Agricultural pollution

Plants need a number of elements (about 20) for their growth. The elements carbon, hydrogen, nitrogen and phosphorus are needed in substantial amounts. Water always provides enough carbon, hydrogen and oxygen: nitrogen and phosphorus are scarcer. Lake water, being poor in nutrients nitrogen and phosphorus, supports little plant life. Since there is no decaying vegetation to use up dissolved oxygen, lake water is saturated with oxygen. Fish can thrive.

Farmers use compounds of phosphorus and nitrogen (phosphates and nitrates) to increase their crop yields. Often, fertilizers are used in large quantities. Plants can absorb only a limited amount through their roots. If land surrounding a lake receives too much fertilizer, unabsorbed fertilizer washes out of the soil into the lake water.

There the fertilizer nourishes the growth of water plants. Weeds flourish, and thick mats of algae cover the surface. If water is taken from the lake for use, water filters become clogged with weeds. The name given to this unintentional enrichment of lake water is eutrophication.

When algae die, they are decomposed by aerobic bacteria. When these bacteria run out of oxygen, they die. Anaerobic bacteria take over, and convert part of the dead matter into smelly decay products. The rest of the debris falls to the bottom. Slowly a layer of dead plant material builds up on the bottom of the lake. Deprived of oxygen, fish die. The lake becomes a dead lake. Many lakes in the USA and Canada are now empty of fish. The tourist industry centered on the lakes has suffered. If the farmers could cut back on fertilizers, they would save money and the fishermen would benefit too.

Excessive fertilizer can also find its way into the ground water. This is the water held underground in porous layers of rock. A third of Britain’s drinking water comes from ground water. The World Health Organization recommends that the level of nitrogen in the form of nitrates should not exceed 11 p.p.m. The level in Britain is far short of this value, but it is increasing, and people are worried that in 20 years’ time it may be dangerously high.

Nitrates affect human health in two ways.

1. Nitrates are converted into nitrites. Nitrites oxidize the iron in hemoglobin from iron (2) ions to iron (3) ions. In its oxidized form, hemoglobin can no longer transport oxygen round the body. Babies are more at risk than adults. The extreme case of nitrite poisoning is the “blue baby” syndrome, in which the baby turns blue from lack of oxygen.

2. Nitrosamines have been found in a number of foods. These compounds cause cancer. Some chemists think that nitrites formed from nitrates may be responsible for the formation of nitrosamines.

Research is being done on ways of reducing the nitrate level. Of course, farmers must use fertilizers, but more care can be taken to match the input of fertilizer to the crop’s ability to use it. The correct amount applied in the right season will be used up by the plants. Agricultural chemists are advising farmers how they can make the most efficient use of fertilizers. This will save the farmers’ money as well as making drinking water safer.

Fertilizers are not the only source of phosphates in water. Soapless detergents contain phosphates as brighteners. Detergents are discharged into rivers. This source of pollution could be avoided. People might be persuaded to use detergents which left their shirts just a shade less white. Then phosphates could be omitted from the detergents and stop polluting our lakes and rivers.

SUMMARY NOTE

When nitrates and phosphates from fertilizers wash into lakes and rivers, they stimulate the growth of weeds and algae. Decaying plant material uses oxygen. This leads to a shortage of dissolved oxygen, and fish die.

Nitrates are also washing into the ground water from which we obtain much of our drinking water. More sparing use of fertilizers is recommended.

Detergents containing phosphates are discharged into rivers and lakes.

2. Make sure if you remember the meaning of the following words. Consult a dictionary if necessary:

Substantial, scarce, to saturate, to thrive, fertilizer, to nourish, weed, mat, algae, to clog, to run out, to take over, debris, excessive, to avoid, to persuade, to omit.

3. Read the following words and pay attention to their meaning:

Absorbed – unabsorbed, intentional – unintentional, aerobic – anaerobic, to compose – to decompose, safe – unsafe, usual – unusual, dependent – independent, direct – indirect, definite – indefinite, necessary – unnecessary.

4. Finish the following sentences according to the content of the text:

1. Farmers use compounds of phosphorus and nitrogen (phosphates and nitrates) to ….

2. If land surrounding a lake receives too much fertilizer, …. 3. When algae die, they

…. 4. Excessive fertilizer can also find its way into …. 5. Farmers must use fertilizers, but more care can be taken to …. 6. Soapless detergents contain ….

5. Answer the questions:

1. What elements do plants need in substantial amounts for their growth?

2. What are major constituents of fertilizer?

3. Why do farmers use compounds of phosphorus and nitrogen?

4. What happens if fertilizers are used in large quantities?

5. Why are fertilizers harmful for lakes and rivers?

6. What harm do nitrates bring to human health?

7. Why are detergents containing phosphates dangerous?

8. What is recommended to avoid land and water pollution with phosphates and nitrates?

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Дата добавления: 2015-10-30