How Soils Formed

Paragraph 1：Soil formation is a dynamic process that takes place in different environments. It is strongly influenced by the parent material, climate (largely vegetation and temperature and water exchanges), topography (the elevations, depressions, directions and angles of slopes, and other surface features of the landscape), and time. 

 

 

Paragraph 2：The parent material is the unconsolidated mass on which soil formation takes place. This material may or may not be derived from the on-site geological substrate or bedrock on which it rests. Parent materials can be transported by wind, water, glaciers, and gravity and deposited on top of bedrock. Because of the diversity of materials involved, soils derived from transported parent materials are commonly more fertile than soils from parent materials derived in place. Whatever the parent material, whether derived in place from bedrock or from transported material, it ultimately comes from geological materials, such as igneous, sedimentary, and metamorphic rocks, and the composition of the rocks largely determines the chemical composition of the soil. 

 

 

 

1. According to paragraph 2, which of the following is true about parent material   found at a given site? 

 

O It was most likely deposited there by glaciers rather than by wind and water. 

 

O It is formed and consolidated entirely from the bedrock on which it rests. 

 

O It can vary in its richness and fertility according to where it originated.

 

O The speed with which it is transported determines its location. 

 

 

 

2. Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.   

 

O Transported parent materials, parent materials derived in place from bedrock, and various kinds of rocks all contribute to the composition of a soil.   

 

O The chemical composition of a soil is ultimately determined by the geological materials from which it forms, regardless of where or how the parent material originates.  

 

O The parent material from which a soil is formed ultimately comes from igneous, sedimentary, and metamorphic rocks.

 

O The composition of the geological materials from which the parent material was originally derived can be determined by studying the chemical composition of the soil. 

 

 

 

 

 

Paragraph 3：Climate is most influential in determining the nature and intensity of weathering and the type of vegetation that further affects soil formation. The soil material experiences daily and seasonal variations in heating and cooling. Open surfaces exposed to thermal radiation undergo the greatest daily fluctuations in heating and cooling, soils covered with vegetation the least. Hill slopes facing the sun absorb more heat than those facing away from the sun. Radiant energy has a pronounced effect on the moisture regime, especially the evaporative process and dryness. Temperature can stimulate or inhibit biogeochemical reactions in soil material. 

 

 

 

Paragraph 4：Water is involved in all biogeochemical reactions in the soil because it is the carrier of the acids that influence the weathering process. Water enters the soil material as a liquid and leaves it as a liquid by percolation (the slow movement of water through the soil’s pores) and as a gas through evaporation. The water regime—the water flow over a given time—in soil material is sporadic, and in many parts of the Earth is highly seasonal. Water that enters the soil during heavy rainfall and snowmelt moves down through the soil. As it moves, it leaves behind suspended material and may carry away mineral matter in solution, a process called leaching. On sloping land, water distributes material laterally (sideways) through the soil. 

 

 

 

3. According to paragraph 4, water passing through the soil can affect the soil’s composition in which of the following ways?   

 

O By stopping certain chemical reactions from taking place in the soil   

 

O By carrying away minerals  

 

O By reducing the acidity of the soil   

 

O By preventing leaching of minerals 

 

 

 

 

 

Paragraph 5：Topography is a major factor in soil development. More water runs off and less enters the soil on steep slopes than on relatively level land. Water draining from slopes enters the soil on low and flat land. Thus soils and soil material tend to be dry on slopes and moist on the low land. Steep slopes are subject to surface erosion and soil creep—the downslope movement of soil material, which accumulates on lower slopes and lowlands. 

 

 

 

4. According to paragraph 5, why does soil material tend to be drier on steep slopes than on flatter land?   

 

O Steep slopes are less likely to experience soil creep.   

 

O Soil material on steep slopes tends to be thicker.   

 

O Water that falls on steep slopes tends to erode the soil there and deposit it on level ground.   

 

O More water runs off steep slopes before it can enter the soil.

 

 

 

 

 

Paragraph 6：Vegetation, animals, bacteria, and fungi all contribute to the formation of soil. Vegetation, in particular, is responsible for organic material in the soil and influences its nutrient content. For example, forests store most of their organic matter on the surface, whereas in grasslands most of the organic matter added to the soil comes from the deep fibrous root systems. Organic acids produced by vegetation accelerate the weathering process. 

