Contents
From genetics to ecology, Biology Topics cover a vast array of life sciences.
Soil – Formation, Importance, Types, Erosion, Composition
Soil is the portion of the Earth’s surface that consists of disintegrated rock and decaying organic material. It provides support for many plants and animals. The thickness of soil on the Earth’s surface ranges from a few millimeters to 3-4 meters. Terrestrial and aquatic plants depend upon the soil and water bed, respectively for their nutrients, water supply, and anchorage.
Formation of Soil
Soil is formed from the rocks undergoing the following two processes:
Weathering:
The breakdown of bigger rocks into smaller mineral particles is called weathering. Weathering occurs by following three means:
(i) Physical Weathering:
Various climatic factors such as temperature, wind, rainwater, ice, snow, glaciers, and running water contribute to physical weathering. Water and high temperature cause corrosive humidity and bring about unequal expansion and contraction of rocks, facilitating their breakdown. Water causes weathering of rocks by three methods: (i) wetting and drying (ii) frost action and (iii) abrasion. The freezing water expands in rock crevices and breaks the rocks. Wind action also causes the weathering of rocks.
Strong winds erode the rock surfaces by rubbing and striking their abrasive particles against the rock surface. River water grinds rock chips and stones into the sand and into a finer form – the silt. Soluble components of rocks such as calcium, chloride, sulphates, etc., are removed by water in solution; they percolate downward. The roots of the plants also have a role in the weathering process. They penetrate into the crevices of the rocks and assist in the rock-breaking process.
(ii) Chemical Weathering:
It involves a variety of chemical processes such as hydrolysis, hydration, oxidation, and reduction. The breaking down of complex compounds by the carbonic acid present in water and acidic substances derived from the decomposition of organic matter in soil are some examples of chemical weathering. The primary end products of chemical weathering are silica, clay, inorganic salts, and hydrated oxides.
(iii) Biological Weathering:
Biological weathering is done by living organisms such as lichens and bryophytes (mosses). Lichens growing on rock surfaces extract minerals from the rocks. This creates small crevices at places where a thin layer of soil builds up. Mosses grow over these crevices. They cause deepening of the crevices and result in a build-up of more soil inside them. Deeper crevices form cracks. These cracks become wider and deeper when the roots of short-lived herbs pass into them. With the passage of time, the roots of bigger plants, (e.g., peepal, banyan tree) pass into the cracks. Cracks gradually widen and cause slow fragmentation and eventually pulverization of rocks.
Paedogenesis (Soil Development)
This process concludes the decomposition by bacteria and fungi, during which organic materials are broken down, leading to humification and mineralization. Detritivores, such as nematodes, earthworms, and arthropods such as scolopendra, millipedes, mites, and ants, consume organic matter and add excretory nitrogen to it. Thus, the addition of organic matter (humus) from dead, and decomposed plants and animals, is the final stage in soil formation. A mature soil, thus, has minerals, stored energy in the form of organic matter (such as starch, sugars, cellulose, lipids, proteins), oxides of nitrogen (NO2, NO3), NH+ ions, water, and air.
Soil Profile and Soil Horizons
The term soil profile represents the vertical section of Earth’s crust, which is made up of a succession of horizontal layers (horizons), each of which varies in thickness, colour, texture, structure, consistency, porosity, acidity, and composition.
The upper or A-horizon is the topsoil. lt contains most of the litter and humus. It also has a zone of leaching, through which dissolved materials seep downward. The roots of small plants are embedded in topsoil. The second or Brhorizon is composed of the mineral soil (subsoil). The third or C-horizon contains the unconsolidated parent material. The last or D-horizon comprises of rock or unmodified parent material.
Composition of Soil
Topsoil mainly consists of the following four types of rock particles, which differ in their size, look, and texture:
- Gravels: These are large particles (small stones) that can be picked up by hand. The particle size of gravels is greater than 2 mm in diameter.
- Sand particles: They are coarse to touch and have particle sizes ranging from 0.05 mm to 2.00 mm. They can be seen easily by the naked eye.
- Silt particles: These soil particles have a particle size between 0.005 to 0.05 mm.
- Clay particles: These are the smallest soil particles, with sizes less than 0.005 mm.
To demonstrate that soil is a mixture
Take some soil in a beaker and put water in it. Make sure that the water is at least five times the amount of soil taken. Now stir the soil and water vigorously with a stirrer and allow the soil to settle down.
