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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.
According to paragraph 4, water passing through the soil can affect the soil’s composition in which of the following ways?
选项 A、By stopping certain chemical reactions from taking place in the soil
B、By carrying away minerals
C、By reducing the acidity of the soil
D、By preventing leaching of minerals
答案B
解析 本题问经过土壤中的水可以通过哪种方式来影响土壤的成分，属于事实信息题。原文第4段倒数第2句提到，当水移动的时候，它会留下悬浮物质，并且可能会带走溶液中的矿物质。故B项“带走土壤中的矿物质”与原文说法一致。为正确答案。A项“通过阻止某些化学反应在土壤中发生”，根据第4段第1句，水只是参与了土壤中的生物地球化学反应，并未说明水是否阻止了化学反应的发生。C项“通过降低土壤的酸度”无依据。D项“通过防止矿物质的浸出”，根据第4段倒数第2句，水会导致淋溶作用，带走矿物质，D项错误。
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