soils poor in plant nutrients and basic elements and acidic in reaction are formed.

4. Metamorphic Rocks:

Gneiss usually weathers to form soil which is dominated by smectite and hydrous mica groups of clay minerals in a cool humid climate. These soils contain a fair amount of plant nutrients and are neutral to slightly alkaline in reaction. But these clay minerals further decompose to kaolinite when the temperature and precipitation is increased. Hence highly permeable coarse textured acidic soils are formed in humid climate.

Soils developed from the weathering of mica schists tend to be silty, and are dominated by illite and vermiculite. Hornblende schist may weather to form the smectite group of clay minerals. Schists usually weather to form loamy to clayey soils, fairly rich in plant nutrients and alkaline in reaction.

Recognizable horizons develop faster on sandy loam to loam soils which are high in easily weatherable minerals, are well drained and aerated and are located on moderate slopes. They develop slowly on sandy parent materials which are poor in easily weatherable minerals. They also develop slowly on highly clayey parent materials.

Relation between Parent Material and Vegetation:

If the parent material is rich in basic elements, it weathers the soil rich in basic elements and the vegetation growing on it is rich in basic elements. Parent materials like sandstones; quartzites etc. are poor in basic elements Coniferous narrow leaved plants usually grow on such acidic soils in cool humid climate.

5. Climate and Climatic Water Balance:

Weather is the state of the atmosphere at any given time and place. Climate is the average weather. In Greek, climate means the angle of the sun’s rays striking the earth. Differential heating causes local variation in the earth’s temperature which cause air mass movement and induces precipitation.

The climate comprises of precipitation and temperature that affect various physical and chemical processes in soil. Climatic water balance is the difference between gain in water mainly through rainfall and loss of water due to evaporation.

This water reached the surface of the earth and took part in soil formation. In arid region, less rain water reaches the ground surface, more water is evaporated and therefore, the climatic water balance decreases.

Climatic water balance may be estimated by various means:

The climatic water balance effect, the decomposition of minerals the formation of clay and the growth of natural vegetation. The soluble products of decomposition are removed along with the percolating and the run-off water.

As the climatic water balance increases, more natural vegetation grows and more organic matter is added to the soil. Hence the organic matter and nitrogen content of soil increase. Primary minerals decompose at increasing rates to form 2: 1 types of clay minerals like smectite (Montmorillonite) hydrous mica and vermiculite groups of clay minerals.

Hence the percentages of clay in soils and their cation exchange capacities increase when the climatic water balance increase.

However, excessive rainfall leaches the basic elements and silica down the soil profile to make soils acidic in reaction when 2:1 types of clay minerals decomposes to form the 1:1 group of clay minerals (Kaolinite) which are further decomposed to hydrous oxides of iron and aluminum, 2:1 types of clay minerals dominated the clay mineralogy of soils of relatively arid regions which are rich in basic elements and alkaline reaction.

A calcium carbonate layer occurs near the surface of the soils if the climate is extremely arid. The depth of this calcium carbonate layer increases with an increase in rainfall. When rainfall is constant and the temperature is increased, less vegetation grows thus less organic matter is added to the soil. Organic matter rapidly decomposes. Hence the humus and nitrogen content of soils decreases.

The increase in temperature also increases the rate of weathering of primary minerals to increase the clay content and cation exchange capacity of young soils. But as the clay decomposes, the cation exchange capacity of old soils decreases.

6. Living Organisms:

The biosphere comprises of billions of microorganisms which improve soil productivity by decomposing rocks, minerals and organic matter. Of special interest in the influence of the biosphere on soil genesis, is the lichens which are symbiotic associations of algae and fungi.

Algae, moss, and lichens grow on bare rock and respire to produce carbon dioxide, which reacts with water to form carbonic acid that dissolves primary minerals and releases the nutrient contained in them for plant growth. Algae use atmospheric nitrogen which is released in the soil upon the death of the algae. Thereafter higher plants grow in an environment richer in nutrients and water.

