Download Lillian Liem 6S 26

Lillian Liem 6S 26
Date: 23-12-2003
Discuss on the undesirable effects of chemicals control of pests and weeds, and
the excessive use of chemical fertilizers.
Chemical control of pests and weeds
Pests are organisms that cause harms to crops, leading to significant
reduction in yield. They include weeds, fungi, insects and other animals. 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. These pesticides
commonly used include herbicides, fungicides and insecticides.
Toxic chemicals taken up by organisms or affect their reproductive
success. Many of them cannot be metabolized or excreted such as DDT.
The accumulated toxin would be passed from one trophic level to another
along the food chains. It can be built up to very high level that will kill the
top consumers. In addition, frequent use of pesticides may promote the
development of new strains of pests which are resistant to these
chemicals. Some widely used insecticides and herbicides no longer protect
the crops in many parts of the world. It is because insects reproduce
rapidly and develop genetic resistance to insecticides. As a result it will be
more and more difficult to control the new strains of pests.
Toxic chemicals kill predators or decrease the efficiency of the
predators in controlling the prey population. On the other hand, the pests
may be the main food source of the predators. When the pests are killed,
these predators are unable to live. If the pests alive again, their natural
enemies are no longer present to them down. Then the condition would be
worst than before. Pesticides can be washed away by rain from the
farmland to aquatic bodies such as lakes and rivers. They may accumulate
in phytoplankton and kill them. Because of reduction of oxygen, there is
not enough oxygen to support other aquatic life. More and more carbon
dioxide will accumulate in the ecosystem, leads to global warming. Toxic
chemicals accumulated in the soil may kill decomposers also. They reduce
the supply of minerals to plants. If nitrifying bacteria are killed, nitrites,
which are toxic to plants, will accumulate in the environment.
Excessive use of chemical fertilizers
Nowadays, quite a lot of farmers use chemical fertilizers (inorganic
salts) to the soil to restore plant nutrients. It is because chemical
fertilizers can be transported, stored and applied more easily. The
fertilizers mainly provide nitrogen, phosphorus and potassium. They are
helpful to crop growth but an excessive use of them causes various
environmental problems.
The excessive use of inorganic fertilizers may cause serious water
pollution problems because they are easily washed into the aquatic bodies
by heavy rainfall. The nutrients enhance the growth of algae leads to
red-tide. This will eventually lead to the depletion of oxygen and death of
fish and other aquatic life. It is because the thick layer of floating algae
reduces the penetration of light that causes the submerged algae die. The
dead algae provide organic food for rapid growth of bacteria. The bacteria
use up the dissolved oxygen, so that fish and aquatic life die due to
suffocation.
The organic fertilizers are replaced by the inorganic fertilizers. The
soil becomes lack of natural matters (humus), and thus the water retaining
ability is greatly reduced. Therefore, the soil becomes compact and less
suitable for crop growth. Since rain water can leach the fertilizers into
underground water, drinking water may become toxic and affect the
health. Also, The process of 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
lead to pollution and green house effect.