英语阅读 美国的气候和农业
Climates and Farming in America
Keywords: climate, North America, region, soil, fertilization
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 do best in soils that are 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 the dryland soils, particularly in the Great Plains, are quite fertile, European ttlers learned over a century ago that 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 far东道主
ming 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 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 modern times has all but reduced this regionalization scheme to the level of the hypothetical. Nonetheless, 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 ri 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 the coniferous trees become mixed with broadleaf deciduous trees as one cross the 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 dert are in the Southwest.
22. According to the passage, the inventors of written language in Mesopotamia were probably the
(A) Babylonians
茅尾海(B) Subarians
(C) Akkadians
(D) Sumerians
23. The word "subjugated" in line 4 is clost in meaning to
(A) distinguished
(B) gregated
(C) concentrated
(D) conquered
24. The phra "synonymous with " in line 5 is clost in meaning to
(A) equivalent to
(B) important for师带徒培养方案
(C) respected in
(D) familiar with
25. According to the passage, by the year 3000 B.C., the Sumerians had already done all of the following EXCEPT:
(A) They had abandoned the area north of the Persian Gulf.
(B) They had established themlves in cities.
(C) They had started to communicate through written language.
(D) They had created bronze tools and weapons.
26. The word "Some" in line 14 refers to
桃子的简笔画
爱情片韩国(A) Archaeologists
(B) Sumerians
(C) years
(D) clay tablets
27. Which of the following can be inferred from the passage concerning the Sumerians?
(A) They were descendants of the Persians.
(B) They were the first people to cultivate the valley of the Tigris.
(C) They were accomplished musicians.
(D) They had the beginnings of an economy.
28. According to the passage, when did archaeologists began to be able to understand tablets inscribed in Sumerian?
(A) In the early nineteenth century
劳动力调查(B) More than 150 years ago
(C) After the 1890's
(D) In the mid-eighteenth century
29. According to the passage, in what way did the Sumerian language remble ancient Greek and Latin?
(A) It was invented in Mesopotamia.
(B) It became well established around 3000 B.C.
(C) It became a classical language.
北京大学心理学(D) It was ud exclusively for business transactions.
无花果的营养价值30. The word "excavating" in line 19 is clost in meaning to
