[求助]2000一月 閱讀 第三篇 Q30

In the field, soil texture can be estimated by extracting a handful of sod and squeezing the damp soil into three basic shapes; cast, a lump formed by squeezing a sample in a clenched fist; thread, a pencil shape formed by rolling soil between the palms; and 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 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.

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