Soil Pollution

Table of contents

Introduction to Soil Pollution

Soil receives large quantities of hazardous wastes from different sources and gets polluted. Soil Pollutant may be defined as any factor which cause changes in the mineral content, texture and the quality of the soil and thereby disturbing normal balance of the organisms in the soil. It also effects the plant growth.
India being agricultural based economy gives high priority to agriculture. The production of agriculture depends primarily on the quality of soil because it is the soil which provides nutrients, water and minerals for the growth of plants. However, India is also facing serious problem of deteriorating quality of soil and resulting into soil pollution. The main sources of soil pollution are dumping of industrial wastes, urban wastes, volatile organic compounds, radio-active wastes and agricultural practices. Today soil gets polluted to a large extent using modern agricultural practices. Increasing the crop yield has also resulted in increasing use of weedicides, pesticides fertilizers, herbicides, and soil conditioning agents because of which soil pollution problems have increased.

Soil gets contaminated by use of Fertilizers which consist of various elements like As, Pb, Cd etc. These elements are used as a raw material in manufacturing of fertilizers and these elements present in rock phosphate mineral get transferred to superphosphate fertilizers. Metals gets accumulated in the soil due to excessive usage of phosphate fertilizers, and because of this reason, metals do not degrade. Similarly, some metal ions such as Co2+, Zn2+, Ni2+, Mn2+etc. are added to the soil in trace amounts as micronutrients. However, excessive addition of these micronutrients may lead to their accumulation above the toxic level. These may also cause soil pollution.
Different kinds of pesticides are used to control pests in the crops. Though these pesticides help in getting boost in food production, yet the excessive use has been of great concern because these cause soil pollution. These pose a potential danger to the human health because eating of food contaminated with these chemicals may cause serious problems. Consequently, many of these chemicals have been banned or restricted in use. 
Thus, the role of pesticides in protecting crops and generating pollution is very significant and discussed below:

Role of Pesticide in Generating Pollution 

The chemicals which are used for destroying pests are called pesticides. These are used to kill or control unwanted dangerous species of plants and animals. Pesticides are basically synthetic toxic chemicals with ecological repercussions. The repeated use of the same or similar pesticides make some pests resistant to that group of pesticides, thereby, making the pesticides ineffective. For Example, when insect resistance of DDT increased, other organic toxins such as Aldrin and Dieldrin, etc. were introduced. Most of the organic toxins are water insoluble and non-biodegradable. Therefore, these high persistent toxins are transferred from lower trophic level to higher trophic level through food chain. As the time passes, the concentration of toxins in higher animals reach a level which causes serious metabolic and physiological disorders. In response to high persistence of chlorinated organic toxins, a new series of less persistent or more biodegradable products have been introduced. These are organo-phosphates and carbamates. But these chemicals are severe nerve toxins and hence are more harmful to human beings. There are some reports of pesticides related deaths of agricultural field workers. 

Pesticide being sprayed in farmsFig. No. 1 Pesticide being sprayed in farms

Classification of Pesticides

The pesticides may be classified in following categories: 

Insecticides

These are used to kill insects in agricultural fields. During World War II, DDT [2, 2 (bis-p-chlorophenyl)-1, 1, 1-trichloroethane] was found to be of great use in controlling malaria and other insect borne diseases. Therefore, after the war, DDT was used in agriculture to control the damages caused by insects, rodents, weeds and various crop diseases. Other common examples of insecticides are: Malathion, carbaryl, BHC. Among these DDT (dichloro diphenyl trichloro ethane) is an organo chlorine compound. Organo chlorine compounds are not very soluble in water, therefore they do not too much harm to humans but are considered very toxic to insects even in trace quantities. Organo chlorine compounds are stable compounds so they persist in the food chain and effect the biological activities for much longer periods.These are persistent and show their biological activity for long periods of time, and they also affect non harmful insects or organisms like earthworm which are considered helpful for farmers.

Herbicides

These are used to kill weeds or undesirable vegetation. For Example: Sodium Chlorate (NaClO3), sodium arsenate (NaAsO3),2,4 dichlorophenoxy acid, triazines, etc. Most herbicides are toxic to mammals but are not as persistent as organo-chlorides. These chemicals decompose in a few months. Like organo chlorides, these also become concentrated in the food web. Same herbicides cause birth defects. 

Fungicides

These are used to control the growth of various types of fungus and check plant diseases. For Example, thiram i.e. (CH3)2 NCSS, phenyl mercury acetate, Bordeaux mixture. Mostly fungicides consist of compounds made up of mercury. And when these compounds breakdown, they have harmful effects and disastrous consequences.  One such incident occurred in Iraq in the year 1971-72 in which many deaths were reported as people consumed bread made grain over which methyl mercury fungicide was used.