A critical examination of this soil water mixture will reveal that the soil has settled down at the bottom of the beaker in layers. It is so because the soil is a mixture and contains different-sized particles of rock in addition to humus (partially dead organic matter). The particles have settled down depending on their size, i.e., one having a larger size at the bottom and lighter, smaller-sized ones on top of it in layers.
You may also find some living organisms or dead partially decayed organic matter floating on the surface of water. Since water is a universal solvent, it must have certain dissolved soluble materials present in soil (e.g., salt). For that, you have to perform certain chemical tests.
Different Types of Soil – Sand, Silt, Clay, and Loam
Based on the relative amounts of particles and types of particles soils are classified into three main types:
- Sandy Soil: This soil contains a large amount of sand particles and a very small portion of silt and clay. It is found in desert areas. It cannot hold much water.
- Clayey Soil: This soil mainly contains clay particles and a small quantity of humus and silt. Clayey soil is compact and can hold water, but cannot trap air.
- Loamy Soil: This soil contains clay, silt, sand, and humus. Loamy soil has good water-holding capacity and is porous, to allow aeration of roots.
From an agricultural point of view, the soils of India are divided into the following types:
Residual Soil:
This soil exists at the place of its formation. Residual soil may be black, red, and laterite.
(a) Black Soil:
It is derived from a basaltic rock. It is rich in iron, calcium, aluminium, and magnesium. Black soil is porous and contains humus. Humus is a dark, finely divided, amorphous organic matter, which consists of decomposed plant and animal material and is rich in C, N, P, and S content. Black soil is best suited to grow cotton and sugar cane and is mainly found in Maharashtra, Andhra Pradesh, Gujarat, and Madhya Pradesh. It is also found in South Western Uttar Pradesh, Eastern Rajasthan, and Haryana.
(b) Red Soil:
This soil is red due to the presence of iron oxide in it. It contains quartz and clay particles and forms the topsoil. Red soil is poor in lime, magnesium, phosphorus, and nitrogen and contains very little quantity of humus. In India, red soil is mainly found in Kerala, Tamil Nadu, southern Karnataka, Andhra Pradesh, Orissa, and Madhya Pradesh.
(c) Laterite Soil:
This soil is also red in colour and is found in regions that receive heavy rainfall. It contains hydrated oxides of aluminium and iron. This soil is rich in nutrients and is good for growing tea, coffee, and coconut. In India, laterite soil is found in the Western Ghats, parts of Tamil Nadu, Andhra Pradesh, Orissa, and Assam.
Transported Soil:
This type of soil gets displaced and settled at places away from its origin due to gravity, flowing water, wind, or glacier. For example, in deserts when sand, is deposited in sand dunes, it is called aeolian soil. Transported soil is of the following three types:
(a) Alluvial Soil:
It is formed by the deposition of silt brought down from the mountains by the flowing rivers. Alluvial soil is layered and consists of smooth round particles. This soil is rich in humus and contains gravel, sand, and clay. This soil is suitable for growing wheat, rice, and sugar cane. Alluvial soil is mainly found in the plains of Uttar Pradesh, Haryana, Bihar, and Bengal. Alluvial soil is also known as Khad’ar.
(b) Desert Soil:
This soil is coarse, sandy, and porous. It is greyish brown in colour and rich in minerals. Desert soil produces rich crops when it is supplied with water. Desert soil is found mainly in Rajasthan, Gujarat, and Ladakh.
(c) Mountainous Soil:
This soil varies in its contents from place to place. It consists of sand, stones, clay, shales, and limestones. It also contains a good amount of humus. Mountainous soil is mainly found in the Himalayan region. (Note. Shales are soft rocks formed from compressed mud or clay that can be split into thin layers).
Soil consists of a sufficient amount of air, water, and many living organisms. Soil air occurs within pore spaces of the soil and contains three gases – O2, CO2, and N2. It is rich in moisture. Soil water, which is available to plants, is the capillary water that exists as a thin film around soil particles. This water, in fact, occurs in smaller soil channels and is held by capillary forces (e.g., surface tension and attraction forces of water molecules).
Water, which is retained as a thin and tightly-bound film around individual soil particles is called hygroscopic water. It is not used by plants. The rest of the water in the soil is called gravitational water as it is free to drain downwards, through the soil, under the influence of gravity. This water causes leaching, i.e., the washing away of minerals (nutrients). The level to which gravitational water drains is called the water table.
Soil becomes the habitat for many living organisms such as bacteria (including nitrogen-fixing bacteria and blue-green algae), fungi, protozoans, nematodes, land snails, millipedes, centipedes, ants, termites, amphibians, reptiles, and mammals (rats, mice, and rabbits).
