30 NATUEB AND FEOPEETIBS OP SOILS 



oxidation and hydration of a part of the combined iron ; and 

 a earbonation and solution of a large proportion of the soluble 

 bases. These processes are hastened and the whole mass 

 evolved into a soil by the admixture and decay of certain 

 amounts of organic matter. ' ' ^ 



21. Variation of soil formation with climate. — It may 

 be seen readily that the activity of the various soil-forming 

 agencies will fluctuate with climate. A comparison of weath- 

 ering and erosion in an arid and a humid region will illustrate 

 the point at issue. Under arid conditions, the physical forces 

 will dominate and the resultant soil will be coarse. Tempera- 

 ture chaiiges, wind action and the influence of animals will 

 be almost the sole agents. In a humid region, however, the 

 forces are more varied and practically the full quota will be 

 at work. Chemical decay will accompany disintegration and 

 the result will be shown in the greater fineness of the product. 

 The separate minerals will also show the change of color and 

 loss of luster so characteristic of chemical action. A granite, 

 for example, is a very insoluble rock, compared with a lime- 

 stone, and in a humid region, where chemical agencies are 

 dominant, it will be markedly more resistant. If, however, 

 these rocks are exposed in an arid region, where physical 

 weathering is potent, the results will be entirely different. 

 The limestone, being homogeneous, will not be affected mark- 

 edly by temperature changes, but the stresses set up in granite 

 must ultimately reduce it to fragments. 



Arid soils, besides being rather coarse, are generally rather 

 Jimlfiim, there being little difference between soil and subsoil. 

 The soils of humid regions are usually of fine texture, par- 

 ticularly in residual sections, since the chemical agencies have 



^It is well to remember that synthetic processes as well as forces of 

 simplification and dissolution are active in soil formation. The soil 

 features that result are of two kinds, hereditary and acquired. The 

 former develop through geological forces^ the latter through the activity 

 of true soil processes. 



