Theories of Geography Part 11 – Soils

Soil Basics

Soil Basics

The fine particles of the solid rocks along with the organic / inorganic matter are called soil.  The naturally occurring soil is influenced by parent material, climate, relief and the physical, chemical and biological agents such as microorganisms living in it. The naturally occurring soil is influenced by Parent rocks, Climate, Organic content, Topography, Land use practices/ Human interference, Time etc. Soil contains mineral particles, decayed organic material, soil water, soil atmosphere, and living organisms, which exist in a complicated and dynamic relationship with one another. Soil is a dynamic natural body made up of the materials covering the earth’s surface in which plants grow. It is composed of both mineral and organic matter.

Complex Nature of Soil

The above mentioned factors do not work on soil independently or in isolation, but in close association with each other, leading to a whole network of inter-relationships of quite a complex nature. The material of the soil or the parent material is derived from the rocks ex-posed on the surface. The relief and slope along with the work of various materials. Soils weathering determine conditions for the disintegration of the rock materials. Soils may be transported by the running water, wind or other agents of the rock materials. Soils remain in the original position.

When the soil remains in its original position, it is said to be in situ, and in that state it is further modified by the climate, particularly moisture supply, plant growth ,and  bacterial activity dependent on these factors .   A brief supply, plant growth, and bacterial activity dependent on these factors. A soil is made up of four elements: inorganic or mineral 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.


The end-product of the breakdown of dead organic material is known as Humus. Humus is a structure-less, dark-brown or black jelly found beneath the soil surface. In uncultivated land, the humus is derived from the natural decay of previous generations of plants, while in the ploughed and cultivated land it is supplied as some kind of manure. The humus of ordinary soil is black, and is thus responsible for making the soil darker than the subsoil. It plays an important but very complicated part in maintaining the fertility of soil.  The amount of humus in different soils varies considerably; some, like the peat soil, consist largely of slightly decomposed organic matter which has not yet become humus.

Soil Texture

A soil is generally characterized by the size of its particles. A clayey soil may thus be described as fine, a sandy soil as coarse, while a silly soil is intermediate. If one handles a moist soil sample of each of these he feels gritty, sticky and silky, respectively.  The standard unit for the measurement of soil particles is the millimeter, but a smaller unit is the micron (1 micron= 0.001 mm), which is applicable, for instance, to the measurement of soil colloids.

Sandy Soil

Sandy soil is a light soil that consists mainly of sand, i.e., grains of quartz with considerable air spaces between them. The sand may either be ‘coarse’ where the particles are between 0.2 and 2 mm in diameter, or ‘fine’ where the grains between 0.05 and 0.2 mm are just visible to the naked eye.  These light soils allow water to drain through rapidly, taking soluble plant foods with it. Sandy Soils are known as ‘hungry’ soils, which not only need constant manuring but may dry out completely during a period of drought so that shallow-rooted crops fail and pastures ‘burn’. They are good for horticulture (vegetables and fruits), legumes (such as moth and pulses), ground nut and bajra.

Clayey Soil

Clayey soil is an exceptionally fine grained soil, very retentive of moisture. It often becomes plastic when mixed with water. The individual grains of clayey soil are 0.002 mm in diameter. These particles consist mainly of hydrated aluminium silicates. Clay contains little air and can hold more water, so forming a sticky mass,but when it dries out completely, it forms a hard, concrete like surface, seamed with numerous cracks. Sometimes, a compacted solid layer of clay in the subsoil is formed, which is known as claypan, and is often hard and difficult to dig or plough. Clayey soils are often rich in plant food and give much better yields than that of sandy soils.  They are devoted to rice, perennial grasses other crops such as clover. Efficient drainage methods, modern machinery and careful liming enable clayey soils to grow roots, green crops and cereals.

Silty Soil

Silty soil is finer than sand but coarser than clay. Its particles are assumed to have a diameter between 0.02 and 0.002 mm. These soils are rich in humus contents and are devoted to numerous cereal and non-cereal crops.

Loamy Soil

It is highly fertile soil consisting mainly of a mixture of sand and day, together with silt and humus. It has the good qualities of both sand and clay, but not their bad qualities. It comprises an almost equal mix of sand and silt with less than 30 per cent clay. It can retain some moisture and plant food even under the adverse weather and climatic conditions. It is well· aerated and drained, and can be readily worked.  It is generally devoted to wheat, barley, legumes, sugarcane, sugar beet, maize, millets, rice, grasses, vegetables and orchards.

