The wide variety of climates in North America has helped spawn a complex pattern of3 |0 J2 O7 [' e6 ^7 M2 A6 P" j
soil regions. In general, the realm’s soils also reflect the broad environmental partitioning l; s. ]0 e" q9 I; b/ w
into “humid America” and “arid America.” Where annual precipitation exceeds 20 inches- J @& P* o: I% `8 G: D
(50 centimeters),soils in humid areas tend to be acidic in chemical content, Since crops
' V' R9 D$ a% e, x(5 ) 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 the5 _. Z6 R4 Q, A; [0 z
two. Arid America’s soils are typically alkaline and must be fertilized back toward& o C- ?# o' G/ o( q% e! ?( L
neutrality by adding acidic compounds. Although many of these dryland soils, particularly
: n4 K: v/ \8 w% h3 a in the Great Plains, are quite fertile, European settlers learned over a century ago that
6 k% o. J% B3 {- w) J(10) water is the main missing ingredient in achieving their agricultural potential. In the
& S, t" M4 \: ?1970’s, certain irrigation methods were perfected and finally provided a real opportunity
: c7 i$ {3 k5 ~6 _ to expand more intensive farming west from the Central Lowland into the drier portions; q" u3 `# v. c' }6 K3 c0 x
of the Great Plains. Glaciation also enhanced the rich legacy of fertile soils in the central
( o) J' o a- T United States,both from the deposition of mineral-rich glacial debris left by meltwater
( w9 ^5 ?. ~& x! Z(15) and from thick layers of fine wind-blown glacial material, called loess, in and around the: c/ q7 P5 J3 p& C2 p
middle Mississippi Valley.
+ R& X- U& g y Natural vegetation patterns could be displayed on a map of North America, but the2 [0 ?$ O- y( @! `8 U
enormous human modification of the North American environment in modern times has
$ E6 I5 F! R/ @$ ]' B" m all but reduced this regionalization scheme to the level of the hypothetical. Nonetheless,( T' D3 n# k. l y) e
(20) the humid America-arid America dichotomy is still a valid generalization: the natural
5 n) n6 v+ z% H8 b$ }! nvegetation of areas receiving more than 20 inches of water yearly is forest, whereas the
# }7 P$ |( `9 W2 h$ Y& A5 u& B drier climates give rise to a grassland cover. The forests of North America tent to make& s7 `; _$ ?% a1 r/ R
a broad transition by latitude. In the Canadian North, needle-leaf forests dominate, but
5 g1 a0 k U- R) B0 lthese coniferous trees become mixed with broadleaf deciduous trees as one crosses the
( S% ?: v, q" d$ V r+ ?; M/ {(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
# [* x# M; H" `' h stepper. The only areas of true desert are in the Southwest.- r D" o: v4 z. d; B
+ T& P% U' A2 n, {" D! M0 V( r
21 What aspect of North America does the passage mainly discuss?: @. w$ a* V$ F! @% q5 ^; `
(A) The wide variety of climates# u& \( r, A( }' u
(B) Soil types and vegetation patterns0 n& S: k* l% |, b
(C) Improved irrigation methods and the expansion of agriculture
# f% [8 r3 }/ ~3 r6 Q(D) The change in precipitation patterns) g% ^/ {1 u$ _
22 The word “spawn ” in line 1 is closest in meaning to
3 y# C3 P K% T$ {8 E(A) distinguish% G6 W5 c) w, Q& I9 k, p0 B
(B) eliminate' V) b+ E! `% t5 G A& ]7 o
(C) protect
2 m: f1 l( ^$ l(D) create |
发表于 2012-8-14 22:34:35
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