THE MINERAL CONSTITUENTS OF THE SOIL SOLUTION 51 



exist in mono-calcium phosphate, CaH 4 (PO 4 ) 2 .H 2 O. Along the 

 line CD the only solid which is stable and can continue to per- 

 sist is the dicalcium phosphate. From the point C the composi- 

 tion of the stable solid varies continuously with the concentra- 

 tion of the liquid solution. Therefore, these solids form a 

 series varying in composition from pure dicalcium phosphate 

 to pure calcium hydroxide. One of these basic phosphates, as 

 they would ordinarily be called, has a less solubility than any 

 other, as indicated by the point B. All solutions to the right of 

 the point B have an acid reaction, while all solutions to the left 

 possess an alkaline reaction. It follows from these facts that 

 if we start with any lime phosphate corresponding to some 

 point to the right of B and dilute it, or what amounts to 

 the same thing in case it has been added to the soil, if we 

 leach it, phosphoric acid will go into solution more rapidly 

 than will lime until the composition of the residue is that of 

 the basic phosphate stable at B: Similarly, if we start with a 

 phosphate more basic, lime will be removed more rapidly than 

 phosphoric acid, until the residue has the composition of the 

 phosphate of lowest solubility. From this point, with continued 

 leaching, the lime and phosphoric acid will dissolve in a definite 

 ratio, which ratio is obviously that of the phosphate of least solu- 

 bility. That is to say, if the leaching process is slow, as would 

 be the case under soil conditions, the solution would have a 

 perfectly definite concentration with respect to lime and phos- 

 phoric acid. What the ratio of lime to phosphoric acid may 

 be, is of no particular interest in this connection, but the order 

 of concentration of phosphoric acid is of interest. Owing to 

 serious analytical difficulties, this has not yet been determined 

 with any great precision, but by interpolating on the experiment- 

 ally determined curve AC, this concentration is found to be some- 

 where in the neighborhood of 5-10 parts per million, figures 

 close to those obtained for the concentration of the soil solution 

 with respect to phosphoric acid by the previously described -in- 

 vestigations. 



Under ordinary circumstances, however, it is not probable that 

 lime is the dominant base controlling the concentration of phos- 



