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Soil
Pollution
We can no more manufacture a soil with a tank of chemicals than
we can invent a rainforest or produce a single bird. We may
enhance the soil by helping its processes along, but we can
never recreate what we destroy. The soil is a resource for which
there is no substitute. (Environmental historian Donald Worster
reminds us that fertilizers are not a substitute for fertile
soil).
The soil is a thin covering over the land consisting of a mixture
of minerals, organic material, living organisms, air and water,
that together support the growth of plant life. Several factors
contribute to the formation of soil from the parent material.
This includes the mechanical weathering of rocks due to temperature
changes and abrasion, wind, moving water, glaciers, chemical
weathering activities, and lichens. Climate and time are also
important in the development of soils. In extremely dry or cold
climates soils develop very slowly, while in humid and warm
climates soils develop more rapidly. Under ideal climatic conditions,
soft parent material may develop into 1 cm of soil within 15
years. Under poor climatic conditions, a hard parent material
may require hundreds of years to develop into soil.
Mature soils are arranged in a series of zones called 'soil
horizons'. Each horizon has a distinct texture and composition
that varies with different types of soils. A cross-sectional
view of the horizons in a soil is called a 'soil profile'.
The top layer or the surface litter layer, called the 'O-horizon',
consists mostly of freshly-fallen and partially-decomposed leaves,
twigs, animal waste, fungi and other organic materials. Normally,
it is brown or black.
The uppermost layer of the soil, called the 'A-horizon', consists
of partially-decomposed organic matter (humus) and some inorganic
mineral particles. It is usually darker and looser than the
deeper layers. The roots of most plants are found in these two
upper layers. As long as these layers are anchored by vegetation,
the soil stores water and releases it in a trickle throughout
the year instead of in a force like a flood. These two top layers
also contain a large amount of bacteria, fungi, earthworms,
and other small insects, which form complex food webs in the
soil, help recycle soil nutrients, and contribute to soil fertility.
The 'B-horizon', often called the subsoil, contains less organic
material and fewer organisms than the A horizon. The area below
the subsoil is called the 'C-horizon' and consists of weathered
parent material. This parent material does not contain any organic
materials. The chemical composition of the C-horizon helps to
determine the pH of the soil and also influences the soil's
rate of water absorption and retention.
Soils vary in their content of clay (very fine particles), silt
(fine particles), sand (medium-size particles) and gravel (coarse
to very coarse particles). The relative amounts of the different
sizes and types of mineral particles determine the soil texture.
Soils with approximately equal mixtures of clay, sand, silt
and humus are called loams.
Causes
of soil degradation
Erosion
Soil erosion can be defined as the movement of surface litter
and topsoil from one place to another. While erosion is a natural
process, often caused by wind and flowing water, it is greatly
accelerated by human activities such as farming, construction,
overgrazing by livestock, burning of grass cover, and deforestation.
The loss of the topsoil makes a soilless fertile and reduces
its water-holding capacity. The topsoil, which is washed away,
also contributes to water pollution by clogging lakes and increasing
the turbidity of the water, ultimately leading to the loss of
aquatic life. For one inch of topsoil to be formed it normally
requires 200-1000 years, depending upon the climate and soil
type. Thus, if the topsoil erodes faster than it is formed,
the soil becomes a non-renewable resource.
Therefore, it is essential that proper soil conservation measures
are, used to minimize the loss of the topsoil. There are several
techniques that can protect the soil from erosion. Today, both
water and soil are conserved through integrated treatment methods.
The two types of treatment generally used are:
- Area treatment, which involves treating the land
- Drainage-line treatment, which involves treating the
natural water courses (nalas).
Area Treatment
Purpose |
Treatment Measure |
Effect |
| Reduces the impact of rain drops
on the soil |
Develop vegetative cover on the
non arable land |
Minimum disturbance and displacement
of soil particles |
Infiltration of water where it falls |
Apply water infiltration measures
on the area |
In-situ soil and moisture conservation |
Minimum surface run-off |
Store surplus rain water by constructing
bunds, ponds in the area |
Increased soil moisture in the area,
facilitate ground water recharge |
Ridge to valley sequencing |
Treat the upper catchment first
and then proceed towards the outlet |
Economically viable, less riskof
damage and longer life of structures of the lower catchments |
Drainage-line treatment
Purpose |
Treatment Measure |
Effect |
| Stop further deepening of gullies
and retain sediment run-off |
Plug the gullies at formation |
Stops erosion, recharges groundwater
at the upper level |
| Reduce run-off velocity, pass cleaner
water to the downstream side |
Create temporary barriers in nalas |
Delayed flow and increased groundwater recharge
|
Minimum sedimentation in the storage
basins |
Use various methods to treat the
catchments |
|
Low construction cost |
Use local material and skills for
constructing the structures |
Structures are locally maintained |
Continuous contour trenches can be used to enhance the infiltration
of water, reduce the run-off, and check soil erosion. These
are actually shallow trencl1es dug across the slope of the land
and along the contour lines, basically for the purpose of soil
and water conservation. They are most effective on gentle slopes
and in areas of low to medium rainfall. These bunds are stabilized
by fast-growing tree species and grasses. In areas with steep
slopes where bunds are not possible, continuous contour benches
(CCBs) made of stones are used for the same purpose.
