MAXTKES. 133 



which is that on a soil of tolerable fertility we can 

 grow large crops of any particular plant by using 

 such manures as are chiefly required for its ashes, as 

 phosphoric acid for a crop of wheat, for instance, 

 or potash for potatoes or tobacco. 



As a universal rule, it may be stated that to ren- 

 der a soil fertile for any particular plant, we must 

 supply it (unless it already contains them) with those 

 matters which are necessary to make the ash of that 

 plant ; and, if we would render it capable of pro- 

 ducing all kinds of plants, it must be furnished with 

 the materials required in the formation of all Xv'/^/.y 

 of vegetable ashes. 



To carry out this system, however, with much 

 nicety or certainty, would require a more thorough 

 knowledge of the composition of the soil and of the 

 feeding of plants than we yet possess. The only 

 safe rule is, by the use of manures and of thorough cul- 

 tivation, to make the soil fertile for all crops ; and 

 then to keep it fertile by the return of all mineral 

 matters removed in its produce. 



A long acquaintance with any field will show 

 its strong and its weak points, and the greatest skill 

 of the farmer should be applied to strengthening its 

 weaker ones and preventing its stronger ones from 

 becoming weaker. In this way the soil may be raised 

 to its highest state of fertility, and be fully maintained 

 in its productive powers. 



2d. Those manures which render available the 

 matters already contained in the soil. 



Silicic acid, (or sand,) it will be recollected, exists 