 

 

 

5. Paragraph 6 supports which of the following ideas about organic material soil?   

 

O Most of the organic material in soil comes from fibrous root systems.   

 

O How organic material is distributed in soil depends on the type of vegetation the soil supports.  

 

O The organic materials in soil prevent acids from forming during the weathering process.   

 

O Animals, bacteria, and fungi consume much of the nutrient content in soil. 

 

 

 

 

 

Paragraph 7：The weathering of rock material and the accumulation, decomposition, and mineralization or organic material require considerable time. Well-developed soils in equilibrium with weathering, erosion, and biotic influences may require 2,000 to 20,000 years for their formation, but soil differentiation from parent material may take place in as short a time as 30 years. Certain acid soils in humid regions develop in 2,000 years because the leaching process is speeded by acidic materials. Parent materials heavy in texture require a much longer time to develop into soils because of an impeded downward flow of water. Soils develop more slowly in dry regions than in humid ones. Soils on steep slopes often remain poorly developed regardless of geological age because rapid erosion removes soil nearly as fast as it is formed. Floodplain soils age little through time because of the continuous accumulation of new materials. Such soils are not deeply weathered and are more fertile than geologically old soils because they have not been exposed to the leaching process as long. The latter soils tend to be infertile because of long-time leaching of nutrients without replacement from fresh material. 

 

6. According to paragraph 7, soil on steep slopes tends to remain poorly developed because   

 

O soil on steep slopes tends to be very acidic   

 

O soil that forms on steep slopes tends to be heavy in texture   

 

O so much of the soil that forms on steep slopes becomes eroded quickly  

 

O soil differentiates very quickly from parent materials 

 

 

 

7. According to paragraph 7, why does floodplain soil tend to be especially fertile?   

 

O Because floodplain soil tends to be deeply weathered   

 

O Because floodplain soil is not continually replaced by the addition of new material   

 

O Because floodplain soil has not had many nutrients removed  

 

O Because most floodplains are in geologically new regions 

 

 

 

8. The purpose of paragraph 7 in the passage is to   

 

O provide evidence that soils in equilibrium maintain nutrients for longer periods of time than other soils   

 

O explain why geologically younger soils are more fertile than older soils, regardless of where they are found   

 

O explain how long it takes for soils to develop and what elements affect their fertility  

 

O summarize the various stages of soil formation discussed earlier in the passage 

 

 

 

 

 

Paragraph 5：Topography is a major factor in soil development. ■More water runs off and less enters the soil on steep slopes than on relatively level land. ■Water draining from slopes enters the soil on low and flat land. ■Thus soils and soil material tend to be dry on slopes and moist on wet on the low land. ■Steep slopes are subject to surface erosion and soil creep—the downslope movement of soil material, which accumulates on lower slopes and lowlands. 

 

 

 

9. Look at the four squares [■] that indicate where the following sentence can be added to the passage. 

 

Its role is demonstrated by the contrast between what happens on steep slopes and what happens on the lower-lying land at the bottom of those slopes.

 

Where would the sentence best fit? 

 

 

 

 

 

10. Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some answer choices do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points. Drag your choices to the spaces where they belong. To review the passage, click on View Text. 

 

There are a variety of elements that influence soil development. 

 

 

 

Answer Choices

 

O By analyzing the chemical composition of developed soil, it is possible to determine the location of the geological substrate from which the parent material was originally derived. 

 

O Both topography and the types of organisms present in the soil as it develops determine its nutrient level.

 

O The length of time that is takes to develop high-quality soil is determined by humidity levels, the makeup of parent materials, and patterns of water flow and erosion.

 

O The weathering process is strongly influenced by temperature and its variation, the nature of the water flow, and by the acids carried in the water.

 

O Where soil material is regularly subject to rapid and extreme variations in temperature and water content, certain biogeochemical reactions necessary for soil formation cannot take place. 

 

O Because high humidity speeds up both leaching processes and erosion, soils in humid regions take much longer to reach equilibrium than soils in dry regions, and are seldom as fertile. 

 

 

 

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