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.

　　语篇解读　一场大雨后河流或小溪里的水颜色的变化是被冲进去的土壤引起的。

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

　　A．litter

　　B．topsoil

　　C．humus

　　D．subsoil

　　【解析】　细节理解题。根据第二段4－7行，可知选B。

　　【答案】　B

　　2．According to the text，which of the following is NOT true?

　　A．Soil forms from weathered rock on the earth surface.

　　B．The deeper layer of soil is darker in colour than the surface soil.

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