Soil pH

Soil pH is a measure of the acidity or basicity in soils.  As we know, the pH below 7 is acidic and above 7 is basic. Soil pH is considered a master variable in soils as it controls many chemical processes that take place. It specifically affects plant nutrient availability by controlling the chemical forms of the nutrient. The optimum pH range for most plants is between 6 and 7.5, however many plants have adapted to thrive at pH values outside this range. The first thing we should note that in cool and moist areas, percolating groundwater leaches out the soluble bases (such as calcium). As a result, the soils gradually become lime-deficient which increases the acidity of the soil. Both the highly acidic and alkaline soils are injurious to crops.  If the soil becomes unduly acidic, the farmers apply lime in various forms to meet the requirements of the soil. In practice, a pH value between 6 and 6.5, i.e., very slightly acidic, is desired. Lime not only helps to neutralize the excess adds and so ‘sweeten’ the soil, but it also encourages bacteria and helps to improve the physical , texture of heavy soils. High soil acidity is typical of cold, humid climates. In arid climates, soils are typically alkaline.

How to Increase soil pH?

Acidity can be corrected by the application of lime, a compound of calcium, carbon and oxygen (CaCO3), which removes add ions and replaces them with the base calcium.

How to Decrease soil pH?

To decrease the pH of the soil, the Iron sulphates or aluminium sulphate as well as elemental sulfur (S) are used through the formation of sulphuric acid.  Further, Urea, urea phosphate, ammonium nitrate, ammonium phosphates, ammonium sulphate and monopotassium phosphate fertilizers have a organic matter in the form of plant litter, compost, and manure will decrease soil pH through the decomposition process. Certain acid organic matter such as pine needles, pine sawdust and acid peat are effective at reducing pH.

The problem of Alkaline Soils

Alkali or alkaline soils are the soils with high pH (> 9). The first visible impact of Alkaline soil is that it has a poor soil structure and a low infiltration capacity. The Alkali soil is generally having a hard calcareous layer at 0.5 to 1 metre depth. Alkali soils have dominated presence of minerals such as Sodium Carbonate which causes the soil to swell. Please note that all alkaline soils are basic, but NOT all basic soils are alkaline. This is because even presence of basic salts, the soil may not become alkaline due to other chemical reactions. For example, pH of a solution can be lowered by the addition of CO2. This will reduce the basicity; however, the alkalinity will remain unchanged. The reason is that net reaction produces the same number of equivalents of positively contributing species (H+) as negative contributing species (HCO3- and/or CO32-).

How Gypsum helps in Treatment of Alkali soils?

Gypsum (calcium sulphate, CaSO4. 2H2O) can be applied as a source of Ca++ ions to replace the sodium at the exchange complex in the soil. However, there must be enough natural drainage to the underground, or else an artificial subsurface drainage system must be present, to permit leaching of the excess sodium by percolation of rain and/or irrigation water through the soil profile, while using Gypsum.

Soil Air

Soil air is vital both to soil 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. On the other hand, too high degree of oxidation may consume so much organic material that the soil becomes increasingly sterile.

Soil fertility

Soil fertility is the ability of soil to sustain plants. Soil has fertility when it contains organic substances and clay minerals that absorb water and certain elements needed by plants. The boundary between horizons is usually visible in the field, using the properties of colour, texture consistency, porosity, the presence or absence of certain minerals, moisture, and chemical processes.

Soil horizons

Soil horizons are the building blocks of soil classification. The various layers exposed in a pedon; roughly parallel to the surface and identified as O,A,E,B, and C are known as soil horizon.

  • The O horizon is the topmost layer of most soils. It is composed mainly of plant litter at various levels of decomposition and humus.
  • A horizon is found below the O layer. This layer is composed primarily of mineral particles and has two characteristics: it is the layer in which humus and other organic materials are mixed with mineral particles, and it is a zone of translocation from which eluviation has removed finer particles and soluble substances, both of which may be deposited at a lower layer. Thus the A horizon is dark in color and usually light in texture and porous. The A horizon is commonly differentiated into a darker upper horizon or organic accumulation, and a lower horizon showing loss of material by eluviation.
  • The B horizon is a mineral soil layer which is strongly influenced by illuviation. Consequently, this layer receives material eluviated from the A horizon. The B horizon also has a higher bulk density than the A horizon due to its enrichment of clay particles. The B horizon may be colored by oxides of iron and aluminium or by calcium carbonate illuviated from the A horizon.
  • The C horizon is composed of weathered parent material. The texture of this material can be quite variable with particles ranging in size from clay to boulders. The C horizon has also not been significantly influenced by the pedogenic processes, translocation, and/or organic modification.
  • The final layer in a typical soil profile is called the R horizon. This soil layer simply consists of unweathered bedrock.

