112 SOILS. 



hence the name of " bunch grass " commonly used, which how- 

 ever means not any one definite kind of grass, but serves to 

 distinguish the grasses of the uplands from those of the moist 

 lowlands, where true sward may be found. Between these 

 bunches of grass the soil is fully exposed, and being free from 

 roots and leaf-covering is compacted, unless its nature is such 

 that the usually gentle rains do not produce a serious crusting 

 of the surface. 



That such is actually the predominant nature of the soils 

 formed under arid influences has already been stated ; and 

 thus the hard-baked soil-surface so often seen in the Eastern 

 United States in unplowed bare land, or during the prevalence 

 of a drought, is rarely seen in the arid region. The clay lands 

 that do exist are usually sufficiently calcareous to possess the 

 property of " slaking " into crumbs whenever wetted after dry- 

 ing. But where this is not the case, the stony hardness brought 

 about by the long dry and warm season is long in being re- 

 moved by the winter rains. 



Charges of soil-volume on wetting and drying. The be- 

 havior of colloidal clay in the above respects has already been 

 described above (see chapt. 4, page 59). It is obvious that 

 whenever soils contain a large proportion of such clay, their 

 behavior on wetting and drying will approximate to those of 

 the pure clay. This is exemplified in the heavy clay, or so- 

 called " prairie soils >! of the United States, which when 

 thoroughly wetted in spring will, during a dry summer, form 

 wide, gaping cracks. These in the long summers of the arid 

 region may extend to the depth of several feet, with a width of 

 as much as three and more inches at the surface of the ground. 

 This, of course, contributes greatly to the drying-out of the 

 soil to the same depth, and results as well in the mechanical 

 tearing of the root-system of growing plants; sometimes 

 causing the total destruction of vegetation. In some clay soils 

 it happens that after a rain or irrigation, the shrinkage occur- 

 ring upon the advent of warm sunshine will cause the surface 

 crust to so contract around the stein, e. g., of grain, as to con- 

 strict and injure the bark, causing serious injury to the cn^. 

 In soils of this character very thorough tillage in preparing 

 for a crop, and the maintenance of a loose surface during its 

 growth, are of course extremely essential. 



