Lesson 8.03 Chemical Soil Pollution

Pesticides

Pesticides are used in farming to rid crops and fields of unwanted organisms, such as weeds, insects, and fungi that can cause harm to crops.

Herbicides are chemicals that are used in agriculture to kill unwanted plants and other vegetation that compete with crops for water, nutrients, and space. These chemicals can be either inorganic or organic in makeup. Most herbicides kill plants by blocking part of a biochemical pathway that prevents the plant from carrying out photosynthesis. They accomplish this by preventing cell division in roots or stems or by breaking down chlorophyll molecules that are used to absorb sunlight during photosynthesis.

Example: Agent Orange is an example of an herbicide that was once used in war and agricultural practices. Agent Orange is made up of 2 chemicals - 2,4,5 trichlorophenoxyacetic acid (2,4,5-T) and 2,4 dichlorophenoxyacetic acid (2,4-D). It was used as a defoliant in Vietnam to destroy dense forests that enemy forces were using for cover. The chemicals in Agent Orange have been linked to different types of cancers in humans.

Insecticides are pesticides used to eliminate insects that cause crop damage. Most insecticides affect the nervous system of the target organism disrupting enzymes that are neurotransmitters enzymes that allow messages to be sent from the brain to the other areas of the body of the organism and from the body to the brain .

Example: DDT (dichloro-diphenyl-trichloroethane) is an example of an insecticide that was once used. It was used as an insecticide to control malaria, typhus, body lice, and the bubonic plague during World War II. Its use was banned in 1972 because of the severe effects on the environment and human health. DDT was found to exist in the environment for long periods of time and caused thinning eggshells in birds and nervous system problems and birth defects in humans.

Fungicides are chemicals that kill or inhibit the growth on fungi or fungal spores. Fungi do not have the ability to make their own food. They are heterotrophic. They must get their food from other sources. Fungi can become parasites organisms that get their energy or food by living on or in another organism at the host's expense on crop plants in order to get the energy/food they need. This can cause loss of valuable food crops. Fungicides work by damaging the cell membranes and by interfering with key processes like energy production and respiration. When applied to an area to kill vegetation, insects, or parasitic fungi, these chemicals can enter the soil and remain in the soil affecting the growth of future vegetation and presenting a danger to animals and humans that become exposed to them.

Pesticides can also kill important beneficial organisms - like bacteria, worms, and fungi - that are involved in breaking down organic matter which supplies needed nutrients to the soil. Using pesticides can actually be counterproductive. Instead of benefiting crops, they can damage the soil which harms crops in the long term by reducing soil quality and fertility. The amount of time that these chemicals can remain in the soil is determined by soil characteristics such as texture, porosity, and permeability.

Although pesticides are designed to help get rid of unwanted organisms, they can also destroy beneficial organisms as well as the soil.

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Fertilizers

Synthetic fertilizers are another example of chemicals that are used to help with crop production that can cause harm to the soil. Crops need certain nutrients - nitrogen, phosphorous, and potassium are the three main elements important to plant nutrition. Some soils do not contain the needed nutrients or the soil has been depleted of these nutrients. Fertilizers are often used to replenish or supply the plant nutrients that are deficient in the soil.

Example: Ammonium nitrate is an example of a common fertilizer that can be added to the soil to promote plant growth. Negatively, it can change the pH of the soil by making it more acidic. A change in soil pH can affect the ability of future plants to grow in that soil. It can cause the current plant to die or not be as productive.

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Alternatives

Chemical pesticides and fertilizers have their place in agriculture practices but can cause a decrease in soil quality as well as other environmental problems if overused or used incorrectly.

Crop rotation is one alternative to overdependence on these chemicals. Through sequencing and alternating the types of crops grown in a particular area, the use of synthetic fertilizers and pesticides can be reduced. Crop rotation can help replenish the important nutrients that plants need in the soil without using chemical fertilizers. Some plants take nutrients from the soil - like corn and tomatoes - and some plants, known as cover crops, help put nutrients back into the soil. By rotating crop types, the nutrient levels of the soil can be maintained without adding as much synthetic fertilizers.

Crop rotation can also help decrease the occurrence of disease and insect infestation. Diseases, such as those caused by fungal infections, often occur in similar plant types, and insects often feed on similar plant types. By sequencing and rotating plant types, effects from pests like fungi and insects can be reduced without having to use chemical pesticides.

Promoting beneficial insects and bugs can help rid crops of pest insects and is another alternative to the use of chemicals. There are several natural predators or parasites to the harmful insects.

Examples: The lady bug is an example of a beneficial insect that preys on aphids, which are insects that can cause leaf damage to crops. The tachinid fly is a beneficial parasitic insect. These flies lay their eggs on harmful caterpillars, beetles, and bugs. Their eggs turn to larvae and tunnel into the harmful insect feeding on it from the inside out, eventually killing it.

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Other Pollutants

Soils can also become contaminated by chemicals, compounds, and elements introduced through

• mining activities,
• hazardous waste disposal from nuclear power plants, and
• atmospheric deposition.

Humans depend on minerals and elements stored in rocks called ore. Copper is an element that is used in electrical wiring. It has to be mined from the Earth's crust. It is trapped in ore which means that is has to be separated from the rock. This is done through chemical and electrical processes that leave harmful chemicals in the soil.

Nuclear power is an alternative source of energy that is used to generate electricity. The nuclear reaction that generates electricity produces a radioactive waste that has to be disposed of. These wastes are often buried underground in containers that can leak and contaminate the soil.

Mining can release chemicals harmful to the soil.

Atmospheric deposition is the process by which the soil is polluted from air pollution. When fossil fuels are burned, they release pollutants into the air. These pollutants do not remain in the air forever. They fall back to Earth as acid rain. Acid rain can enter the soil and change the pH of the soil which can harm plant and animal life in the soil. Mercury is another pollutant that enters the air when fossil fuels are burned. This mercury also falls back to Earth where it can contaminate the soil. Soil can also become polluted through deposition in other ways. Soil on roadsides can become polluted from the buildup of fuel spills, brake dust, and heavy metals from engines. These chemicals can accumulate in the soil and undergo reactions where they are changed into even more toxic substances.

Acid rain can destroy forests, monuments, cars, and make its way into the soil.

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