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 focusof examination in studies of soil texture. Texture is the term used to describe theline composite sizes of particles in a soil sample, typically several representative handfuls.(5) To measure soil texture, the sand, silt, and clay particles are sorted out by size andweight. 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 sod andsqueezing the damp soil into three basic shapes; (1) cast, a lump formed by squeezing (10) 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, provides the basis for a general textural classification. The behavior of the soil in the hand test is determined by the (15) 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 (20) 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(25) 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 andevaporated, leaving a residue of clay, which can be weighed.
23. 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
24. The author mentions "several representative
(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
25. The phrase "sorted out" in line 5 is closest in meaning to
26. It can be inferred mat 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
27. The word "dampened" in line 15 is closest in meaning to
28. 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
29. The word "they" in line 23 refers to
(C) larger particles
(D) clay particles
30. 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
31. 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
32. The word "fine" in line 24 is closest in meaning to
33. All of the following words are defined in the passage EXCEPT
(A) texture (line 3)
(B) ribbon (line 11)
(C) sediment sieves (line 19)
(D) evaporated (line 27)
参考答案： DAD BCBBD BA