Science IX | Natural Resources: Our Environment | Part 3

Natural Resources: Our Environment | Part 3


Soil is the portion of the earth’s surface consisting of disintegrated rock and decaying organic material. It provides the support for many plants and animals. Thickness of soil on the earth’s surface ranges from a few millimeters to 3-4 meters. Terrestrial plants depend for their nutrients, water supply and anchorage upon the soil. Even for the aquatic plants, the soil is important as chief storage of all the nutrients which are made available to the water medium.

1. Formation of soil :

Soil is formed from the rocks by undergoing the following two processes :

1. Weathering ;

2. Paedogenesis

A. Weathering.

Breakdown of bigger rocks into fine smaller mineral particles is called weathering. Weathering occurs by following three means :

(i) Physical weathering :

This is done by various climatic factors such as temperature, wind, rain water, ice, snow, glaciers and running water. Water and high temperature cause corrosive humidity and bring about unequal expansion and contraction or rocks, facilitating their breakdown. Rock pulverizing glaciers, low temperature and water grind the rocks. The freezing water expands in rock crevices and breaks the rocks. Wind action also causes the weathering of rocks. River water grind rock chips and stones into sand and into more fine form – the slit. 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 weathering process. They penetrate into the crevices of the rocks and enhance rock-breaking process.

(ii) Chemical weathering :

It involves a variety of chemical processes such as hydrolysis, hydration, oxidation and reduction. Chemical weathering, for example, involves the breaking down of complex compounds by the carbonic acid present in water and by acidic substances derived from the decomposition process of organic matter in soil. The main end products of the chemical weathering are silica, clay, inorganic salts and hydrated oxides.

(iii) Biological weathering :

Lichnes, bryophytes (mosses) and other plants live on rocks and produce acids, which accelerate the process of rock weathering.

B. Paedogenesis (soil development) :

This process involves the decomposition process by bacteria and fungi by which organic material s are broken down and leads to humification and mineralization. Detritivores such as nematodes, earthworms and artropods such as scolopendra, millipede, mites and ants consume organic matter and add excretory nitrogen to it. Thus, addition of organic matter (humus) from dead and decomposed plants and animals is the final stage in the formation of soil. A mature soil 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 Erosion

The removal and transportation of top layer of soil from the original position to another place with the help of certain agents such as strong winds and fast running rainwater is called soil erosion. The top layer of soil is fertile. It provides anchorage (firm support) to plants and is also source of nutrients and water to the plants.

Soil erosion normally occurs n bare areas, i.e., areas without plant cover. It is so because the bare topsoil is loose and thus can easily carried away by strong winds or fast moving water of heavy rains or rivers.

Biogeochemical Cycles

A constant interaction between the biotic and abiotic components of the biosphere makes it a dynamic, but stable system. These interactions consist of a transfer of matter and energy between the different components of the biosphere. Let us look at some processes involved in the maintenance of the above balance.

The Water or Hydrological cycle

Water evaporates from the hydrosphere (oceans, seas, rivers, streams, lakes, ponds, moist soil) with sun’s heat and forms clouds. By the help of wind, the clouds are blown over the land, where they are cooled enough to drop the water as rain, hail and sleet (called precipitation). Rain may fall directly into the oceans also.

The organisms get water from and return it to the global water cycle. Plants absorb water from the soil or water reservoir and ad it to the air as vapour by transpiration. Water transpired by trees cools the surrounding air, and plays a role in determining the microclimate around them. Animals take water from the water reservoir or with food (plants, or other animals or their products). They return it to the air as vapours by respiration or to the soil as fluid by excretion. Mammal excrete water also as sweat which evaporates from their bodies. Water is also added to the environment by death and decay of organisms. Water vapor formed by transpiration and respiration form clouds and enter global water cycle.

The Nitrogen Cycle

In our atmosphere nitrogen gas makes up 78% nitrogen one of the essential part of living molecule like protein, nucleic Acid (DNA & RNA) and vitamins. It also found in alkalides and urea which are important biologically compounds.

Nitrogen is thus in essential nutrient for all life form and life would be simple if all these life forms could use the atmospheric nitrogen directly. However, other than a few form of bacteria, life form are not able to convert the comparatively inert nitrogen molecule into form like nitrates and ‘nitrites fixing’ bacteria are found in roots of legumes plants (Generally the plants which give us pulses). The other manner in which the nitrogen molecules are converted into nitrates and nitrites by physical process. During lighting the high temperature and pressure created in the air convert nitrogen into oxides of nitrogen. These oxides dissolves in water to form nitric and nitrous acid and fall an land along with rain. These re them utilized by various life forms.

Plants generally take up nitrates and nitrites and convert them into Amino Acid which are used to make proteins and other complex biological compound are subsequently consumed bacteria Animals. Once the animals or the plant dies, other bacteria in the soil convert the various compound of nitrogen back into nitrates and nitrites. A different type of bacteria converts the nitrates and nitrites into elemental nitrogen. In this way nitrogen cycle occur in our atmosphere.

The Carbon Cycle

Carbon is found in various forms on the Earth. It occurs in the elemental form as diamonds and graphite. In the combined state, it is found as carbon dioxide in the atmosphere, as carbonate and hydrogen carbonate salts in various minerals, while all life forms are based on carbon containing molecules like proteins, carbohydrates, fats, nucleic acids and vitamins. The endoskeletons and exoskeletons of various animals are also formed from carbonate salts. Carbon is incorporated into life forms through the basic process of photosynthesis which is performed in the presence of sunlight by all life forms that contain chlorophyll. This process converts carbon dioxide from the atmosphere or dissolved in water into glucose molecules. These glucose molecules are either converted into other substances or used to provide energy for the synthesis of other biologically important molecules.

The utilization of glucose to provide energy to living things involves the process of respiration in which oxygen may or may not be used to convert glucose back into carbon dioxide. This carbon dioxide then goes back into the atmosphere. Another process that adds to the carbon dioxide in the atmosphere is the process of combustion where fuels are burnt to provided energy for various needs like heating, cooking, transportation and industrial processes. In fact, the percentage of carbon dioxide in the atmosphere is said to have doubled since the industrial revolution when human beings started burning fossils fuels on a very large scale. Carbon, like water, is thus cycled repeatedly through different forms by the various physical and biological activities.

The Oxygen Cycle

Oxygen is a very abundant element on our Earth. It is found in the element form in the atmosphere to the extent of 21%. It also occurs extensively in the combined form in the Earth’s crust as well as also in the air in the form of carbon dioxide. In the crust, it is found as the oxides of most metals and silicon, and also as carbonate, sulphate, nitrate and other minerals. It is also an essential component of most biological molecules like carbohydrates, proteins, nucleic acids and fats (or lipids).

But when we talk of the oxygen-cycle, we are mainly referring to the cycle that maintains the levels of oxygen in the atmosphere. Oxygen from the atmosphere is used up in three processes, namely combustion, respiration and in the formation of oxides of nitrogen. Oxygen is returned to the atmosphere in only one major process, that is, photosynthesis. And this forms the broad outline of the oxygen-cycle in nature.

Though we usually thing of oxygen as being necessary to life n the process of respiration, it might be interest to you learn that some forms of life, especially bacteria, are poisoned by elemental oxygen. In fact, even the process of nitrogen-fixing by bacteria does not take place in the presence of oxygen.


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