Rodenticides

These are used to kill rodents (rats and mice). For Example: Warfarin, Zinc Phosphate. In addition pesticides also include nematicides (which inhibit nematodes), molluscicides used to kill molluscs mainly snails and slugs), piscicides (used to control undesirable fish species and other aquatic animals).

A Major Air Pollution Accident-Bhopal Gas Tragedy (MIC)

A Major Air Pollution Accident took place on the night of December 2, 1984 in Bhopal (Madhya Pradesh) in the Union Carbide Factory. This is known as Bhopal gas tragedy. On the chilly night, when the city of Bhopal was virtually asleep, at about 11.30 pm a dense cloud of deadly Methyl Isocyanate (MIC) gas leaked from a storage tank of Union Carbide Ltd. plant. This caused intense aching in the eyes of people around the factory. This resulted into deaths of thousands of people and many suffered permanent medical disability. Many women were badly affected and the worst victims being pregnant women.
Methyl isocyanate (MIC) was used to manufacture the insecticide, carbaryl marked under the commercial name sevin in this plant. It was prepared by the reaction of methylamine and phosgene.

CH3NH2 + COCl2          →    CH3N=C=O + 2HCl

Phosgene                                MIC

Chaos caused during Bhopal Gas TragedyFig. No. 2 Chaos caused during Bhopal Gas Tragedy

MIC is stored in refrigerated moisture free tanks in Union Carbide Factory. It is very sensitive to moisture and itself is a volatile liquid (B.p - 43.45°C). It has been investigated that probably pressure rose in one of the tanks and opened its release value thereby allowing methyl isocyanate to escape into the atmosphere. As the clouds of methyl isocyanate rose over the city, it caused immense loss of life and injury to people. It proved to be one of the most fatal tragedy.

Monitoring and Control of Pesticide Pollution

Excessive use of pesticides pose a potential hazard to man, animal, fish, vegetation, etc.These spread most widely and alarming danger to the environment. The use of many toxic pesticides have been banned. India is one of the few producers of DDT in the world, which has been banned by USA since 1972 for environmental reasons.

Other Causes of Soil Pollution

Most of the soil pollution is caused by harmful activities of humans. Some of them are:

Deforestation

Cutting down of trees loosens the topmost layers of soil which is held by roots of trees, thereby increasing soil erosion. They are easily carried away that is, soil erosion takes places because of various factors like wind, water etc. Thus the soil is unable to support further any kind of vegetation as these activities make it barren.

Garbage Pollution

Garbage consist of various wastes like plastics, polythene, domestic waste and many other things which can neither be recycled nor get decomposed by the passage of time(Some of these waste may take upto thousands of years to decompose). Collection of such waste results in foul smell and contamination of soil.

Non-Degradable WasteFig. No. 3   Non-Degradable Waste

Industrial Waste

Industries contribute to most of the soil pollution, releasing a large number of chemicals which contaminating environment. Pollutants may be either in liquid or in a solid state.

Effects of Soil Contamination

  • Loss of soil fertility: Increasing population has resulted in increased food demand with the rapid growth of human population, we need all the food we can get. Chemicals used on soils reduce soil fertility so food production drops.

  • Impact on human health: Soil contamination has resulted in number of death mainly due to eating food which is grown on contaminated soil.

  • Climate change: Soil contamination causes deforestation which disturbs the ecological as well as rain cycle which causes climate changes thereby increasing the temperature worldwide and leading to loss of biodiversity.

How to Reduce Soil Contamination

  • Reduce the use of pesticides and fertilizers: Pesticides and fertilizers are major contributors to soil contamination so cutting down on their usage could do a world of good to the soil.

  • Reforestation: This is an effective method to curb soil erosion. There are many countries in the world which citizens are told to plant more number of trees in the place where one has been cut. This reduces soil erosion.

  • Controlled farming practices: Over exploiting of any resource is considered dangerous. So, same thing is with farming, over cultivation puts too much pressure, so over farming must be avoided and various techniques and concept like green farming. Overgrazing must also be reduced as they increase soil pollution.

Over cultivationFig. No. 4 Over cultivation

  • Use Biodegradable Products:Government must encourage its citizen to use more and more biodegradable products in day to day life because when biodegradable products are these are disposed, they break into simple form and easily get mixed into the soil.

  • Reduce, Recycle, and Reuse: Various items aluminum, paper, glass etc. can be recycled and reused and therefore they should not be thrown off. We should also avoid the usage of polythene as it harms the environment and make use of paper or cloth bag which can recycled or reused.

  • Bioremediation: Bioremediation is method in which use certain microorganisms which are helpful in breaking down the contaminants of the soil. Bioremediation being an environmental friendly method, it helps the nature in restoring ecological balance

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