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The mineral particles found in soil range in size from microscopic clay particles to large boulders. The most abundant particles — sand, silt, and clay — are the focus of examination in studies of soil texture. Texture is the term used to describe the composite sizes of particles in a soil sample, typically several representative handfuls.

To measure soil texture, the sand, silt, and clay particles are sorted out by size and weight. The weights of each size are then expressed as a percentage of the sample weight. In the field, soil texture can be estimated by extracting a handful of soil and squeezing the damp soil into three basic shapes; (1) cast, a lump formed by squeezing a sample in a clenched fist; (2) thread, a pencil shape formed by rolling soil between the palms; and (3) ribbon, a flatfish shape formed by squeezing a small sample between the thumb and index finger. The behavioral characteristics of the soil when molded into each of these shapes, if they can be formed at all, provide the basis for a general textural classification. The behavior of the soil in the hand test is determined by the amount of clay in the sample. Clay particles are highly cohesive, and when dampened, behave as a plastic. Therefore the higher the clay content in a sample, the more refined and durable the shapes into which it can be molded.

Another method of determining soil texture involves the use of devices called sediment sieves, screens built with a specified mesh size. When the soil is filtered through a group of sieves, each with a different mesh size, the particles become grouped in corresponding size categories. Each category can be weighed to make a textural determination. Although sieves work well for silt, sand, and larger particles, they are not appropriate for clay particles. Clay is far too small to sieve accurately; therefore, in soils with a high proportion of clay, the fine particles are measured on the basis of their settling velocity when suspended in water. Since clays settle so slowly, they are easily segregated from sand and silt. The water can be drawn off and evaporated, leaving a residue of clay, which can be weighed.

1. What does the passage mainly discuss?

(A) Characteristics of high quality soil

(B) Particles typically found in most soils

(C) How a high clay content affects the texture of soil

(D) Ways to determine the texture of soil

2. The author mentions "several representative handfuls" in line 4 in order to show

(A) the range of soil samples

(B) the process by which soil is weighed

(C) the requirements for an adequate soil sample

(D) how small soil particles are weighted

3. The phrase "sorted out" in line 5 is closest in meaning to

(A) mixed

(B) replaced

(C) carried

(D) separated

4. It can be inferred that the names of the three basic shapes mentioned in paragraph 2 reflect

(A) the way the soil is extracted

(B) the results of squeezing the soil

(C) the need to check more than one handful

(D) the difficulty of forming different shapes

5. The word "dampened" in line 14 is closest in meaning to

(A) damaged

(B) stretched

(C) moistened

(D) examined

6. Which of the following can be inferred from the passage about a soil sample with little or no clay in it?

(A) It is not very heavy.

(B) It may not hold its shape when molded.

(C) Its shape is durable

(D) Its texture cannot be classified

7. The word "they" in line 21 refers to

(A) categories

(B) sieves

(C) larger particles

(D) clay particles

8. It can be inferred from the passage that the sediment sieve has an advantage over the hand test in determining soil texture because

(A) using the sieve takes less time

(B) the sieve can measure clay

(C) less training is required to use the sieve

(D) the sieve allows for a more exact measure

9. During the procedure described in paragraph 3, when clay particles are placed into water they

(A) stick to the sides of the water container

(B) take some time to sink to the bottom

(C) separate into different sizes

(D) dissolve quickly

10. The word "fine" in line 24 is closest in meaning to

(A) tiny

(B) many

(C) excellent

(D) various

11. All of the following words are defined in the passage EXCEPT

(A) texture (line 3)

(B) ribbon (line 10)

(C) sediment sieves (line 18)

(D) evaporated (line 25)

答案：

DADBC BBDBA D

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