92 OP THE SUBSOIL. 



kind of plant, while a plant of another kind may find a 

 quantity of food amply sufficient for its perfect develop 

 ment. A third sort of plant may thrive upon the same 

 soil, after the second, if the remaining mineral constitu 

 ents are sufficient for a crop of it. And if, during the 

 cultivation of these crops, a new quantity of the sub 

 stance wanting for the first, for instance, of soluble silica 

 for Indian corn, has been rendered available by weather 

 ing, then, if the other elements be found in sufficient 

 quantity, the first crop may be again grown upon the 

 same land. 



CHAPTER X. 



OF THE SUBSOIL. 



310. Immediately below the soil lies the subsoil. It 

 may be and often is composed of the same kind of earth 

 as the proper soil ; or it may be entirely different. A 

 sandy soil may rest upon a subsoil of clay, or upon cal 

 careous rock, or rock of any other kind, or upon gravel. 



311. The influence of the subsoil upon vegetation is 

 often very great, especially when the soil is not deep 

 enough for the free growth of the roots of the plants cul 

 tivated. In that case, when the subsoil is of such a 

 nature as to admit of it, the soil should be deepened by 

 ploughing. This should be done gradually and with 

 judgment, because, as the subsoil has no mould or loam 

 in it ? turning too much of it up to the surface at once, 