Soil Taxonomy

Soil classification based on observable soil properties actually seen in the field is known as soil taxonomy. There are a number of soil classifications presented by the experts of soil science. The major types of the soils have been discussed here:

  • Oxisols: These soils develop in the hot and humid climates of the equatorial region. These soils are called oxisols because they have distinctive horizon with a mixture of iron and aluminium oxides. Related vegetation is the luxuriant and diverse tropical and equatorial rain forest. Typical are red-dish and yellowish from the iron and aluminium oxides left behind, with a weathered clay-like texture. In fact, these are the lateritic soils in which the leaching process is very strong. The Laterite can be quarried in blocks and used as building material. They are traditionally being used for shifting cultivation. When oxisols are disturbed, soil loss can exceed a thousand tones per sq km per year. The regions dominated by oxisols and rain forests are attracting the much worldwide environmental attention.
  • Aridisols (desert soils): The largest single soil order occurs in dry regions of the world. These soils occupy nearly 19 per cent of the earth’s land surface. Pale and light near the surface, deficit in moisture. Lack in organic matter. Salinisation is the main problem of these soils. Salinisation complicates farming in Aridisols.
  • Mollisols (grassland soils): They are most productive soils of the earth. They are rich in humus content. They have dark –colored surface. Mollisols are soft, even when dry, with granular pads, loosely arranged when dry. These humus rich organic soils are high in basic cations and have high fertility. Soils of the steppes and prairies of the world belong to this group. These soils are being utilized for large-scale commercial grain farming and grazing. The process of calcification is very strong in these soils. When cemented or hardened, these deposits are called calche or kankar.
  • Alfisols (moderately weathered forest soils): These are the most widespread of the soils orders, extending form near the equator to high latitudes. Pale, grayish brown to reddish in colour and are considered moist versions of Mollisols soil group. Alfisols have moderate-to-high reserves of basic cations and are fertile. How-ever, their productivity depends on moisture and temperature. They are supplemented by the moderate application of lime and other chemical fertilizers. Some of the best agricultural farms of USA have this type of soil.
  • Ultiosols: These highly weathered forest soil are found in the temperature climates. These soils tend to be reddish in colour because of residual iron and aluminium oxides in the a horizon. The increased precipitation in ultisol regions means greater mineral alteration, more leaching, and therefore, a lower level of fertility. Fertility is further reduced by certain agricultural practices and the effect of soil damaging crops such as cotton and tobacco. These soils need substantial management.
  • Spodosols (coniferous forest soils): Found in the humid continental mild summer climates. Their distribution is found in North America and Eurasia. They are not found in the southern hemisphere. Spodosols lack in humus and clay in the horizons. The leaves of the conifers add acidity in soil. Their colour is ash gray and they are also known as podozal soils. These are not very fertile soils. To enhance the fertility, the application of lime required.
  • Entisols (recent, underdeveloped soils): Usually young or underdeveloped. Lack vertical development of horizons. These are less fertile soils. The sand dunes ergs outwash glacial plains, and the poorly drained tundra, tidal mud flats, etc. are the examples of Entisols.
  • Inceptisols (weakly developed soils): These soils are inherently infertile. They are usually the weakly developed young soil though they are more developed than entisols. They include the soils of most of the arctic tundra and outwash moraines.
  • Andisols (volcanic parent materials): The term andisols has been derived from volcanic ash and glass. Highly fertile and have a high water holding capacity. These soils occupy relatively smaller area, especially around the volcanic ring of fire in the pacific rim. Examples are the fertile soils of Hawaii that produce sugarcane and pineapple as important cash crops.
  • Vertisols (expandable clay soils): Composed of more than 30 per cent clays. Vertisol clays are black when wet and become iron hard when dry. When drying, Vertisols crack and the cracks widen and deepen as the soil dries; this produces cracks 2-3 cm wide. These are productive soils. The regur soils of India are an example of vertisols.
  • Histosols (organic soils): Formed from accumulation of thick organic matter. Bog marsh are the examples of Histosols. Dried Histosols are used as low-grade fuel.
January 13, 2018

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