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 Soft

    Soil is made up of a number of layers(层)，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(腐殖质)，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 l2．000 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 t}le 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．

    A．1itter    B．topsoil    C．humus D．subsoil

68．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 t}le 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 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

70．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

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 Soft

Soil is made up of a number of layers（层），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（腐殖质），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 l2．000 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 t}le 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．

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

A．1itter    B．topsoil    C．humus D．subsoil

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 t}le surface soil．

C．Air pollution is partially responsible for acid soil．

D．Groundwater tends to carry away nutrients for plant growth．

3.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

4.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

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 Soft

Soil is made up of a number of layers（层），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（腐殖质），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 l2．000 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 t}le 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．

A．1itter    B．topsoil    C．humus D．subsoil

68．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 t}le 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 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

70．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

Across countryside, non-food crops are growing alongside wheat．Some are used in new alternatives to plastics and other materials, but others will simply be burned．

Burning crops is becoming more popular because it is good for the planet．Doing so is also cheaper than burning fossil fuels．As more land is devoted to non-food crop production, the economics of crops for fuel are likely to become even more favorable．

Humans have been burning such biomass—organic materials from plants or animals— since they discovered fire．But that burning fossil fuels could have catastrophic consequences has brought biomass back into fashion．

Even allowing for emissions (排放) of carbon dioxide from fuel used in planting, harvesting, processing and transporting biomass fuel, replacing fossil fuel with biomass can typically reduce greenhouse gas emissions by more than 90 percent．

Power stations around the world are experimenting with forms of biomass to add to their coal or oil, with encouraging results．Indeed, power companies could profit by turning to biomass, especially when the subsidies (补助) many governments offer for using renewable energy are taken into account．

Farmers can benefit from growing biomass．In Europe, the reduction of subsidies for certain crops is exposing farmers to market forces．Instead of being paid for whatever they produce, farmers must seek a clear demand for their product．Many believe that the demand for alternatives to fossil fuels could be just such a driver．

But while biomass offers a variety of potential alternatives, the world's infrastructure (基础设施） has developed around burning fossil fuels to such a stage that switching to biomass involves a change in conception that many companies have not accepted．

Also, there are problems with using biomass．Although most coal-fired power stations could take a small amount of their fuel from biomass without significant adjustment, few are built to run on biomass alone．Burning some forms of biomass also causes environmental problems．Oils and waste can be smelly, while wood produces smoke unless burned properly．

The supply chains for fossil fuels are set up well, but those for biomass are just the beginning. Sources of supply for biomass rely on farm production and can therefore be less reliable. They are necessarily limited and it wouldn't make sense to turn a very large amount of valuable agricultural land away from food crops．But as the world adjusts to the need to control greenhouse gas emissions, these problems may receive more attention．

1.What do we know about biomass?

A．It is a useful way of burning crops．

B．It is plant and animal matter used to provide power．

C．It is a new alternative to be used widely．

D．It is a popular approach to fighting against pollution．

2.What are the advantages of using biomass?

a．It gives off less greenhouse gases．

b．It helps farmers receive subsidies for what they produce．

c．It has met the clear demand for energy．

d．It has brought subsidies for some power companies．

e．It is cheaper than burning fossil fuels．

A．a, b, c            B．b, c, d            C．c, d, e            D．a, d, e

3.For coal-fired power stations, ______．

A．more effective ways to use biomass have been tried

B．using renewable energy has become their daily task

C．burning biomass alone would require considerable changes

D．setting up dependable supply channels needs governments' support

4.What is the author's attitude towards biomass fuels?

A．Defensive．       B．Doubtful．        C．Negative．        D．Objective

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 Soft
Soil is made up of a number of layers（层），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（腐殖质），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 l2．000 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 t}le 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．
【小题1】The layer of soil that provides necessary nutrients for plant growth is called．