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  3. SOIL QUALITY 1 Soil is a renewable resource, but only on a very long time scale, as it takes hundreds or thousands of years fo

SOIL QUALITY 1 Soil is a renewable resource, but only on a very long time scale, as it takes hundreds or thousands of years fo

admin2012-01-14  17
问题                         SOIL QUALITY
1   Soil is a renewable resource, but only on a very long time scale, as it takes hundreds or thousands of years for the natural processes of erosion, organic decay, and accumulation to create soils．Soil quality and the potential to produce crops can vary enormously from region to region and among various soil types．
2   One important factor affecting the productivity of soils ever time is agriculture．Top-quality lands are brought into production earlier because of their higher potential to produce food．As more and more land has been brought under agricultural production, the average quality of land has decreased, reducing potential productivity per hectare．Crops deplete soil fertility by consuming nutrients, and this eventually reduces crop yields．Poor management practices lead to soil compaction and soil pollution as well as loss of soil cover．Without proper management and the constant addition of nutrients and energy in the form of fertilizers and irrigation, crop production falls over time．
3   Within the scientific community, there is little doubt that soil quality is diminishing in many areas around the world．Scientists have found that the quality of one quarter of the world’s soils has experienced some degradation, and the pace of degradation has accelerated over the past 50 years．The loss of soil fertility has caused a slowing in the growth of agricultural productivity．Annual crops tend to degrade soils more than perennial crops, and common property lands generally suffer more degradation than private lands．
4   Today, irrigated cropland produces about one-third of the world’s food．Approximately 18 percent of the world’s cropland is irrigated, and scientists project this amount to double by 2020．Irrigation can increase crop yields per hectare to two or three times the yields of land watered only by rain．However, there are also some harmful side effects．Besides increasing crop productivity in the short run, irrigation can lead to sharp drops in crop productivity in the long run by causing excessive salt buildup and rising water tables．
5   One of the most critical soil quality problems related to irrigation is the increase in concentration of dissolved salts．This process, salinization, affects an estimated one-fourth of the world’s irrigated cropland．In some places, irrigation water contains as much as 3.5 tons of salt per 1.000 cubic meters．As the water flows over and through the ground, it dissolves salts, increasing the salinity of the water．Since some crops require 6.000 to 10.000 cubic meters of water per hectare, land can receive tens of tons of salt per hectare． As the water evaporates, high concentrations of salts such as sodium chloride are left behind in the topsoil．Salt buildup can stunt crop growth, decrease yields, kill crop plants,  and eventually make the land unproductive．
6   A problem that often accompanies salinization in dry regions is waterlogging, which often occurs when farmers apply heavy amounts of irrigation water in an attempt to prevent salts from accumulating．However, unless the water drains properly, it collects underground and gradually raises the water table closer to the surface, thereby bringing salts to the surface and concentrating them．Saltwater then envelops the fragile root systems of plants,  killing the plants and converting fertile fields to wet deserts．This is a particularly serious problem in California’s heavily irrigated San Joaquin Valley, where soils contain a clay layer that prevents water from flowing through the ground．Worldwide, at least one-tenth of all irrigated land is subject to waterlogging．
7   Another serious soil problem is erosion, the loss of soils from water and wind action． Soil erosion occurs on agricultural land without vegetative cover for protection or because of poor agricultural management．Scientists estimate that topsoil on cultivated land is being lost 16 to 300 times faster than it is being replaced．In many parts of the world, a shift away from traditional agricultural practices is placing greater pressure on the soil, which in turn is causing an increase in soil erosion rates．Studies in the United States suggest that the loss of just 2.5 centimeters of topsoil reduces corn and wheat yields by 6 percent．
Glossary:
   degradation: loss of quality
The word which in paragraph 7 refers to
选项 A、world
B、practices
C、pressure
D、soil
答案C
解析 The referent of which is something that is causing an increase in soil erosion rates. Logic tells you that which refers to pressure on the soil. (1.3)
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