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 l2.000 years ag0.
Water Beneath the Soil
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.
9. The layer of soil that provides necessary nutrients for plant growth is called.
A.1itter B.topsoil C.humus D.subsoil
10. 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 color than the surface soil.
C.Air pollution is partially responsible for acid soil.
D.Groundwater tends to carry away nutrients for plant growth.
11. We can infer from the passage that the water table lies .
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
12. The underlined word “dissolve” is used to express the idea that organic matter and minerals from soil are .
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
TAG:高考英语