... keep…free from/of 使…免受（影响；伤害等） keep the soil fertile 使土壤肥沃 every few years 每隔几年 ...
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Burning the forest just once can leave behind enough black carbon to keep the soil fertile for thousands of years.
森林仅仅燃烧一次就能留下足够的黑碳，来保持数千年土壤肥沃。
Organic farmers also use many other methods to keep the soil fertile.
有机耕作的农民也使用很多其他的方法来保持土地肥沃。
Organic farmers also use many other methods to keep the soil fertile.
因此，有机耕作的农民通常喜欢把天然的动物粪便当作肥料。
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