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 word “assessing” in paragraph 2 could best be replaced by ...........

• It can be inferred from the passage that the courtship of great horned owls ................

• It can be inferred from the passage that _____________

ZUNIA12

• What does the writer’s brother do?

• From the passage we can infer that: .........

• What does the word "run" in the last paragraph mean?

• Which of the following is NOT true?

• The word “thoroughfares” is closet in meaning to ..........

• The phrase "held back” in paragraph 1 means________.

• Which of the following would be the most appropriate title for this text?

• The word “haphazard” is closest in meaning to which of the following?

• The word formulaic is closest in meaning to___________.

• The author states that the reason that infants need to avoid certain foods related to allergies has to do with the infant's ___________ .

• What should a diver do when ascending?

• The best way to protect children from improper material is .............

• How do theorists believe a person can remember more information in a short time?

• All of the following are advantages of business “Intranets” mentioned in the passage EXCEPT__________

• The passage discusses a way to deal with an offensive noise by_________.

• When does television encourage people to read?

• What does the writer mean by saying ''education quite often produces surprises"?