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  3. HOW SOIL IS FORMED (1) Soil formation is a dynamic process that takes place in different environments. It is strongly influe

HOW SOIL IS FORMED (1) Soil formation is a dynamic process that takes place in different environments. It is strongly influe

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问题                                                 HOW SOIL IS FORMED
    (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.
    (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 materials, 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.
    (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.
    (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 snow melt 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 materials laterally (sideways) through the soil.
    (5) Topography is a major factor in soil development. [A] More water runs off and less enters the soil on steep slopes than on relatively level land. [B] Water draining from slopes enters the soil on low and flat land. [C] Thus soils and soil material tend to be dry on slopes and moist and wet on the low land. [D] Steep slopes are subject to surface erosion and soil creep—the down slope movement of soil material, which accumulates on lower slopes and lowlands.
    (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.
    (7) The weathering of rock material and the accumulation, decomposition, and mineralization of 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.
An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that eocpress the most important ideas in the passage. Some sentences do not belong in the summary because they eocpress ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.
There are a variety of elements that influence soil development.
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Answer Choices
(A) 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.
(B) Both topography and the types of organisms present in the soil as it develops determine its nutrient level.
(C) The length of time that it takes to develop high-quality soil is determined by humidity levels, the makeup of parent materials, and patterns of water flow and erosion.
(D) 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.
(E) 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.
(F) 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.
选项
答案B,C,D
解析 本题属于文章总结题。本文主要介绍了影响土壤形成的几个因素，分别是母质、气候、地形和时间。B项“地形和覆盖土壤的生物类型决定了土壤的养分水平”是对第5段和第6段的整合，这两段均阐述影响土壤形成的重要因素(地形)。C项“优质土壤形成所需的时间长短取决于湿度水平、母质的成分以及水流和侵蚀的模式”是对第7段的整合，该段阐述了时间是影响土壤形成的重要因素。D项“风化过程受温度及其变化、水流的性质和水中所携带的酸性物质的强烈影响”是第3段和第4段的整合，这两段均阐述影响土壤形成的重要因素(气候)。B、C、D三项均为文章主要观点。A项“通过分析已成形的土壤中的化学成分可以确定其母质来源的基质位置”，第2段末句只提到地质材料在很大程度上决定了土壤的化学成分，没有提到可根据土壤的化学成分来确定母质来源的基质位置。E项“如果土壤材料的温度和含水量经常产生快速和极端的变化，那么土壤形成所必需的生物地球化学反应就不会发生”，第3段末句只提及“温度能够刺激或者抑制土壤材料中的生物地球化学反应”，并未点明生物地球化学反应不会发生的前提。F项“由于高湿度加速了淋溶过程和侵蚀，湿润地区的土壤达到平衡所需的时间就比干燥地区的土壤长得多，而且也比干燥地区的土壤贫瘠”，末段第5句说“与潮湿地区相比，干旱地区的土壤形成得更慢”，F项与原文相悖。
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