CHAPTER XX. 



SOILS OF THE ARID AND HUMID 1 REGIONS. 



Composition of Good Medium Soils. In the preceding 

 tables examples have been given of rather extreme types of 

 soils, both rich and poor throughout, and also of such as are 

 deficient in one or several of the important ingredients. In the 

 table below are given the analyses of some of the good aver- 

 age farming lands ; uplands of several states, both of the humid 

 and arid regions. In the former, the representative timber 

 trees of such lands are the black, red, white and (less charac- 

 teristically) the post, black-jack, Spanish, overcup and locally 

 some other oaks ; grading higher in proportion to the presence 

 of more or less hickory, and lower as the latter is replaced by 

 pine. In the states south of Ohio, the " oak and hickory up- 

 lands " are what the farmer usually looks for, outside of the 

 valleys or bottoms. 



Criteria of Lands of the Two Regions. In the country west 

 of the Rocky Mountains, the timber, while locally very char- 

 acteristic, cannot be as broadly used as a criterion, partly on 

 account of its scarcity, partly because the dominant factor in 

 the growth of trees is moisture, which is measurably independ- 

 ent of chemical soil-composition. The latter, moreover, on ac- 

 count of climatic conditions, already alluded to (chapter 16), 

 does not vary as materially in the arid as the humid region, on 

 account of the almost universal presence of larger proportions 

 of lime carbonate ; the variations of which in the humid region 

 govern largely the vegetative changes. For we there find the 

 timber grozvth of the lowlands ascending into the uplands so 



1 In the discussion in this chapter the " humid region " referred to is always that 

 of the temperate zones, unless expressly otherwise stated. The most humid region 

 of all the tropics is treated under a special head. 



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