386 



SOILS. 



binations other than kaolinite, viz., hydrous silicates, or soil zeolites, 

 formed either with the bases other than alumina shown in the analysis 

 or, more probably, containing some of the alumina itself in essential 

 combination. 



We are thus enabled to obtain from the determination of the soluble 

 silica an estimate of the extent to which these soil zeolites, that form 

 so important a portion of the soil in being the repositories of the reserve 

 of more or less available mineral plant-food, are present in the soils of 

 the several regions. A glance at the table shows that the general 

 average of soluble silica is very much greater in the soils of the arid 

 regions than in those of the humid, approximating one to two in favor 

 of the arid division. 1 



Differences in the Sands of the Arid and Humid Regions. 

 In chapter 5 mention has been made of the fact that while 

 in the humid regions, " sand " as a rule means quartz grains, 

 mostly with a clean surface and very frequently rounded and 

 polished, in the arid regions even the coarse sand grains consist 

 of, or are covered with, a great variety of minerals in a parti- 

 ally decomposed condition. This is owing to the absence of 

 the abundant rainfall which in humid climates continually 

 washes down the finely divided, half-decomposed mineral mat- 

 ter into the subsoil ; while in arid climates the light rains can- 

 not ppiduce any such washing effect and hence the sand grains 

 remain incru.-ted with the products of either their own decom- 

 position, or of that of neighboring particles; it being therefore 

 not concentrated in the finer portion only, viz., the clay and 

 finest silts. This fundamental difference, which is illustrated 

 in the analytical table below, at once explains why in the arid 

 regions generally, sandy soils are found so highly productive 

 that, owing to their easy cultivation they are preferred to the 

 clayey lands, in which tillage and irrigation are more difficult. 



1 Looking at the details of the several states, we find that on the arid side 

 Washington has a relatively low figure for soluble silica, which in the average, h>\v 

 ever, is overborne by the high figures for California and Montana. The explana- 

 tion of this fact probably lies in the derivation of the majority of the Washington 

 soils examined, from lake deposits brought down gradually from the humid region 

 at the heads of the Columbia drainage, where sandy beds are very prevalent, 

 while the country rock the basaltic eruptives are very basic, and moreover slow 

 to disintegrate. In California and Montana the rocks are infinitely varied, and the 

 general outcome of their weathering is plainly a predominance of complex hydrous 

 silicates in the soils, as compared with humid regions. 



