64 ON INCREASING THE GROWTH 



Any stone which contains phosphoric acid in con- 

 siderable quantity may be turned to account, as an 

 excellent addition to other composts, and more par- 

 ticularly to such as are rich in nitrogen. In England 

 many such minerals, which have already imparted 

 great benefit to agriculture, have recently been dis- 

 covered ; and in Germany, also, some of similar 

 character might certainly be found, were they but 

 sought for. 



5. Lime and Magnesia. Both these earths are in- 

 deed absolutely indispensable to the growth of plants, 

 but they belong to those mineral substances which 

 are most universally distributed over the earth, and 

 are accessible to the farmer wherever it supplies lime- 

 stone, chalk, marl, or gypsum, or where oyster-shells 

 can be obtained. Hence in all places he possesses 

 the means of providing an abundant supply of these 

 substances for the cultivation of his plants. On the 

 numerous and diversified benefits which they impart, 

 — partly as direct means of nourishment, partly as 

 agents which improve or loosen the ground, or, on 

 the other hand, as substances which neutralize acids 

 or hasten decay, — more specific information will be 

 given in our observations upon mineral composts. 



6. Silica. All plants contain silica ; many (as, 

 for example, the various species of grain in their 

 straw) in very considerable quantity. Hence we in- 

 fer that it is an essential constituent of plants, and 

 that they cannot dispense with it in their food. But 

 it does not of necessity follow that the farmer must 



