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

• Why does the author mention “artistic, political, or pedagogic communication” in the passage?

• If there was a preceding paragraph to this passage it would most likely be concerned with which of the following topics?

• What does Bethan conclude about Facebook?

ZUNIA12

• Why did John Mills fly in an aeroplane?

• Sixty years ago, most women ...................

• According to the passage, what are children NOT usually taught outside the classroom?

• According to the passage, some Americans choose to live in the country because ___.

• Approximately how much of the Earth’s atmosphere is nitrogen?

• Which is the most popular sport in the US?

• Which of the following is closest in meaning to the word “nocturnal”?

• According to the passage, which of the following sentence is NOT true?

• According to the passage, the depression of the 1930's caused which of the following?

• What use was made of the nineteenth-century women's history materials in the Schlesinger Library and the Sophia Smith Collection?

• What is the simplest way to apologize?

• Why are beta clockers not prescribed regularly?

• The phrase “these years” in the third paragraph refers to ________.

• The main subject of the passage is ...........

• What does the passage mainly discuss?

• Which of the following can be inferred from the passage?

• Why does the author mention "artistic, political, or pedagogic communication" in line 14-15?