!62 SOILS. 



within the humic surface soil, and is often carried off in 

 amounts sufficient to obstruct drain tiles by its deposition in 

 contact with air (see chapt. 3). In the case of moderate rains, 

 however, it is carried no farther than the subsoil, and is there 

 redeposited, in consequence of the penetration of air, following 

 the water, and causing the carbonic gas to diffuse upward; 

 thus leaving the lime carbonate behind. In the majority of 

 cases this results simply in a gradual enriching of the subsoil in 

 this substance; while the surface soil may become so depleted 

 as to require its artificial replacement by liming or marling. 

 The same general process occurs to a less extent, in the case of 

 magnesia. 



Calcareous Subsoils The fact that subsoils are more cal- 

 careous than the corresponding surface soils is often of great 

 practical importance, in enabling the farmer to enrich his de- 

 pleted surface soil in lime by subsoil plowing. The accumula- 

 tion of lime carbonate in the subsoil also tends in a measure to 

 offset the extreme heaviness sometimes resulting from the ac- 

 cumulation of clay. 



Calcareous Subsoils and Hardpans. When soils are very 

 rich in lime, and rains occur in limited showers rather than con- 

 tinuously, the lime carbonate dissolved from the surface soil 

 may accumulate in the subsoil so as to either form calcareous 

 " hardpan " by the cementing of the subsoil mass; or it may 

 accumulate and partly crystallize around certain centers and 

 thus form white concretions, known to farmers as " white 

 gravel." The latter is the form usually assumed in the re- 

 gions of summer rains ; while in the arid regions the deficient 

 rainfall causes this substance to accumulate, and calcareous 

 hardpan to form, at definite depths depending upon the maxi- 

 mum penetration of the annual rainfall ; sometimes in crystal- 

 line masses of veritable limestone ("kankar" of India), or 

 sometimes merely as crystalline incrustations loosely cementing 

 the subsoil. 



"Rawness" of Subsoils in Humid Climates. From the 

 greater compactness of the subsoil which is almost universal in 

 the humid regions, the absence of humus and of the resulting 

 formation of carbonic and humic acids, it follows that its 

 minerals are less subject to the weathering process than are 