Minerals. Earth has limited non-renewable quantities of minerals distributed unevenly in different parts of the globe. Coal, petroleum copper, aluminium, zinc, lead, iron, silver, and gold are important natural resources to humankind. Fossil fuels (e.g., coal and petroleum) are being widely used by human beings in industry, transport, household, and agriculture. The ever-increasing demand for minerals may lead to their depletion within a few decades. Therefore, all non-renewable minerals (metals and non-metals) should be used judiciously.
Soil Erosion Causes, Types, Ways to Reduce and Prevent
The removal and transportation of the top layer of soil from its original position to another place, under the effect of strong winds and fast-running rainwater, is called soil erosion. The top layer of soil is very fertile. It provides anchorage (firm support) to plants and is also a source of nutrients and water for the plants.
Soil erosion normally occurs in bare areas, i.e., areas without plant cover. It is so because the bare topsoil is loose and thus can be easily carried away by strong winds or fast-moving water of heavy rains or rivers.
Causes of Soil Erosion
The soil that we see today in one place has been created over a very long period of time. However, some of the factors that created the soil in the first place and brought the soil to that place may be responsible for the removal of soil too.
1. Strong Winds:
The soil which is uncovered and loose, is eroded, when it is exposed to strong winds. The winds carry away the fine soil particles to other places.
2. Heavy Rains:
When rain falls on the unprotected topsoil, rainwater washes it down into the streams and rivers, etc.
3. Improper Farming and Suspended Cultivation:
Farmers loosen the topsoil of the agricultural fields either for cultivation or for removing weeds. Such soil can get eroded, due to the winds or rains. Sometimes due to certain reasons, ploughed agricultural fields remain fallow (not-cultivated) for a long time and is affected by soil erosion.
4. Human Actions:
Human activities such as the expansion of urban areas have led to the removal of vegetation from certain regions. The bare land is thus exposed to agencies (winds, rains) of soil erosion. Thus, large-scale deforestation and overgrazing by our domestic animals, not only destroy biodiversity but also lead to soil erosion.
5. Dust Storms:
Dust storms shift huge amounts of loose soil from one place to another.
6. Frequent Floods:
Frequent flooding of rivers is another cause of soil erosion. Fast-moving water in the rivers removes the topsoil of the fields near the river banks and carries it away.
Effects of Soil Erosion
Soil erosion results in the following human crises:
1. Loss of Fertility and Desertification:
Soil erosion results in the displacement of the topsoil from one region to another, thus, reducing its fertility. When the top fertile soil is constantly removed from a region, only infertile subsoil is left behind. In such soil, only sparse vegetation can grow. This way, soil erosion gradually turns lush green areas into deserts.
2. Landslides in Hilly Areas:
Barren hills or hills with sparse vegetation are constantly exposed to heavy rainfall that makes the top soils of hills lose. Due to soil erosion, rock pieces of various sizes and loose soil from hills, suddenly slide down the steep slopes of mountains/hills. This phenomenon is called landslides. When these rock pieces and soil block the narrow river bed they result in floods. Sometimes, landslides block the roads and disrupt hill life. Landslides occasionally kill the people living in downhill areas and destroy their dwellings. Toxic agricultural and industrial chemicals that move downhill and downstream with the displaced soil tend to disturb the soil quality.
3. Flash Floods:
Vegetation in hilly regions absorbs a lot of rainwater and keeps the topsoil intact. Barren hills or hills with sparse vegetation cannot absorb much rainwater and thus can not keep the soil intact. So, heavy rains result in rapid movement of water in the areas resulting in flash floods in lower areas causing enormous loss of life and property.
4. Famines (excessive shortage of food):
Continuous soil erosion from a region removes the fertile topsoil leaving behind only infertile subsoil. Texture change in eroded soil reduces its water-holding capacity. Crops, thus, cannot grow in such infertile, dry soil leading to a shortage of food grains in the region. Ultimately such a situation leads to famine in an area.
5. Silting of Water Reservoirs:
Topsoil, when washed down by water, clogs drains, water channels, etc., due to deposition. Silt pollutes the water. The problem of silting in water reservoirs lowers the water level in them which ultimately leads to a shortage of production by the hydroelectric power stations.
Positive Effects of Soil Erosion:
Fertility may be enhanced when soils are washed down from hills into river valleys and deltas or are deposited on prairies by wind.
Prevention of Soil Erosion
1. Intensive Cropping:
If the fields remain covered with crops throughout the year, their topsoil will not be exposed to winds or rains. In such a condition, no soil erosion will occur.
