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 . 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 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 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.

The word "they" in the passage refers to ______.

Câu hỏi liên quan

• The speaker’s main topic is ...................

• The word “doubtless” in the passage mostly means ...............

• Which of the following best expresses the main idea of the second paragraph of the passage?

ZUNIA12

• What can the word invokes be best replaced by?

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

• Which of the following can be inferred from the first paragraph?

• What can be the best title of the reading passage?

• The word “emaciated” in the passage mostly means .......

• The “pounds” in the second paragraph are probably_______.

• When we apologize, ..............

• According to the passage, approximately how many non-native users of English are there in the world today?

• The passage implies that Melville stayed in Tahiti because____________.

• According to the passage, one exceedingly clever student has ___________

• The word "These" in paragraph 3 refers to ..........

• The word "obscurity" could best be replaced by __________

• The word “they” in the last paragraph refers to ............

• The phrase “of no consequence” means ___________.

• Which of the following would the writer support?

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

• It can be inferred from the passage that brain size is assumed to ___________.