The wide variety of climates in North America has helped spawn a complex pattern of' D" G. k. a) G4 p a( G# R: V
soil regions. In general, the realm’s soils also reflect the broad environmental partitioning
+ j3 I0 ?$ d6 J( @( Y$ t' i into “humid America” and “arid America.” Where annual precipitation exceeds 20 inches
2 b' @& o6 a8 ~" D# n& x' Y! V(50 centimeters),soils in humid areas tend to be acidic in chemical content, Since crops . x8 e9 o( R, @3 V5 i/ y
(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 the
1 p+ X4 Y6 k$ y+ G7 T4 d two. Arid America’s soils are typically alkaline and must be fertilized back toward* E+ Q" F" A: L
neutrality by adding acidic compounds. Although many of these dryland soils, particularly d3 `" |7 B, o2 n
in the Great Plains, are quite fertile, European settlers learned over a century ago that * Z8 Q( L- J# s' B) ^
(10) water is the main missing ingredient in achieving their agricultural potential. In the ' e- X) u1 Z3 ] w2 D; `; g$ |" G: ~
1970’s, certain irrigation methods were perfected and finally provided a real opportunity
4 Y- n+ a: L4 U2 i: w0 D5 \ to expand more intensive farming west from the Central Lowland into the drier portions$ i1 c2 F7 ? S( ?* u& a+ O
of the Great Plains. Glaciation also enhanced the rich legacy of fertile soils in the central6 k* _ `* h5 `" L0 `5 c" Y. g7 b! \
United States,both from the deposition of mineral-rich glacial debris left by meltwater
" j2 Y6 z: s" Q(15) and from thick layers of fine wind-blown glacial material, called loess, in and around the
" ?9 `+ q7 i" a4 J: [6 t# } middle Mississippi Valley.
0 A% E/ e' X- u% [! i- | Natural vegetation patterns could be displayed on a map of North America, but the
& B7 W3 n* y4 O. z& V8 F enormous human modification of the North American environment in modern times has8 @' X% e. H9 W5 [% }, F' S
all but reduced this regionalization scheme to the level of the hypothetical. Nonetheless,, b- e0 t4 l& O6 m
(20) the humid America-arid America dichotomy is still a valid generalization: the natural j) ?9 K( Q2 _6 v
vegetation of areas receiving more than 20 inches of water yearly is forest, whereas the
! n- q' r6 G) W8 g" r. B0 R# r drier climates give rise to a grassland cover. The forests of North America tent to make% p2 M/ z" H {7 i/ C" }
a broad transition by latitude. In the Canadian North, needle-leaf forests dominate, but ! m4 z+ G6 i8 @. b2 `
these coniferous trees become mixed with broadleaf deciduous trees as one crosses the
) P3 u" h6 v/ P" z' I. C(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
" y) p5 r6 p n0 n* E$ R0 Y stepper. The only areas of true desert are in the Southwest.
: k6 K1 S: j' L/ s, C , ~8 k' x4 w$ @+ W. _# e
21 What aspect of North America does the passage mainly discuss?# p" q0 o. y8 a8 x1 Z. X g
(A) The wide variety of climates
: X8 M( A8 n8 L(B) Soil types and vegetation patterns
9 v/ E( H% Q s- o9 K& `(C) Improved irrigation methods and the expansion of agriculture
* E4 ]4 }( t6 w# K* Q7 M(D) The change in precipitation patterns
- R# M( g+ ?6 U5 F$ D. ]" ~5 C3 ^22 The word “spawn ” in line 1 is closest in meaning to 3 U" k4 h# ?' ^! U2 J
(A) distinguish6 y' i# ]! Y+ m/ N8 Q
(B) eliminate+ i) D: R6 E6 ?/ T
(C) protect
/ x, U6 e" a- u0 Y9 u+ L" G6 f(D) create |
发表于 2012-8-14 22:34:35
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