Potentially toxic elements from pollution

Potential poisoning of humans by arsenic, cadmium, lead, and mercury are real concerns today. Arsenic has accumulated in soils from sprays used to control insects and weeds and to defoliate crops before harvesting. Although arsenic accumulates in soils and has injured crops, it has not created a hazard for humans or animals. Cadmium poisoning has occurred in Japan from the dumping of mine waste into rivers, where fish ingested the cadmium from the mine waste. Cadmium also appears in some sewage sludges. Using soils for the disposal of sewage represents a potential danger. Mine spoils may have toxic levels of cadmium, but natural agricultural soils do not contain harmful levels of cadmium. Lead is discharged into the air from automobile exhausts and other sources, and it eventually reaches the soil. In soil, lead is converted to forms unavailable to plants. Any lead that is absorbed tends to remain in plant roots and is not transported to the shoots. Soils must become very polluted with lead before significant amounts move into the tops of plants.

 

Mercury is discharged into the air and water from use in pesticides and industrial activities. Under conditions of poor aeration, inorganic mercury is converted to methyl mercury, which is very toxic. Plants do not take up mercury readily from soils; however, soils should not be used to dispose of mercury because of the highly toxic nature of methyl mercury.

 

The soil is being used increasingly for sewage disposal. The application of high levels of heavy metals, and their potential uptake by plants, represents an important limitation in the use of soils for sewage and industrial waste disposal. Sewage known to be lacking in heavy metals can be added to soils without any danger of heavy metal poisoning of humans or other animals.

 

RADIOACTIVE ELEMENTS

Rocks and soils naturally contain radioactive elements. Currently, there is concern about radon (Rn). Many radioactive elements have very short half-lives, and contamination of the soil with these elements is of little concern. Radioactive cesium (Cs) is produced by atomic bombs and has a long half-life. Radioactive cesium appears to be fixed in vermiculite minerals much like potassium. This limits its availability to plants. The soil tends to slow down the movement of radioactive cesium from the soil into plants and, later, into animals.

 

The contamination of soils is via the atmosphere, and radioactive elements that fall on vegetation are absorbed by the leaves. Testing of nuclear weapons prior to 1962 resulted in significant contamination of tundra vegetation in Alaska. Increased radioactivity was found in the caribou that grazed on the tundra. People whose major food was caribou were found to have about 100 times more radioactivity in their bodies than people in the continental United States. More recently, a similar contamination occurred in northern Europe as a result of the Chernobyl nuclear reactor explosion in the Soviet Union.

 

Nuclear bomb tests were conducted on Bikini Atoll of the Marshall Islands between 1946 and 1958. The soils were heavily contaminated with radioactive cesium-137. Several approaches have been suggested to decontaminate the soil. First, mix a large quantity of potassium with soil and irrigate it with seawater to increase the likelihood of potassium uptake by plants relative to cesium. A second approach is to remove the top 16 inches of topsoil and replace it with uncontaminated soil. The Bikinians favor the second approach.

(H.D. Foth, Fundamentals of Soil Science)

 

Find in the text above English equivalents for the following words and expressions and memorize them.

 

1) соответствовать/отвечать потребностям растений

2) в меньшей мере/степени

3) окислённые/оксидированные формы

4) загрязнять, заражать

5) декоративные растения

6) уменьшать, снимать симптомы

7) останавливать, замедлять рост

8) способствующий фактор

9) мышьяк

10) кадмий

11) свинец

12) ртуть

13) уничтожать растительность (удалять листья)

14) отходы горнодобывающих предприятий (шахтные отходы)

15) выхлопные газы автомобилей

16) избавляться от…(удалять/утилизировать)

17) метилртуть

18) сброс сточных вод

19) период полураспада

20) очищать от загрязнений, обеззараживать

TEXT 6

Soil fertility is the soil's ability to provide essential elements for plant growth without a toxic concentration of any element. Soil fertility is assessed by use of deficiency symptoms, plant tissue analysis, and soil testing. Plant tissue analysis and soil tests are the basis for making fertilizer recommendations. Fertilizer nutrients are absorbed most efficiently when they are applied near the time of maximum plant uptake, owing to the potential loss of nitrogen by leaching and volatilization and the fixation of phosphorus and potassium. These factors are also important considerations for placement of fertilizers. In some cases, the acidity and salinity effects of fertilizers are important considerations in the selection and application of fertilizers. Manure contains nutrients and is a substitute for chemical fertilizers. Manure is also an energy source for biogas generation and as a substitute for fuel wood. Much of the sewage sludge produced in the United States is applied to land. This sludge contains nutrients that can substitute for fertilizers. Some sludges contain heavy metals, and the amount of sludge that can be applied is limited to the heavy metal content of the sludge. Soil pH should be maintained at 6.5 or higher to inactivate heavy metals. Modern agriculture is energy intensive and has been becoming increasingly so. Sustainable agriculture concerns stem from the uncertainty of the future supply and cost of energy.

Soil genesis deals with the factors and processes of soil formation. The formation of soil is the result of the interaction of five soil-forming factors: parent material, climate, organisms, topographic position or slope, and time. The dominant processes in soil genesis are: (1) mineral weathering, (2) humification of organic matter, (3) leaching and removal of soluble materials, and (4) translocation of colloids (mainly silicate clays, humus, and iron and aluminum oxides). The net effect of these processes is the development of soil horizons, that is, the genesis of soil.