The effect of chemical
control of pests and weed
And
The excessive use of chemical
fertilizers
The
effect of chemical control of pests and weed
As the human
population grew and agriculture spread, people began to look for ways to
protect their crops. They did so by developing a variety of pesticides which
are chemicals that can kill pests. Pests are organisms that cause harms to
crops, leading to significant reduction in yield. They include weeds, fungi,
insects and other animals. The pesticides commonly used include herbicides,
fungicides and insecticides
.
The widespread use
of pesticides has led to great harmful effects on the ecosystem and environment
in the followings:
Problems
with chemical pesticides.
Some
pesticides are persistent, they do not break down in the environment or within
the tissues of living organisms. This give rise to two potential problems.
Bioaccumulation is the
accumulation of a substance in a biological tissue. Organisms at any trophic
level may be capable of bioaccumulation.
Biomagnification is the
increasing concentration of a substance up a food chain - i.e. from one trophic
level to the next . Animals at the higher trophic levels will be most affected.
Example
Some
of the earliest insecticides were organochlorines or chlorinated hydrocarbons.
These insecticides kill by both contact and stomach ingestion.
Dichlorodiphenyltrichloroethane (DDT) was the first known contact insecticide,
synthesized in 1874. This organochlorine compound has saved more human lives
than any other invention; by killing mosquitoes it has helped to eradicate
malaria in many parts of the world. Unfortunately, DDT persists in fatty
tissues (bioaccumulation). Larger, long lived predators at the end of a
food chain may accumulate a lethal quantity of DDT as a result of eating large
numbers of smaller species (biomagnification).
Upsetting the balance of the ecosystem
The wide variety
of pesticides can upset the ecosystem in two main ways. Firstly, most pests
have natural enemies which prey on them. Pesticides may kill these predators or
decrease the efficiency of the predators in controlling the prey population.
Secondly, the pests may be the main food source of the predators. When the
pests are killed, these predators are unable to survive to. In both situations,
the enemies of pests are removed from the ecosystem. If the pests start to
flourish again, their natural enemies are no longer present to keep them down.
Moreover, the wiping out of the natural predators can also unexpectedly unleash
new pests which have held in control previously by those predators.
Resistant strains
Frequent use of
pesticides may promote the development of new strains of pests which are
resistant to these chemicals. Insects breed rapidly and develop genetic
resistance to insecticides through natural selection. Weeds may also develop
genetic resistance to herbicides similarly. As a result, it will be more and
more difficult to control the new strains of pests. Today, some widely used
insecticide and herbicides no longer protect the crops in many parts of the
world.
Effect on balance of the atmosphere
Pesticides are
washed away by rain from the farmland to aquatic bodies such as lakes and
rivers. They may accumulate in phytoplankton and significantly reduce their
amount. Consequently, the oxygen released as a result of photosynthesis by
phytoplankton may no longer be enough to support other aquatic life. At the
same time, more and more carbon dioxide will accumulate in the ecosystem,
contributing to global warming.
Effect on cycling of materials
Pesticides
accumulated in the soil may happen to kill decomposers. This reduces the
availability of minerals to plants. If nitrifying bacteria are killed,
nitrites, which are toxic to plants, will accumulate in the environment.
The
excessive use of chemical fertilizers
In general, crops
require minerals for development. These minerals include nitrogen, phosphorus,
potassium, calcium, magnesium and sulphur. They are required in varying
quantities according to the crops being cultivated. In general, up to two-third
of the nitrogen and phosphorus taken up by crops are lost from the soil, within
the crops harvested and the crop residues. Therefore, fertilizers play an
important role in replenishment of these nutrients.
Farmer may apply
either organic fertilizers commercial chemical fertilizers to the soil to
restore plant nutrients. Today, many farmers, especially in developing
countries, rely on chemical fertilizers due to their easiness in transport,
storage and application. Chemical fertilizers mainly provide nitrogen,
phosphorus and potassium. Other plant nutrients may also be provided in small
or trace amounts. Although inorganic fertilizers are helpful to crops growth,
an excessive use of them causes various environmental problems.
Soil destruction
The application of
chemical fertilizers implies that fertilizers are being replaced. The soil
becomes lack of humus or organic matter, and thus the water retaining ability
is greatly reduced. So, the soil becomes compact and less suitable for crop
growth.
Water pollution
The intensive use
of chemical fertilizers may cause serious water pollution problems, especially
for slopped farmlands near streams and lakes. The fertilizers are easily washed
into the aquatic bodies by heavy rainfall. The nutrient enrichment may cause
rapid growth of algae, a phenomenon known as algal bloom.
Other effects
The production,
transportation and application of chemical fertilizers consume a large amount
of energy. In the manufacture of chemical fertilizers, harmful gases are
released into the atmosphere. This may contribute to air pollution and global
warming. Further, rain water seeping through the soil can leach the fertilizers
into underground water. Drinking water may be contaminated, become toxic and
affect the health of infrants.