TRANSFORMATION OF MINERALS IN THE SOIL 



659 



and magnesium carbonates, the silicates and salts of organic acids, in 

 preference to the phosphate. The reaction of the medium has to be 

 distinctly acid (pH 3.0) for the phosphate to become soluble. Such 

 acidity is injurious to plant growth. Figure 50 shows the correlation 



H2SO4 





 cc. 



10 



20 



30 



40 



50 



60 





pH 52. 5.0 



4.0 



3.0 



2.0 



1.0' 



Fig. 50. Hydrogen-ion concentration at which rock phosphate becomes avail- 

 able (from Rudolfs). 



between soil reaction and transformation of tri-calcium phosphate. 41 

 When rock phosphate is used in place of tri-calcium phosphate, the 

 reactions involved are more complicated, since it contains aluminum 



41 Rudolfs, W. Influence of sulfur oxidation upon growth of soybeans and its 

 effect on bacterial flora of soil. Soil Sci., 14: 247-262. 1922. 



