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  3. The texture of the soil depends on the relative amounts of different-sized particles that combine to make up the soil. These par

The texture of the soil depends on the relative amounts of different-sized particles that combine to make up the soil. These par

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问题     The texture of the soil depends on the relative amounts of different-sized particles that combine to make up the soil. These particles can be as large as stone and gravel or as small as clay.
    A typical clay soil is composed of approximately 60 percent actual clay, 20 percent silt, and 20 percent sand. The particles in a sandy soil are so fine that it tends to be compact and interferes with the oxygen supply for plant roots. Water has trouble entering this impervious soil, and runoff is very common during rainfalls.
    A typical light sandy soil is composed of approximately 70 percent sand, 20 percent silt, and 10 percent clay. The particles in a sandy soil are comparatively large, permitting water to enter the soil and to pass through it so quickly that it often carries nutrients with it and dries out very rapidly. The texture of sandy soils is generally very difficult to modify because huge amounts of organic material must be added.
    A typical loam soil is composed of approximately 40 percent sand, 40 percent silt, and 20 percent clay, making it an ideal garden soil. It is easily worked and retains water and nutrients, which fire slowly absorbed by plant roots.
    A typical adobe soil is a clay soil present in hot, dry areas of the country and is often very hard and cracked. It has all the disadvantages of a heavy clay soil and, being much drier, is more difficult to correct.  
According to the passage, the texture of sandy soil can be improved by mixing in ______.
选项 A、water
B、gravel
C、chemistry nutrients
D、organic material
答案D
解析 作者在第四段末尾说，在一般情况下，沙土的结构很难改变，因为必须在里面添加大量的有机物质。
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