FOOD rilYSICALLY AND CHEMICALLY COMBINED. 71 



and pliosphoric acid in two difTcrcnt forms, namely, in 

 chemical and in physical combination : in the one form, 

 infinitely difiused over all the surface of the porous par- 

 ticles of the soil ; in the other, in the shape of granules 

 of phosphorite, or apatite and felspar, very unequally 

 distributed. 



In a soil abounding in silicate and in phosphate of lime, 

 which has for thousands of years been exposed to the dis- 

 solving action of water and carbonic acid, the component 

 particles vnll be found every^vhere physically saturated 

 'vvitli potash, ammonia, sihcic acid, and phosplioric acid ; 

 and it may occur, as in the case of the so-called Kussian 

 black-earth, that the phosphate of lime dissolved but not 

 absorbed is deposited again in concretions, or in a crystal- 

 hne form in the subsoil. 



In this state of physical combination the alimentary 

 substances are manifestly in the most favourable condition 

 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 readmess as if these substances were in 

 solution m 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 solution 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 subsoil. 



Nothing can be more certain than that the mechanical 