Higher plants absorb-nutrients from the lower layers of the soil and leave them on the surface in the form of decaying leaves, which decompose, forming humus. Carbon dioxide produced from the decomposition of organic matter and respiration of microorganisms and plant roots continues to decompose primary minerals to form more clay and convert insoluble minerals to soluble ones.

When plants grow on soils the phosphorus and potassium content of soils increases but the calcium and magnesium content and the soil pH usually decreases.

Root system of grasses are profusely developed and are uniformly distributed throughout the A horizon and the upper portion of the B horizon.

These roots die and decay to form the humus which is incorporated with the soils of the A and the upper portion of the B horizon thoroughly enough to impart dark colour to them. Forest trees shed huge quantities of leaves on the forest floor. Hence forest land soils possess the thick “O” horizon.

Usually grassland soils contain more organic matter and nitrogen and are heavier in texture and darker in colour, of higher cation exchange capacity and pH then the forest and soils.

Pine trees which contain low amount of basic elements, grow on soils poor in basic elements and acidic in reaction.

When both grassland and forest land soils have developed from the same parent material under the same climatic conditions, the rate of removal of bases and the consequent development of soil acidity is more in forest land soil than in the grassland soil probably due to the three following reasons.

Firstly the forest trees return fewer basic elements in the form of decayed leaves to the surface soil. Different species of trees return different amounts of bases to the soil.

Secondly, rainwater leaches the basic elements and plant nutrients to a greater depth in before they are absorbed by roots in forest land soil then in grassland soil.

Thirdly, water entering to rest soils is more acidic in reaction because it contains organic acids formed from decomposition of forest vegetation. The organic acids react with the primary minerals of the forest soil to release the basic ions, which are washed down by high rainfall.

So in forest land soil, the percentage of clay in the B horizon is usually more than that of the A horizon whereas the clay contents of the A and the B horizons of grassland soils are more or less equal.

The decomposition of the organic matter in the forest land soils usually produces organic acids that humus and clay are washed down to B horizon. This horizon therefore possesses illuvial deposits of humus and iron and aluminum compounds in addition to the illuvial deposit of clay.

Soil animals like moles, earthworms, ants and termites bring the soil from B horizon to A horizon. Excessive ploughing of the land, burning of the natural vegetation and overgrazing of the land, decreases the soil productivity and retards soil development.

The application of lime and fertilizers to the soil also retards the development of the soil profile but increases soil productivity. In the humid regions, this practice makes the soil environment more favourable for the growth of grasses then for trees. The soil profile also develops slowly when the soil is constantly mixed up by the animal and the people.

Living organism are influenced both by parent material and climate. Vegetation growing on soils developed from basic parent material is usually acidic parent material. Grasses are the dominant form of vegetation in semi-arid areas whereas the forests are the dominant vegetation in semi humid areas.

Earthworms are usually found in soil richer in basic element and organic matter and alkaline in reaction in the humid region. Ants and termites usually occur in soils of temperate regions and tropical and sub-tropical regions respectively.

7. Topography:

The relief of the land refers to the differences of elevation within it. The relief shown on the topographic map is called the topography of the land. The parent material which is located on the fairly level to very gently sloping lands quickly decomposes to form the soil profile because a major proportion of the rain water washed down some of the soluble salts and humus and fine clay down the soil.

If the parent material has any hard pan to restrict the downward flow of water through it, the Soil profile development will be slower. Water may be ponded on the surface of the level land where the organic matter may accumulate to form the peat and the muck soil.

The rain water rapidly flows down the parent material which is located on the steep slope and carries away considerable amounts of the weathered materials. So less water passes down the parent material on steeper slopes. So less of parent material is decomposed to form less soil on steeper slopes.

When the slope of the land is increased, more clay and basic ions move to the bottom land. Hence the bottom land soils are darker in colour heavier in texture, deeper and more alkaline in reaction than soils on steeper slopes, the soil located on steeper slopes are highly oxidized, red, and acidic in reaction and of shallow depth.

A group of soils which have developed from the same parent material in same climate but under different topographical condition is called Soil Catena. Topography influences vegetation. Grasses occupy the elevated areas and forest plants occupy the lower areas where the climate is neither too dry nor too wet.