Gradonies can also be used to convert wastelands into agricultural
lands. In this, narrow trenches with bunds on the downstream
side are built along contours in the upper reaches of the catchment
to collect run-off and to conserve moisture from the trees or
tree crops. The area between the two bunds is used for cultivating
crops after development of fertile soil cover.
Some of the ways in which this can be achieved are:
- Live check-dams, in which barriers are created by planting
grass, shrubs and trees across the gullies.
- A bund constructed out of stones across the stream can
also be used for conserving soil and water.
- An earthen check-bund constructed out of local soil across
the stream to check soil erosion and the flow of water.
- A Gabion structure, which is a bund constructed of stone
and wrapped in galvanized chain link. A gabion structure
has a one-inch thick, impervious wall of ferrocement at
the center of the structure, which goes below the ground
level up to the hard strata. This ferrocement partition,
supported by the gabion portion, is able to retain the water
and withstand the force of the runoff water.
- An bandhara is an underground structure across a nala
bed that functions as a barrier to check the movement of
groundwater.
Excess use of fertilizers
Approximately 25% of the world's crop yield is estimated
to be directly attributed to the use of chemical fertilizers.
The use of chemical fertilizers has increased significantly
over the last few decades and is expected to rise even higher.
Fertilizers are very valuable, as they replace the soil nutrients
used up by plants. The three primary soil nutrients often in
short supply are potassium, phosphorus and nitrogen compounds.
These are commonly referred to as macronutrients. Certain other
elements like boron, zinc and manganese are necessary in extremely
small amounts and are known as micronutrients. When crops are
harvested, a large amount of macronutrients and a small amount
of micronutrients are removed with the crops. If the same crop
is grown again, depleted levels of thee nutrients can result
in decreased yields. These necessary nutrients can be returned
to the soil through the application of fertilizers. In addition
to fertilizers, a large amount of pesticides (chemicals used
to kill or control populations of unwanted fungi, animals or
plants often called pests) are also used to ensure a good yield.
Pesticides can be subdivided into several categories, based
on the kinds of organisms they are used to control. Insecticides
are used to control insect populations, while fungicides are
used to control unwanted fungal growth. Mice and rats are killed
by rodenticides, while plant pests are controlled by herbicides.
Problems with pesticide use
Pesticides not only kill the pests but also a large variety
of living things, including humans. They may be persistent or
non-persistent. Persistent pesticides, once applied, are effective
for a long time. However, as they do not break down easily they
tend to accumulate in the soil and in the bodies of animals
in the food chain.
For example, DDT, one of the first synthetic organic insecticides
to be used, was thought to be the perfect insecticide. During
the first ten years of its use (1942-1952), DDT is estimated
to have saved about five million lives primarily because of
its use to control disease-carrying mosquitoes. However, after
a period of use, many mosquitoes and insects became tolerant
to DDT, thus making it lose its effectiveness. In temperate
regions, DDT has a half-life (the amount of time required for
half of the chemical to decompose) of 10-15 years. This means
that if 100 kg of DDT were to be sprayed over an area, 50 kg
would still be present in the area 10-15 years later. The half-life
of DDT varies according to the soil type, temperature, kind
of soil organisms present, and other factors. In tropical parts
of the world, the half-life may be as short as 6 months. The
use of DDT has been banned in some countries. However, India
still permits the use of DDT, although only for purposes of
mosquito control. Persistent pesticides become attached to small
soil particles which are easily moved by wind and water to different
parts thus affecting soils elsewhere. Persistent pesticides
may also accumulate in the bodies of animals, and over a period
of time increase in concentration if the animal is unable to
flush them out of its system, thus leading to the phenomenon
called bioaccumulation. When an affected animal is eaten by
another carnivore, these pesticides are further concentrated
in the body of the carnivore. This phenomenon of acquiring increasing
levels of a substance in the bodies of higher trophic level
organisms is known as 'biomagnification'. This process, especially
in the case of insecticides like DDT, has been proved to be
disastrous. DDT is a well-known case of biomagnification in
ecosystems.