2. Sowing Grasses and Planting Xerophytes:
Soil should not be left uncovered. Sowing grasses on barren soil or planting xerophytes will bind the loose soil. The roots of grasses and xerophytes hold the soil in place. Vegetative cover on the ground also helps in percolating water into deeper layers of soil.
3. Terrace Farming (Terracing):
In terracing the slopes are divided into a number of flat fields to slow down the flow of water. In hilly regions, small crop fields are thus formed in the form of steps or terraces for the cultivation of crops. Such terrace farms reduce the flow of rainwater down the slopes of hills. Moreover, eroded soil from the upper regions of hills gets deposited in lower terraces.
4. Contour Bunding:
Small bunds (embankments or dikes) are raised on the edges of fields to prevent the loss of topsoil through wind or water.
5. Conservation Tillage:
Instead of conventional tillage, reduced or no tillage can be practiced. It prevents soil erosion.
6. Wind Breaks:
Rows of trees and shrubs are planted at right angles to the prevailing wind flow, to check erosion of soil by the wind.
6. Proper Drainage Canals around the Fields:
This method involves the removal of excess rainwater through small drainage canals formed around the fields.
7. Making Strong Embankments along the River Banks:
Formation of strong embankments of stones, sandbags, etc., on both sides of the rivers particularly in erosion-prone areas, will check soil erosion caved due to the fast-moving river water.
Soil Degradation due to Extensive Farming
Modern farming practices involve the use of large amounts of fertilizers and pesticides. The use of these substances over long periods of time can destroy the soil structure by killing the soil microorganisms that recycle nutrients in the soil. It also kills the earthworms which are helpful in making the rich humus. Fertile soils can quickly become barren if sustainable practices are not followed. Removal of useful components (nutrients) from the soil and the addition of other substances (fertilizers, pesticides, etc.) which adversely affect the fertility of the soil and kill the diversity of organisms that live in it, is called soil pollution.
To demonstrate the role of plants in checking soil erosion
Take two identical trays A and B and fill them with soil. Plant mustard, green gram, barley or paddy seedlings in tray A and keep tray B as such (control). Water both trays regularly for a few days. When the growth of herbage becomes visible, tilt both the trays at an angle as shown in Fig. Make sure that both trays are tilted at the same angle. Now, pour an equal quantity of water gently on both trays in such a way that the water flows out of the trays from the other end.
Study the amount of soil that has been carried out of the trays with water. Now, pour equal quantities of water on both trays from a height. The water must be 3-4 times the amount that had been poured earlier. Study the amount of soil that is carried out of the trays with water.
You will observe that in the first set of experiments, more soil will be carried out of tray B than out of tray A. In the second set of experiment, when more water is poured on the trays and that too from a height, the flowing of water further loosen the soil. More soil will be loosened in tray B without plants and will be carried out water than from tray A having plants. It is so because in tray A roots of plant seedlings play an important role in checking the soil erosion by binding the soil.
Soil compaction, resulting from intensive cultivation with ever larger and heavier farm machinery, definitely reduces yields. About half of the irrigated lands of the world are damaged to some extent by salinization (salt accumulation) or alkalinization (alkali accumulation).
What is Soil Pollution
Soil pollution is caused by solid wastes and chemicals. The slag heaps from mines spoil the beauty of the sites of mines. Pulp and paper mills, sugar mills, oil refineries, power plants, chemicals, and fertilizer manufacturing units, iron and steel plants, and plastic and rubber-producing complexes are some major contributors to soil erosion. Most industrial furnaces and thermal power stations produce fly ash, which is a grey, powdery residue of unburnt material, and causes pollution. This fly ash hampers the growth of crop plants and also decreases crops of orchards.
Domestic waste also adds a large amount of solids. They include food scraps, vegetable remains, packing materials, cans, rags, papers, cinders, ash, broken gadgets, (electronic wastes such as old computers) wood, metals, bones of dead animals; plastics, polythene bags, ceramics, glass, aluminium, rubber (vehicle tires, tubes), leather (old shoes, belts, purses, suitcases, etc.), construction rubbish, bricks, sand, and other junk. This solid waste has two types of pollutants:
Degradable pollutants include domestic wastes and sewage that decompose easily.
Non-degradable pollutants which are not degraded and persist in the environment, e.g., plastics, inorganic metallic compounds, oxides, pesticides (DDT), and radioactive substances.
Most of the solid wastes are used in a sanitary landfill, some of these are recycled and some are burned in the incinerator.