356 THE EEGOLITH 



use of fertilizers on an extensive scale, has been mneh more 

 rapid and effective in the hot climates of the equatorial belts, 

 thus rendering available so large a proportion of the soil's in- 

 trinsic stores of plant food that the need of artificial fertilization 

 is there restricted to those soils of which the parent rocks were 

 exceptionally deficient in the mineral ingredients of special 

 importance to plants that ordinarily form the essential material 

 of fertilizers. ' ' ^ 



Concerning the concentration and leaching out of certain con- 

 stituents by the action of meteoric waters, the same authority 

 says: 



**When, however, the rainfall is either in total quantity or in 

 its distribution insufficient to effect this leaching, the sub- 

 stances which otherwise would have passed into the sea are 

 wholly or partially retained in the soil stratum, and when in 

 sufficient amount may become apparent on the surface in the 

 form of efflorescences of 'alkali' salts. One of the most im 

 portant modifications produced by scantiness of rainfall on soil 

 formation is the great retardation of formation of clay from 

 feldspathic rocks (kaolinization) and the sediments derived 

 therefrom. As a result, it is observed that the soils of the 

 Atlantic slope are prevalently loams, containing considerable 

 clay, and even in the case of alluvial lands, oftentimes very 

 heavy, while the character of the soils of arid regions is pre- 

 dominantly sandy or silty with but a small proportion of clay, 

 unless derived directly or indirectly from clay or clay shales. 

 In the former case, the clay, becoming partially diffused in 

 the rain water when a somewhat heavy fall occurs, percolates 

 through the soil in that condition and tends to accumulate in 

 the sub-soil, the result being that almost without exception, 

 the sub-soils of the humid regions are very decidedly more 

 clayey than the corresponding surface soils. Not only does 

 this clay water tend to make the sub-soil more compact and 

 heavy, making it less pervious to water and air, but it is as- 

 sisted materially in this by the action which tends to leach the 



^ WMle the action of frost in bringing rock masses into the condition of 

 soil is, in temperate climates, of very great importance, there seems to be 

 a limit beyond which it accomplishes little in the way of directly promoting 

 decomposition, and presumably disintegration as well. Collier's (8th Ann. 

 Eep. New York Exp. Station, 1889) experiments showed that 47 successive 

 freezings and thawings of a soil did not perceptibly increase the percentage 

 pf soluble potash. 



