1．Have you ever noticed the colour of the water in a river or stream after a heavy rainfall？What do you think caused this change in colour？It is soil that has been washed into the river from the riverbank or from t}le nearby fields．
Components of Soil
Soil is made up of a number of layers（層），each having its own distinctive colour and texture．The upper layer is known as the litter．It acts like a blanket．limiting temperature changes and reducing water loss．The topsoil layer is made up of small particles of rock mixed with rotten plant and animal matter called humus（腐殖質(zhì)），which is black and gives the topsoil its dark colour．This layer is usually rich in nutrients，oxygen，and water．Below the topsoil is the subsoil，a layer that contains more stones mixed with only small amounts of organic matter．This layer is lighter in colour because of the lack of humus．Beneath the soil lies a layer of bedrock．
Soil forms from the bottom up．Over time bedrock is attacked by rain，wind，frost，and snow．It is gradually broken down into smaller particles in a process called weathering．Plants begin to grow，and rotten materials enrich the topsoil．Most of the soil in Eastern Canada．for example．Was formed from weathered rock that was exposed when the ice disappeared l2，000years ago．
Water Beneath the Soil
Surface water collects and flows above the ground in lakes．ponds．a(chǎn)nd rivers．Once in the soil or rock，it is called groundwater．Gravity pulls groundwater through the soil in a process called percolation（滲透）．Eventually the water reaches a layer called the water table．Under this is bedrock through which water cannot percolate．
As water percolates downward，it dissolves organic matter and minerals from the soil and carries them to deeper layers．This causes a serious problem because plants require these nutrients for growth．
Soil pH
Soil can be acidic．neutral．or basic．The pH of the soil is determined by the nature of the rock
from which it was formed．a(chǎn)nd by the nature of the plants that grow and rot in it．
The acidity of rain and snow can lower the pH of the groundwater that enters the soil．By burning fossil fuels such as coal，oil and gasoline，humans have been contributing to higher levels of acidity in many soils．When fossil fuels are burned．gases are released into the air and then fall back to earth as acid rain．Acid soil increases出e problem of carrying nutrients to lower soil levels．As nutrients are removed，soil is less fertile．Plants grow more slowly in acidic soil，and also become easily attacked by diseases．
67．The layer of soil that provides necessary nutrients for plant growth is called．B
A.1itter
B．topsoil
C．humus
D．subsoil
68．According to the text．which of the following is NOT true？B
A．Soil forms from weathered rock on the earth surface．
B．The deeper layer of soil is darker in colour than the surface soil．
C．Air pollution is partially responsible for acid soil．
D．Groundwater tends to carry away nutrients for plant growth．
69．We can infer from the passage that the water table liesC．
A．between the topsoil layer and the subsoil layer
B．in the subsoil layer above bedrock
C．between the subsoil layer and bedrock
D．in the bedrock layer beneath the subsoil
70．The underlined word"dissolve"is used to express the idea that organic matter and minerals from soil areD．
A．rushed away into the river
B．cleaned and purified by water
C．destroyed and carried away by water
D．mixed with water and become part of it．
分析 文章講述了在大雨過后，小河里的水會變顏色，這都是由于雨水將河岸的泥土沖進河里的緣故．
解答 BBCD
67．B 細節(jié)理解題 根據(jù)文章里The topsoil layer is made up of small particles of rock mixed with rotten plant and animal matter called humus（腐殖質(zhì)），which is black and gives the topsoil its dark colour． This layer is usually rich in nutrients，oxygen，and water．可知，給植物生長提供必要營養(yǎng)的土層叫做topsoil，故選B．
68．B 推理判斷題 根據(jù)文章里This layer is lighter in colour because of the lack of humus．Beneath the soil lies a layer of bedrock．可以得知，深水層的顏色比表層要深，故選B．
69．C 推理判斷題 根據(jù)Eventually the water reaches a layer called the water table．Under this is bedrock through which water cannot percolate．可以得知，在subsoil layer and bedrock之間的這部分叫做the water table，故選C．
70．D 細節(jié)理解題．根據(jù)該段的內(nèi)容介紹可知，水滲透下來以后，泥土中有機質(zhì)和其他物質(zhì)和水按常識是和水混在一起，然后分解，推出答案為D．
點評 閱讀這類文章時，也應(yīng)該在整體把握文章結(jié)構(gòu)的前提下，主要注意細節(jié)信息．細節(jié)題題干都是相應(yīng)原文的變形（如同義改寫、詞性轉(zhuǎn)換等），因此要找到答案一定要找到題干在原文中的出處，再把原文和選項相比較．做題時，根據(jù)所提問題用尋讀、跳讀的方法可達到事半功倍的效果．
