英语翻译When nature is left alone,a balance is reached among the

英语翻译
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 knowl- edge 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 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.

当我们放任大自然不管时,生活在一个地区的动植物之间会达到一种平衡.但是当人类开始干涉打字人,这种平衡很可能就会被摧毁.人种植一种作物然后取走食之,之后就不会再有枯叶落向地面,使落于地表的水得到涵养,腐烂给土壤增加腐殖质.除非农民凭借知识和技巧行事,否则他极可能因此而使得土壤更加贫瘠.为了取代植物中他抛弃的有用物质,他使用某些肥料.化肥确实很有帮助,但动物所产生的废料以及植物腐烂的遗体也应该使于土地.在某些地方,焚烧四处可见的废料是一种习惯,但是这种焚烧摧毁了死掉的植物中的有用物质.尽管当那些留下来的灰烬施于土地时是有价值的,但是填埋废物以致其腐烂、增加土壤腐殖质是更好的办法.在过去,当世界人口较之现在还很少时,人们平时在种植所需食物时几乎没有困难.当一块土地被使用一些年后,就会变得贫瘠,农民还能转向别的地方.贫瘠的土地之后慢慢就恢复了.渐渐地,草类和其他植物便会重现在土地上,其生产能力,经过植物的腐烂,亦会渐渐回到正常水平.然而,自然,若放任不管,它要经过很长时间才能将土地恢复到正常状态；且所需的时间有多长常常取决于当地条件,但很可能是十年时间.在同一处土地种植同一种作物不是一种好的做法.如果作物更换的话,土地将承受的伤害少些,因为它被以不同的方式使用.不同的植物对土地产生的效果各不相同.因此,作物更替产生的伤害比年复一年地种植单一作物产生的伤害要小些；有规律的改变所种草类会对土壤有所益处.如果不同的作物一个接一个种植,这样便会达到更好的效果-----这种方法被称为作物轮作.
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