对于很多正在准备托福考试的人们来说，不知道准备的怎么样了呢？今天就和出国留学网的小编一起来回顾一下2018年4月21日托福阅读机经预测整理。
（图片转自百度）
How Soil is Formed
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.
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.
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.
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