insoluble. These results were construed as proving the rever- 

 sion of phosphoric acid by soils to insoluble phosphate of lime. 

 In another experiment precipitated phosphates of iron and 

 aluminum were added to soils with the result that none of the 

 phosphoric acid became insoluble in ammonium citrate during 

 eight days. 



Petermann 1 attempted to determine the crop producing 

 power of these various forms of phosphoric acid. The "citrate 

 soluble" was in many cases superior to the "water soluble," 

 though the results were not altogether satisfactory. The low 

 value of the water soluble was due apparently to leaching, while 

 the citrate soluble, being insoluble in water, was retained for the 

 use of the plant. 



Solubility determinations of the compounds supposed to 

 exist in superphosphate were made by Albert and Wagner 2 . Di- 

 calcium phosphate was found to be soluble in water charged with 

 carbon dioxide. Sulphates and chlorides appeared to have very 

 little solvent action, while nitrates and carbonates were more 

 effective. The solubility of precipitated phosphates of iron and 

 aluminum in carbon dioxide water was a great deal less than 

 that of precipitated calcium phosphate. Drying these salts les- 

 sened their solubility. Regarding the absorption of phosphoric 

 acid by soils they found a solution of the phosphates in carbon 

 dioxide water similar in its action to a solution of "soluble phos- 

 phoric acid. ' ' Absorption was most complete in clays poor in 

 lime, less complete in calcareous clays and organic soils, and 

 very small in sands poor in lime. An increase of humus in the 

 soils rich in lime and clay was always accompanied by an increase 

 in absorptive power. 



In contradiction to the work just quoted, Fiedler 3 states that 

 solutions of nitrates dissolve less phosphoric acid from soils than 

 pure water. His results showed absorption to be favored silghtly 

 by sodium nitrate. A solution of sodium nitrate did not affect 

 the solubility of tri-calcium phosphate but it acted on di-calcium 

 phosphate and phosphates of iron and aluminum. 



1 Bied. Centr., 1880, 87. 



2 Ibid., 1880, 640. 



3 I y andw. Versuchs-Stat., 26, 135. 



