Questions 23-33
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
line 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 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 sod and
squeezing 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 and
evaporated, leaving a residue of clay, which can be weighed.
【題組】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
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