五、任 务 型 When nature is left alone,a balance is reached among

五、任 务 型
When nature is left alone,a balance is reached among the animals and plants
living in one area.But when man starts his work in nature,the balance is
likely to be destroyed.He grows a crop and takes it away to eat ;then there are
no dead leaves to fall on the ground,holding water while it sinks into the
surface,or decaying(腐烂) and adding humus(腐殖质) to the soil.Unless a farmer acts
with knowledge and skill,he is therefore most likely to make the land poorer.
To take the place of the useful matter in the crops that he removes,he uses
some kind of chemical fertilizer(肥料).Chemical fertilizers are of great help,
but the waste products of animals and decaying remains of plants should also be
put on the land.In some places,it is a habit to burn waste material lying
about,but such burning destroys the useful matter in the dead plants.Although
the ashes that are left are valuable when put on the land,a better practice is
to bury the waste so that it decays and increases the humus in the soil.
In the past,when the world population was much lower than it is now,a man
had little difficulty in ordinary times in growing the food that was needed.
When a field had been used some years and had become tired,the farmer could
move to another place.The tired land then slowly recovered.Gradually grasses
and other plants would appear on it and its productive power would slowly return
to normal through their decay.But nature,left alone,would take a long time to
bring back the land to its former state; the length of time required would
depend on local conditions,but it might well be ten years.
It is a bad practice to grow the same crop in a field year after year.If the
crop is changed,the land will suffer less because it is treated and used in a
different way.Different plants have different effects on the soil.Therefore,a
change of crop will do less harm than the growing of the same crop year after
year and a regular change to grass will do good to the soil.Much will therefore
be gained if different crops are grown one after another,a method known as the
rotation(轮作) of crops.
When a field had been used some years,it’ll __79_ a long time for the tired
land to return to its
__80__ state.
The balance of nature
We __71___ in nature along with animals and plants.
But when man starts his work in nature,they are likely to ___75___the
balance.
We should
__72__ the
balance of
nature.The
farmers should
know how to
__73__the soil.
It’s better
to grow
___74____
crops in a
field year
after year.
It’s a bad idea
to _78__ the
crops,for it
destroys the
useful matter
in the dead
plants.
Chemical fertilizer are
___76___
to farmers,
but it does
___77____ to the land.

79：take.80：former.71：live.75：destroy.72：keep.73：protect.74：different.78：grow.76：help.77：put、我擦,我尽力了.你这编号好乱,我好晕.
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再答： 别客气@=@