8. Time:

Time has been regarded as one of the factors influencing formation soil because even chemical weathering of rocks to form soil requires some time to complete itself.

The parent material passes through the following five stages of soil formation when it gradually decomposes to form soil:

(i) Initial stage: The parent material has not yet been weathered.

(ii) Juvenile stage: Weathering of parent material has just started.

(iii) Verile stage: Now the easily decomposable parent material has decom­posed to form clay. So the percentage of clay in the soil has increased.

(iv) Senile stage: Slowly decomposable primary minerals have also decomposed.

(v) Final stage: Soil profile development is practically complete.

If conditions are favourable, the parent material may be transformed into immature soil or young soil in relatively less time in which organic matter has accumulated in the surface soil and a lesser quantity of parent material has decomposed to form the A horizon; very little of clay sesquioxide and humus has been eluviated from the A horizon.

Only A and C horizons now exist. The soil inherits most of its character from the parent material. As time passes more parent material gradually decomposes to form the B horizon and some clay sesquioxide humus etc. have also been illuviated in the B horizon.

All the horizons have been clearly developed and so the soil is mature. As time passes more primary minerals decompose to form the secondary minerals which are also gradually decomposed to form the hydrous oxides of iron and aluminum. When all the primary minerals have been converted to hydrous oxides of iron and aluminum, the soil has reached the old age stage.

Soil Formation in Arid Regions:

Arid regions are commonly characterized by high temperature and low rainfall where chemical weathering of rocks and minerals predominates over their physical weathering. Little decomposition of primary minerals takes place under extremely low rainfall i.e. less than 125 mm per annum to release their constituents in the soluble form that remain in the extremely coarse textured surface soil.

The desert soil thus formed is rich in calcium and magnesium and at some place also in sodium and is therefore alkaline in reaction. A little increase in rainfall i.e. up to 375 mm/annum induces short grasses to come up which get withered during the dry season and rejuvenate with the advent of the short moist season of scanty rainfall.

A little quantity of organic matter in the form of dead roots and shoots of grasses is added to the soil that decomposes to form the humus. The moisture and carbon dioxide generated from the respiration of grass roots and decomposition of dead root facilitates the decomposition of some meager amount of primary minerals to form little amount of clay, Horizons are feebly developed.

Whatever meager amounts of clay and humus have been formed, combine together to impart gray colour to the soil which is known as Sierozems which in Russian mean, gray earth. Its A and B horizons are of gray colour. White specks of lime and gypsum have also been seen in the B horizon. The soil is alkaline in reaction.

Soil Formation in Semi-Arid Region:

Temperature is less and annual rainfalls (375 to 1250 mm) are a little more in the semi-arid region than in the arid region. So chemical weathering of primary minerals takes place a little more readily, in the semi-arid region than in the arid region.

Increase in rainfall permits short grasses to come up. They add some organic matter in the form of their dead roots and shoots during the ensuing dry season moisture coupled with the carbon dioxide generated from the respiration of gross roots and decomposition of dead grasses facilitates the decomposition of primary minerals to form some clay.

Humus combines with the clay to impart gray coloured surface soil. This is known as chestnut soil which is rich in calcium and magnesium and is therefore of alkaline in reaction. A lime layer occurs near the surface. A gypsum layer may be present below the lime layer. Hence the soil forming processes is known as calcification.

A little increase in annual rainfall i.e. up to about 1250 mm induces tall grasses to grow densely enough to profusely develop their underground stems and roots. Some roots are regularly sloughed off and decomposed to form humus.

The availability of relatively higher amount of moisture coupled with carbon dioxide generated from the respiration of the underground roots and stems of grass vegetation and also from the decomposition of dead grass roots and stems facilitates the chemical weathering of primary minerals to form the clay.

Humus combines with clay to form the clay humus complex. Hence soils of A and B horizons are uniformly coloured dark gray. The soil is rich in calcium and magnesium and is therefore alkaline in reaction.

