47 



V. The presence of calcium carbonate in relatively large 

 amounts, appears to have no effect on the solubility, in am- 

 monium citrate, of the compound formed by fixation. 



VI. Insoluble humic acid, calcium humate, and insoluble 

 ammonium humate offer no resistance to the extraction of the 

 phosphoric acid of superphosphate by water. Soluble ammonium 

 humate forms with superphosphate solution, an insoluble com- 

 pound whose composition and properties have not been deter- 

 mined. The absorptive property of humus is due probably to 

 the humus compounds soluble in water. 



VII. Soluble ammonium humate does not effect solution of 

 precipitated tri-calcium phosphate. Insoluble humic acid renders 

 a portion of the phosphoric acid soluble in water. 



VIII. Some of the zeolites and similar hydrous double sil- 

 icates are capable of abstracting phosphoric acid from solution. 

 The property is not destroyed by strong ignition. The com- 

 pounds formed are comparatively soluble in i per cent citric acid. 



IX. The solubility of tri-calcium phosphate, ferric phos- 

 phate, apatite, and wavellite, in water and in i per cent citric 

 acid, was not increased by the addition of ferrous sulphate. 

 With tri-calcium phosphate, the solubility in i per cent citric acid 

 was decreased, indicating that the application of ferrous sul- 

 phate under some circumstances may be detrimental. 



X. Ferrous sulphate forms with superphosphate a compound 

 soluble in i per cent citric acid. In the presence of ferrous sul- 

 phate, and ferric hydroxide, the compound formed with super- 

 phosphate, has practically the same solubility in i per cent citric 

 acid as the compound formed when ferrous sulphate is absent. 

 The latter possesses therefore no especial value as a retentive 

 agent for phosphorics acid. 



