50 THE SOIL SOLUTION 



waters. Further attention will be given the matter in another 

 chapter. 



Another but quite different line of evidence as to the probable 

 concentration of the soil solution is furnished by the investiga- 

 tion of the solubility of certain phosphates. 1 It is popularly 

 supposed that when superphosphate containing mono-calcium 

 phosphate, CaH 4 (PO 4 ) 2 .H 2 O, is added to a soil there is a more 

 or less permanent increase of readily soluble phosphoric acid in 

 the soil, although a part "inverts" to the somewhat less soluble 

 dicalcium phosphate, CaH(PO 4 ).2H 2 O. Such probably is far 

 from a correct view of what actually takes place. The results 

 obtained by studying the solubility of the different lime phos- 

 phates in water at ordinary temperature (25 C.) can be ex- 

 pressed in a diagram similar to the accompanying sketch, which 

 is much distorted for convenience in lettering. As the diagram 

 indicates, when the concentration of the solution increases with 

 respect to phosphoric acid, the lime is at first less and less solu- 

 ble and the point represented by B is reached, then becomes 

 more and more soluble until the point D is reached, from then 

 on becoming less and less soluble, until the solution reaches a 



P 2 5 in solution 



Fig. 2. 



syrupy consistency. In contact with all solutions represented 

 by points on the line DH the stable solid substance which can 



1 For reference to the literature and detailed discussion see : The 

 action of water and aqueous solutions upon soil phosphates, by F. K. 

 Cameron and J. M. Bell, Bull. No. 41, Bureau of Soils, U. S. Dept. Agri- 

 culture, 1907. 