Relatively higher rainfall is responsible for lowering the lime and gypsum layer. As the soil is rich in lime, so the soil forming process is known as calcification.

However melanization which is the process of darkening of the soil mass by the in-cooperation of humus with the soil, is also responsible for the genesis of the soil which is the process of darkening of the soil mass by the in-cooperation of humus with the soil, is also responsible for the genesis of the soil which is known as chernozem.

Soil Formation in Humid Climate:

Podzolisation is the chemical process of migration of iron and aluminum to the B horizon and precipitation of silica in the A horizon as described below:

The acidic parent material rich in silica and poor in basic elements has developed from the weathering of acidic rocks like quartzite that is rich in silica and poor in basic elements. The weathering of this acidic parent material results in the formation of acidic soils poor in basic elements in cold and moist climates.

Some plants like Hemlock, pine, Heath etc. grow on these soils and deliver organic matter to the soil which is poor in basic elements and therefore decomposes to produce considerable amounts of organic acids, which decomposes clay to liberate silicon, aluminum, iron, and other mediums of the A₂ horizon which becomes ash grey in colour.

Iron, aluminum ions, clay and humus are washed down to the B₂ horizon, where the positively charged iron and aluminum ions react with the negatively charged clay and humic micelle and precipitate there. In Russia, pod means under, zola means ash. They call the process podzolisation, and the soil, podzol because it possesses an ash agey A₂ horizon.

Latosolisation (Laterisatton):

It is the chemical migration of silica from the solum and precipitation of iron and aluminum in the solum as described below.

Basic rocks like Basalt decompose when both rainfall and temperature are high, to form basic parent material, which in turn decomposes to form soil rich in basic the elements, on which some broad leaved forest plants grow and organic matter is added to the soil as dead leaves. The basic parent material is rich in ferromagnesian minerals.

The organic matter is also rich in basic elements. The ferromagnesian minerals and organic matter decompose to release silicon, aluminum, iron and basic ions. The basic ions make the medium alkaline, which keeps the silicon ions in solution which is washed down by high rainfall.

Iron is precipitated and later oxidized to ferric oxide which is red in colour. So the soil may be coloured red. Later, basic elements are gradually washed down when the soils become acidic in reaction, and kaolinite decomposes to hydrous oxides of iron and aluminum.

This process is called latosolisation. Whenever drainage is restricted, a soft deposit of iron oxide occur at or near the water table. It is cut in the form of bricks which harden on drying. In Latin later means bricks. So this process is known as Laterisation.

Gleization:

When soils remain saturated with water for long periods in the presence of organic matter, ferric compounds like ferric phosphate and ferric sulphide are reduced to ferrous compounds like ferrous phosphate or sulphide which are bluish grey in color.

So this sub-merged soil develops a bluish colour which changes to brown when the soil is re-exposed to the atmosphere. This process of soil formation is known as gleization. Gleization usually takes place in low lying areas where water accumulates.

Salinization, Alkalization and De-Alkalization:

Soluble neutral salts of sodium, calcium and magnesium originate mainly from the decomposition of primary minerals in the soils of arid regions. When water evaporates from the surface of the soil, the water containing soluble salts moves from the deeper layers to the surface of the soil and deposits them at the surface of the fields, which are covered with a white crust of soluble salts in patches. This process of soil development is known as salinization (or formation of solonchak).

If these soluble salts are removed to the lower layer by a limited amount of rainfall occurring in the arid regions, then calcium and other ions are replaced from the clay and humic micelle by sodium ions.

Consequently the soil becomes sticky and plastic when wet and very hard when dry, black due to the dis­solution of humus in the alkaline medium. This process of soil formation is known as alkalization (or formation of solonez).

If the rainfall increases a little, sodium ions are replaced from the clay and humic micelle by hydrogen ions, the silicate clay is decomposed to release silica which is deposited on the soil particles.

Consequently an ash grey colour develops on the soil. The process a soil formation is known as de-alkalization (or formation of Solod).

Soil is a Natural Body:

Soil is the collection of natural bodies occupying portions of the earth’s surface, possessing properties due the integrated effects of climate and living matter, action upon parent material as conditioned by relief over periods of time.

