Phosphates Their Sources 77 



water, and solvent substances in the soil. Furthermore, 

 the phosphate of the animal bone is always a phosphate 

 of lime, which, while not soluble, is in itself more readily 

 attacked by the natural agencies than a mineral phos- 

 phate which has associated with the bone phosphate 

 other minerals that are not readily attacked by those 

 agencies. That is, the mineral phosphates, while they 

 are made up chiefly of phosphate of lime, are associated 

 with other minerals, as iron and alumina, that are more 

 slowly attacked than the phosphate of lime itself, and to 

 some extent, too, prevent the full effect of the solvents, 

 rather than encourage their action, as is the case with 

 bone. 



Influence of fineness of division. 



In the second place, fineness of division has an im- 

 portant bearing upon availability, since the finer the 

 substance is ground, the greater will be the surface area 

 exposed to the natural agencies which cause decay. 

 Thus the application of a coarsely ground phosphate 

 may not show any results the first season, while the same 

 substance ground to a powder may have a good effect 

 the first season ; that is, its fineness permits of the solu- 

 bility of a considerable portion of its phosphoric acid. 



The character of soil as a factor influencing availability. 



In the third place, the kind of soil to which the phos- 

 phate is applied may influence the rate at which the 

 plants may obtain it. A soil which is open and porous, 

 and thus permits the free access of air and circulation of 

 water, and one which contains a large portion of other 

 matter capable of decay, vegetable or animal, presents 

 more favorable conditions for the solubility of phosphates 



