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INTRODUCTION TO PLANT GEOGRAPHY 



[chap- 



is under either cultivation or rotation cropping is computed to stand 

 at some 478 million acres, or about one-quarter of the total land area. 

 Perhaps another 238 million acres could be added to Classes I to 

 IV lands (see Chapter XVII) that are at present neither cultivated 

 nor in rotation cropping but are yet capable of being cropped, 

 especially with intensive practices. But it has been estimated that 



Fig. 183. — The same as Fig. 182, two years later. Livestock have been excluded 

 by fencing, whereupon by natural succession a cover of soil-protecting vegetation 

 soon developed and controlled soil loss. Instead of a wasteful and dangerous 

 gully (cf. Fig. 181), the area was restored to potential usefulness. (Courtesy of 

 U.S. Soil Conservation Service.) 



already some 25 million acres originally suitable for cultivation have 

 been lost through water-erosion and soil-blowing, another ten 

 million acres have been lost through other types of soil deterioration, 

 and more than half of the remaining cropland has been damaged 

 (critically as to 121 million acres and seriously as to 128 million 

 acres) by one or another of these scourges (according to the 1954 

 figures given out by the U.S. Department of Agriculture's Soil 

 Conservation Service). Other computations have indicated that 

 more than one-third of the productive top-soil of the United States 