Other problems associated with insecticides are the ability
of insect populations to become resistant to them, thus rendering
them useless in a couple of generations. Most pesticides kill
beneficial as well as pest species. They kill the predator as
well as the parasitic insects that control the pests. Thus,
the pest species increase rapidly following the use of a pesticide,
as there are no natural checks to their population growth. The
short-term and the long-term health effects to the persons using
the pesticide and the public that consumes the food grown by
using the pesticides are also major concerns. Exposure to small
quantities of pesticides over several years can cause mutations,
produce cancers, etc.
So the question that comes to mind is that: if pesticides have
so many drawbacks then why are they used so extensively and
what are the substitutes for them? There are three main reasons
for the use of pesticides. First, the use of pesticides in the
short term has increased the amount of food that can be grown
in many parts of the world as the damage by pests is decreased.
The second reason for its extensive use is based on an economic
consideration. The increased yields more than compensate the
farmer for the cost of pesticides. Third, current health problems,
especially in developing countries, due to mosquitoes are impossible
to control without insecticides.
However, more and more farmers are increasingly opting to replace
chemical fertilizers and use different methods of controlling
pests without affecting their yield. Thus, several different
approaches that have slightly varying and overlapping goals
have been developed as an alternative to using fertilizers and
pesticides. Alternative agriculture is the broadest term that
includes all non-traditional agricultural methods and encompasses
sustainable agriculture, organic agriculture, alternative uses
of traditional crops, alternative methods for raising crops,
etc.
Sustainable agriculture advocates the use of methods to produce
adequate safe food in an economically viable manner, while maintaining
the state of the ecosystem. Organic agriculture advocates avoiding
the use of chemical fertilizers and pesticides. A wide variety
of techniques can be used to reduce this negative impact of
agriculture. Leaving crop residue on the soil and incorporating
it into the soil reduces erosion and increases soil organic
matter. The introduction of organic matter into the soil also
makes compaction less likely. Crop rotation is an effective
way to enhance soil fertility, reduce erosion and control pests.
There have been arguments both for and against organic farming.
Critics argue that organic farming cannot produce the amount
of food required for today's population and it is economically
viable only in certain conditions. However, supporters of organic
farming feel that if the hidden costs of soil erosion and pollution
are taken into account, it is a viable approach.
Besides, organic farmers do not have to spend money on fertilizers
and pesticides and also get a premium price for their products,
thus making it financially viable for them.
Another way to reduce these impacts is through the use of integrated
pest management (IPM). This is a technique that uses a cottlplete
understanding of all the ecological aspects of a crop and the
particular pests to which it is susceptible to establish pest
control strategies that uses no or few pesticides. IPM promotes
the use of biopesticides. Biopesticides are derived from three
sources: microbial, botanical and biochemical. Microbial pesticides
are microorganisms such as bacteria, fungus, virus or protozoa
that fight pests through a variety of ways. They produce toxins
specific to the pests and produce diseases in them. Biochemical
pesticides contain several chemicals that affect the reproductive
and digestive mechanisms of the pests. The most commonly used
biopesticides are Bacillus thuringiensis (Bt), neem (Azadirachta
indica) and trichogramma. Although they are available in the
market, they are yet to become market favorites.
Excess salts and water
Irrigated lands can produce higher crop yields than
those that only use rainwater. However, this has its own set
of ill effects. Irrigation water contains dissolved salts and
in dry climates much of the water in the saline solution evaporates
leaving its salts, such as sodium chloride in the topsoil. The
accumulation of these salts 15 called salinization, which can
stunt plant growth, lower yields and eventually kill the crop
and render the land useless for agriculture. These salts can
be flushed out of the soil by using more water. This practice,
however, increases the -cost of crop production and also wastes
enormous amounts of water.. Flushing out salts can also make
the downstream irrigation water saltier.
Another problem with irrigation is waterlogging. This occurs
when large amounts of water are used to leach the salts deeper
into the soil. However, jf the drainage is poor this water accumulates
underground gradually raising the water table. The roots of
the plants then get enveloped in this saline water and eventually
die.
Thus, in the long run it is better for us to adopt sustainable
farming practices to prevent the degradation of soil. |
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