SOILS IN DRY AREAS 71 



is also considerably higher in those river basins than on 

 the benches and the rainfall is usually somewhat less. 

 These conditions add to the difficulty of getting good 

 crops from such soils in the absence of irrigation. 



Volcanic ash soils. Volcanic ash soils, as the name 

 implies, are composed of very fine particles resembling 

 ashes in their fineness and in the ease with which they 

 may be dissolved and carried away by the action of the 

 water. They owe their existence to the action of vol- 

 canoes in eruption in primeval centuries. The particles 

 which compose them are very fine, finer than the par- 

 ticles found in silt soils. These, more than any other 

 soils found in the west, are flocculated in character, which 

 means that the exceedingly fine particles which compose 

 them are gathered together into little flocks, as it were, 

 through the action of lime, which tends to bind them 

 together. Were it not for this binding process, plants 

 would be unable to live in the soil. Organic matter 

 also helps these soils by keeping asunder the particles 

 of the same. 



These soils cover considerable areas of the far west- 

 ern states, especially of the Inter-mountain regions. 

 They are found not only in the valleys, but also on the 

 higher elevations. In many instances they are found 

 without intermixture and of much depth. In other in- 

 stances they are more or less mixed with the substances 

 which tend much to modify their character. 



Volcanic ash soils are exceedingly high in the ele- 

 ments of plant food, especially in the mineral elements 

 of the same. They have great wearing power, and as a 

 result under fair treatment will grow many successive 

 crops without showing any indications of a waning fer- 

 tility. They are also easily tilled. They do not bake 

 readily in the sense in which hard clay soils bake, but 

 they do incrust 'on the surface more or less after rain, 

 as nearly all soils do that are low in organic matter. 



