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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 5, why does soil material tend to be drier on steep slopes than on flatter land?
选项 A、Steep slopes are less likely to experience soil creep.
B、Soil material on steep slopes tends to be thicker.
C、Water that falls on steep slopes tends to erode the soil there and deposit it on level ground.
D、More water runs off steep slopes before it can enter the soil.
答案C
解析 本题要求回答为什么在陡坡上的土壤材料比在平地上的土壤材料更干燥，属于事实信息题题干与第5段第4句相对应，从该句开头的Thus可以知道，该句前面的内容能解释这种现象；而最后一句是对第4句的进一步解释。从第5段第2、3、5句可知，陡坡易受表面侵蚀的影响，水分流失的多，渗入土壤的少。从斜坡上排出的水会流向洼地和平坦的土地，而土壤蠕变又使得土壤沉积在较低的斜坡和低洼地。C项“落在陡坡上的水容易侵蚀那里的土壤，并将其沉积在平坦的地面上”符合原文说法。A项“陡坡不太可能发生土壤蠕变”，第6段末句说陡坡易受土壤蠕变的影响，A项与原文相悖B项“陡坡上的土壤材料往往更厚”在文中没有依据。D项“在渗入土壤前，更多的水流出陡坡”只是在描述一个现象，并未点出水流出陡坡之后的状况(即水会流到平地)，所以无法从此项看出二者的对比；此外，D项也未点明有关土壤材料的问题。
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