SOILS OF THE ARID AND HUMID REGIONS. 



397 



more abundant in the soils of the arid region than in those of 

 the humid, sometimes to the extent of influencing injuriously 

 the quality of certain crops, such as tobacco and sugar beets. 

 Nevertheless, nitrogen is ordinarily, in the arid region, the sub- 

 stance requiring replacement next to phosphoric acid. And 

 when considered in connection with the small humus-content, 

 so liable to burning-out, this places green-manuring with le- 

 guminous plants among the first and most vital improvements 

 to be employed there. 



The Transition (semi-humid or semi-arid) Region. The 

 sloping plains country lying between the Rocky Mountains 

 and the Mississippi, quite arid at the foot of the mountains, 

 but with rainfall increasing more or less regularly to eastward, 

 form a transition-belt between the arid and humid region of 

 which but a small portion has been systematically studied in 

 respect to its soil formations. The analyses made of soils of 

 the two adjacent states of Minnesota and North Dakota, have 

 been placed in the general table (p. 377) to show how far in 

 their general relations their soils correspond to the generaliza- 

 tions deduced from the comparison of the decidedly arid and 

 humid soil areas chiefly represented in the table. Although it 

 has not been possible, for lack of detailed data, to eliminate 

 the soils originating from calcareous formations, it will be 

 seen that those of semi-arid Dakota differ from those of more 

 humid Minnesota, almost throughout, as would be anticipated 

 from the studies of the extremes, given in this chapter. 



