CHAP, n.] CONSTITUENTS OF SOILS. 21 



and hydrogen also by its roots, through the 

 medium of water ; and, as plants cannot be 

 maintained in a healthv condition unless they 

 are supplied freely with these elements, no soil 

 can be fitted for growing plants in that is not 

 sufficiently open to admit air, and allow water 

 to percolate through it. 



Carbon, and the elementary erases, are also 

 found more or less abundantly in the different 

 kinds of soil, by which they are yielded to the 

 plants ; and soils supply certain portions of 

 inorganic or earthy matter, which, though found 

 in plants in very small quantities, are abso- 

 lutely necessary to complete the process of 

 vegetation. 



All the different kinds of soil found on level 

 ground consist of two parts, which are called 

 the surface soil and the subsoil ; the subsoil 

 being what may be considered the natural rock 

 of the particular district ; and the surface soil 

 being compounded of dust crumbled from the 

 subsoil, and mixed with other mineral sub- 

 stances, and various animal and vegetable mat- 

 ters in a state of minute subdivision or decay. 



It would be useless to attempt to give an ac- 

 count of all the various kinds of subsoil; and, in 

 fact, such an account could only be given in a 

 work on geology. It will be sufficient for mv 

 present purpose to mention that the principal 

 rocks which enter into the composition of sub- 

 soils are the quartz rocks, which form the basis 

 of the siliceous or sandy soils; clay, on which lie 

 the argillaceous or clayey soils ; and limestone, 

 which forms the basis of calcareous or chalky 

 soils. 



