Prof. (Dr.) R. D. GUPTA
“The word soil, is composed of four letters viz., s, o, i and l which evince soul, of, infinite and life, respectively. Soil is, thus, considered to be a “soul of infinite life”.
All the civilization is always rooted through the soil. When the soil is properly looked after, it sustains life to an extent, keeps toxic substances itself under check, and helps to protect all its creatures from deadly elements. The soil, therefore, serves as sink. However, when soil is abused, misused, mismanaged, overused or overexploited, it retaliates and the very existence of life gets endangered. This is what, which has an exactly taken place in our country these days. With an advancement of civilization, burgeoning human and livestock population, change in the feeding habits of humans, alterations have been brought about in the agricultural development methodology.
“Green Revolution” (GM), which ushered during 1967-68, brought about considerable increase in food production. However, application of blanket/ indiscriminate use of high analysis chemical-fertilizers and pesticides have not only yielded air and water pollution but also the soil pollution. Creation of soil pollution has now caused stagnation in the production of various crops, especially in rice and wheat cropping system, where the yield of these crops in India has either become constant or decreased. As for example, at the national level, rice yields are hovering between 1.9 t ha-1 in 2000-01 and 2.1 t ha-1 in 2004-05 (Mahajan and Gupta, 2009). Yields of wheat have already become static at 2.7 t ha-1 since 1999-2000. It, therefore, becomes imperative to know the various causes responsible for producing soil pollution which stand elaborated in this paper. Apart from this, concept of soil and various control measures of soil pollution have also been elucidated.
CONCEPT OF SOIL
Importance of soil
The soil is a natural medium for growth of plants. It not only gives prop to plants/trees but also supplies all the essential plant or mineral nutrients for growing plants. The plants then manufacture feed for animals, food, fibre, timber and medicinal plant for the human beings. Soil also serves as an abode of both microorganisms and macroorganisms which are very essential for the decomposition of organic matter. The decomposed organic matter i.e., humus (humic acids, fulvic acids, hymatomelanic acid and humin), which becomes the store house of nutrients for the growing plants.
Definitions of soil
There are a number of definitions of soils, some very long and some short. Here are given some such definitions.
Dokochaev, a well trained Russian geologist, for the first time gave the concept about “soil as the natural resource” like air and water. The soil, therefore, may be defined as, “a natural body of mineral and organic constituents, differentiated into horizons of variable depth, which differs from the parent material below in morphology; physical, chemical and biological properties, and capable of giving support and sustenance to crops and plants.”
“From agricultural point of view, soil is the medium which is capable to grow plant life.”
“From physical point of view, the soil is composed of solid, liquid and gaseous materials.”
“Chemically, the soil is made up of a number of chemical constituents such as silica, iron oxide, alumina, lime, magnesia, phosphate, nitrogen and organic matter.”
SOIL AS DEPOLLUTANT
The soil itself keeps toxic substances under control and as such it acts as depollutant. The ability of the soil to act as depollutant is attributed to the presence of inorganic (clay minerals both crystalline and non-crystalline) and organic (humic substances) colloids. These colloids are capable of adsorbing certain pesticides and other chemicals due to the presence of large specific surface area of these colloids. The soil, thus, becomes a natural medium for disposal of a number of wastes. However, certain organic compounds which are very resistant to decomposition like DDT, BHC and many others which are less degradable and as such they persist in the soil for a number of years and eventually pollute the soil, and become hazardous.
SOIL AS POLLUTANT
Soil as pollutant is a source of dust in the air. Wind erosion contaminates the air with large amount of dust particles. Fast wind movement in deserts and cold arid zones take out silt and clay particles of soil leaving behind pebbles, gravels, stones. Soil sediments brought about by the rivers, streams and rivulets become a source of heavy metal pollution.
CAUSES OF SOIL POLLUTION
Soil pollution is produced by a number of causes. Some of them are:- (i) Urban and rural wastes, (ii) Industrial wastes, (iii) Mining wastes, (iv) Radioactive subtances, (v) Plastic wastes, (vi) Chemicals and toxic metals, (vii) Agricultural practices, (viii) Soil sediments, (ix) Acidic rains. The details of each of these factors with regard to their contribution in causing soil pollution, have been described elsewhere (Gupta, 2005). In addition to these causes, the role of sewage and sludges, cultivation of crops on hilly slopes, heavy metal accumulation, mines and quarries, production of gases and nitrates, as well as deforestation and overgrazing of the pastures are the other causes which generate pollution in soils. Their brief details has been mentioned below (Gupta, 2006).
Sewage and sludges
Untreated sewage and sludges may contain enough live viruses and viable worms. And, as such when used in crops, cause harmful effects not only to soils and plants but also to humans and animals. To push up the animal weight and increase in milk production has promoted the practice of increasing soluble salts content in the animal feeds. As a consequence, the animal wastes obtained may contain more salt content which becomes a source of soil pollution. While the disposal of garbage and other field trash by burial in sanitary landfills or by using as a fuel or for composting, the danger of pollution occurs in groundwater because of leaching of NO3 anion and some cations of heavy metals and in air due to volatile gases.
Extensive cultivation of crops
Due to extensive cultivation of crops on hill slopes and deforested/denuded areas, soil itself becomes a spectacular pollutant when large amounts of the soil erodes down to cover fertile arable land or when receding flood waters leave behind muddy masses and reservoirs get filled with debris. The suspended solids in water reduce sunlight penetration, thereby reduction of organisms which constitute the aquatic food chain.
Heavy metal accumulation in soil
Accumulation of heavy metals in soils and water are the principal causes of environment concern due to excessive use of fertilizers in agriculture. Heavy metals such as Cd, Pb, Cr and Ni accumulate in the soil over a period of time to toxic level (Gupta and Sharma, 2004). These metals finally get accumulated in human beings through agricultural products grown on such soils and thus create complicacies. Various diseases caused by metal pollution have been detailed in a separate publication titled, “Environmental Pollution, Hazards and Control” (Gupta, 2006). Indiscriminate dumping of industrial wastes, improper treatment of the urban solid waste, sewage materials, hospital wastes, lack of appropriate regulatory mechanisms for agricultural land use, etc., have all been contributing to accumulation of heavy metal ions in soils in toxic amounts (Gupta, 2005). Infact, the soil is the primary recipient and from the soil the metallic pollutants make their way into the living beings easily.
Deforestation and overgrazing
Deforestation and overgrazing of the pastures, waterlogging and salinization are also indirectly amenable to originate soil pollution. Deforestation has increased soil erosion and sedimentation. Sedimentation not only reduces the capacity of irrigation reservoirs, canals, rivers and dams but is also a potential source of heavy metal pollution in soils. Infact, sediments carry more nutrients particularly phosphorus and heavy metals, which besides soil pollution cause water pollution also. Soil erosion and sedimentation thus account for the degradation of land, thereby, leading to a loss of its potential growth in foodgrains output. Similarly, overgrazing of the pastures accelerates soil erosion. Overgrazing of the pastures is also responsible in declining various plant species. Eroded and overgrazed soil renders unfit for cultivation.
(To be continued)
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