THE COMPOSITION OF PLANTS. 



33 



66. The ash constituents, though small and vari- 

 able in quantity, are of prime importance in the 

 growth of the plant ; and just here we find the whole 

 theory of fertilizing. The moisture and volatile com- 

 pounds formed during combustion float off unseen 

 in the air when a plant is burned or decays, and, by 

 this same air where these compounds always exist, 

 they are carried to other plants and serve as proper 

 food, while the ash constituents can not be thus trans- 

 ported. Every plant removed from the soil carries 

 with it the elements of this ash, and, as some of these 

 elements exist in minute quantities, the soil is sure 

 to become exhausted where means are not used to 

 have them restored. 



67. The old system of carrying away and never 

 restoring these necessary elements has well been 

 called by Liebig a system of spoliation. Some 

 elements, such as iron and silicon, and, in miny soils, 

 calcium and sodium, are so abundant that, practically, 

 they may never become deficient, and yet they may 