Soils are natural bodies that exhibit three dimensional sequences of characteristics. First, the properties gradually change downwards from the surface to the bed rock.

The unconsolidated material lying above the bed rock is called the regolith. The upper portion of the regolith is different from the lower portion.

Being nearer the atmosphere, this upper zone has been subjected to the weathering action of wind, water and heat. Plant roots are found in the zone. Organic matter delivered as fallen leaves on the surfaces and dead roots below the surface are decomposed to form humus.

The humus and partially decomposed organic matter is mixed with the soil at the surface layer. Primary minerals are decomposed to form clay. This upper and biochemically weathered portion of the regolith is called the solum.

Scientists have considered the soil to be a natural body possessing both depth and surface area. The properties of soil change vertically because the intensity of weathering of primary minerals decreases from the surface downwards and also because the organic matter is incorporated into surface layer, and decomposed there.

The characteristics of soils also changes in the horizontal direction due to change in topography and parent material e.g. soils on gently sloping upland is well drained and oxidized whereas the soil on the adjacent bottom land may be poorly drained and, consequently in a reduced condition. Soil developed from granite tends to be coarse textured, whereas soil developed from limestone tends to be fine textured.

Scientists have recognized the variation in characteristics of the soil in the horizontal direction. They have set up classification systems in which the soil is considered to be composed of a large number of individual soils, each with its own distinguishing characteristics.

An individual soil body is bounded laterally by other soil bodies. Adjacent soil bodies may be differentiated on the basis of the depth of solum e.g. a soil individual may have depth of 60 cm to 90 cm which means that it is bounded laterally by two other individual soils whose depths are less than 60 cm and more than 90 cm respectively.

All these concepts are applied to find out the minimum size of a soil individual, which is called a Pedon. The lower limit of a pedon is the depth of the solum, its lateral dimensions are large enough to permit the study of the nature of any horizon present because a horizon may be variable in thickness, or may even be discontinuous.

Its area range from 1 to 10 square meters, depending upon the variability in the horizon. Where horizons are intermittent or cyclic and recur at a linear interval of 2 to 7 metres (7 to 23ft), the pedon includes one-half of the cycle. Thus each pedon includes the range of horizon variability that occurs within these small areas.

Where the cycle is 2 metres or all the horizons are continuous and of uniform thickness, the pedon has an area of 1 square metre. Again, under these limits, each pedon includes the ranges of horizon variability associated with that small area. The shape of the pedon is roughly hexagonal. One lateral dimension should not differ appreciably from any other.

Soil as a Medium for Plant Growth:

Plant growth depends on the following six factors:

(i) Light

(ii) Mechanical support

(iii) Nutrient supply

(iv) Water supply

(v) Oxygen supply an

(vi) Heat.

Plant growth depends on the soil for all except the first factor. Roots go deeper in to the soil in search of nutrients and water, Roots anchored in the soil enable growing plants to remains erect.

They cannot grow deep in poorly drained soil and absorb nutrients and water from the soil. Not less than sixteen elements are essential for the growth of crops. They get carbon and oxygen from the air, hydrogen from water and the remaining 13 elements from the soil which are called plant nutrients.

Nitrogen, phosphorus, potassium, calcium, magnesium and sulphur which are required by crops in relatively larger amounts are called major elements or macronutrients and the remaining seven elements i.e. iron, manganese, zinc, copper, boron and molybdenum and chlorine, called minor elements or micronutrients are required in relatively smaller amounts.

About 500 ppm or more of macronutrients are required whereas about 50 ppm or less of micronutrients is required by crops. Nitrogen, phosphorus and potassium which are added to the soil in the form of fertilizer, are called fertilizer elements. Calcium and magnesium, are added to the soil in the form of lime are called lime elements.

Primary minerals and organic matter are decomposed to release the nutrients to the soil water in the form of ions. This soil water containing nutrient ions is known as soil solution. Roots take up nutrients in the form of ions mentioned in Table 3.1.