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美文赏析 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 additional① crops.American researchers say that can happen with greater use of an agricultural system called perenniation.It mixes food crops with trees and perennial② 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 yield③.” 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 practices④ where they're not putting in organic⑤ 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 is called evergreen agriculture.This is where farmers plant trees with their crops.Farmers in Africa have been doing this for sixty years，but it seems to be growing in popularity. | 
