FOOD PHYSICALLY AND CHEMICALLY COMBINED. 81 



the dissolving action of water and carbonic acid, the 

 component particles will be found everywhere physi- 

 cally saturated with potash, ammonia, silicic acid, and 

 phosphoric acid ; and it may occur, as in the case of the 

 so-called Russian black-earth, that the phosphate of 

 lime dissolved but not absorbed is deposited again in 

 concretions, or in a crystalline form in the subsoil. 



In this state of physical combination the alimentary 

 substances are manifestly in the most favourable condi- 

 tion to serve as food for plants; for it is clear that the 

 roots, in all places where they are in contact with the 

 soil, will find the necessary nutritive substances in the 

 same state of diffusion and readiness as if these substan- 

 ces were in solution in water, but at the same time not 

 movable of themselves, and retained in the soil by so 

 slight a force that the most trifling dissolvent cause 

 brought to bear upon them suffices to effect their solu- 

 tion and transition into the plant. 



If it is true that the roots of cultivated plants have 

 no inherent power to overcome the force which retains 

 together potash and silicic acid in the silicates, but that 

 those elements of food only which are in physical com- 

 bination with the soil can be taken up and made avail- 

 able for nutriment, this explains the difference between 

 cultivated and uncultivated ground, or barren sub- 

 soil. 



Nothing can be more certain than that the mechan- 

 ical treatment of the soil and the influence of the 

 weather serve to strengthen the causes which bring 

 about the disintegration and decomposition of the 

 minerals, and the uniform distribution of the elements 

 of food contained in them and rendered soluble. The 

 elements chemically combined in the minerals, are re- 

 leased from that combination, and in the arable S'oil 

 gradually resulting from this decomposition acquire the 

 form in which they are available as food for plants. It 

 is evident that only by degrees the rough ground can 

 attain the properties of arable soil, and that the time 

 required for this change depends upon the quantity of 

 nutritive substances present, and upon the obstacles 



