阅读理解　　 The wide variety of climates in North America has helped spawn a complex pattern of 　　 soil regions. In general, the realm''s soils also reflect the broad environmental partitioning 　　 into "humid America" and "arid America." Where annual precipitation exceeds 20 inches 　　 (50 centimeters),soils in humid areas tend to be acidic in chemical content, Since crops (5) do best in soils that ale neither acidic(higher in acid content) nor alkaline(higher in salt 　　content).fertilization is necessary to achieve the desired level of neutrality between the 　　two. Arid America''s soils are typically alkaline and must be fertilized back toward 　　neutrality by adding acidic compounds. Although many of these dryland soils, particularly 　　in the Great Plains, are quite fertile, European settlers learned over a century ago that (10) water is the main missing ingredient in achieving their agricultural potential. In the 　　1970''s, certain irrigation methods were perfected and finally provided a real opportunity 　　to expand more intensive farming west from the Central Lowland into the drier portions 　　of the Great Plains. Glaciation also enhanced the rich legacy of fertile soils in the central 　　United States, both from the deposition of mineral-rich glacial debris left by meltwater (15) and from thick layers of fine wind-blown glacial material, called loess, in and around the 　　middle Mississippi Valley. 　　 Natural vegetation patterns could be displayed on a map of North America, but the 　　enormous human modification of the North American environment in modem times has 　　all but reduced this regionalization scheme to the level of the hypothetical. Nonetheless, (20) the humid America-arid America dichotomy is still a valid generalization: the natural 　　vegetation of areas receiving more than 20 inches of water yearly is forest, whereas the 　　drier climates give rise to a grassland cover. The forests of North America tent to make 　　a broad transition by latitude. In the Canadian North, needle-leaf forests dominate, but 　　these coniferous trees become mixed with broadleaf deciduous trees as one crosses the (25) border into the Northeast United States As one proceeds toward the Southeast, broadleaf vegetation 　　becomes dominant. Arid America mostly consists of short-grass prairies or 　　stepper. The only areas of true desert are in the Southwest.