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.