Surface water collects and flows above the ground in lakes,ponds,and rivers. Once in the soil or rock,it is called groundwater. Gravity pulls groundwater through the soil in a process called percolation(渗透). 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,and 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 the 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.

　　1. The layer of soil that provides necessary nutrients for plant growth is called　.　

　　A. litterB. topsoilC. humusD. subsoil

　　2. According to the text,which of the following isNOT true?

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