gradational difference of fertility [土壤] [肥料] 肥力级差
gradational difference of soil fertility 土壤肥力级差
difference of soil fertility 土壤肥力差异
·2,447,543篇论文数据，部分数据来源于NoteExpress
And in Iran, public policy and investment in family-planning services made the difference between rising fertility between 1979 and 1984 and falling fertility thereafter, not the rise in national incomes, nor female education.
ECONOMIST: Population booms, and busts
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