答案：　A2017·江西省赣中四校高三第二次联考Studies show farmland in Africa is often lacking in important nutrients.But researchers say a combination of farming methods may help.

　　Researchers wonder how to increase production without necessarily clearing more land to grow additional crops.American researchers say that can happen with greater use of an agricultural system called perenniation.It mixes food crops with trees and perennial plants—those that return year after year.

　　Soil scientist John Reganold says，“One of the major problems is that the soils are fairly poor in most of the regions.So how do you grow food on poor soils？There have to be food

　　production systems that can build the soil and improve the

　　yield.”

　　Mr Reganold says poor soil may have resulted from years

　　of weathering that washed away many nutrients.He says some

　　farmers may have done more harm than good.

　　“They have been actually using farming practices where

　　they’re not putting in organic matter.They’re not putting in

　　fertilizers.They can’t afford those things.And it just runs the

　　soil down.So they’re worsening the situation.”

　　The scientist says the word perenniation defines three

　　systems that are already used in Africa.The oldest of the three

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