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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.  
What is the author’s main purpose in this passage?
选项 A、To tell gardeners how to modify their soil.
B、To classify soil types on the basis of their composition.
C、To illustrate the effects of soil type on plant growth.
D、To show how water is absorbed by different soil.
答案B
解析 作者写本文的目的是根据土壤的构成情况，对它们进行分类。在本文中，作者将土壤分成黏土(clay soil)、轻质沙土(light sandy oil)、肥土(loam soil)和制土砖的土 (adobe soil)。
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