6S Elaine Leung 25

4. Discuss on the undesirable effects of chemical control of pets and weeds, and the excessive use of chemical fertilizers.

As the human population grew and agricultural spread, people began to look for ways to protect their crops. They did so by developing a variety of pesticides, which are chemical that can kill pests. Pests are organisms that cause harm to cropss, leading to significant reduction in yield. They include weeds, fungi, insects and other animals. The pesticides commonly used include herbicides (which kill weeds), fungicides (which kill fungi) and insecticides (which kill insects).

The wide variety of pesticides can upset the ecosystem in two mainly 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 th prey population. Secondly, the pests may be the main food sources of the predators. When the pests are killed, these predators are unable to survive too. In both situations, the enemies of pests are removed from the ecosystem. If the pests start to flourish again, their natural enemies predators can also unexpectedly unleash new pests which have been held in control preciously by those predators.

Frequent use of pesticides may promote the development of new strains of pests which are resistant to those chemicals. Insects breed rapidly and develop genetic resistance to insecticides through natural selection. Weed 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 insecticides and herbicides no longer protect the crops in many parts of the world.

Pesticides applied as powders and sprays may disperse in air and water, reaching various non- target areas. Pesticides taken up by organisms will enter into the food chains. They may by toxic to the organisms or affect their reproductive success. As many of these pesticides cannot be metabolized or excreted, they would accumulate in the body of organisms and be passed from one trophic level to other. The consumers at higher trophic levels usually feed on a large number of organisms at lower trophic levels. Due to the successive accumulation along the food chain, the concentration of pesticides will become exceedingly high at the top consumer level. This is known as bioaccumulation.

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 beenough to support aquatic life. At the same time, more and more carbon dioxide will accumulate in the ecosystem, contributing to global warming.

Pesticides accumulated in the soil may happen to kill decomposers. This reduces the availability of minerals to plants. If nitrifying bacteria are killed, which are toxic to plants, will accumulate in the environment.

For the undesirable effect of excess 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.

Farmers may apply either organic fertilizers (animal manure and plant wastes) or commercial chemical fertilizers (inorganic salts) to the soil to restore plant nutrients. Today, many farmers, especially in developed 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 crop growth, an excessive use of them causes various environmental problems.

The application of chemical fertilizers implies that organic fertilizers are being replaced. The soil become lack of humus or organic matter, and thus the water retaining ability is greatly reduced. As a result, the soil becomes compact and less suitable for crop growth.

The intensive use of chemical fertilizers may cause serious water problems, especially for slopped farmlands near steams 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 knows as algae bloom. This will eventually lead to the depletion of oxygen and depth of fish and other aquatic life.

The production, transportation and application og 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, rainwater seeping through the soil can leach the fertilizers into underground water. Drinking water may be contaminated, become toxic and affect the health of infants.