20 



SUMMARY 



So far as the writer has been able to ascertain, the literature 

 cited above includes practically all the work relating directly to 

 the subject under investigation. Briefly stated, the following 

 conclusions have been drawn from the work referred to : 



I. When soluble phosphates are added to soils the phos- 

 phoric acid is usually changed to insoluble forms. The action in 

 most cases is due to chemical change, by virtue of the presence 

 of compounds of iron, aluminum, and calcium, and possibly 

 magnesium. These compounds are easily decomposable salts 

 which can be extracted with hydrochloric acid. 



II. Hydrated oxides of iron and aluminum, and carbonate 

 of calcium unite with phosphoric acid very rapidly. The phos- 

 phates of iron and aluminum formed are from two to five times as 

 insoluble in pure water as calcium phosphate. They are also less 

 soluble in water containing carbon dioxide and various alkali 

 salts. 



III. It is the tendency of phosphoric acid to form the more 

 difficulty soluble compounds in the soil, but not all the phosphoric 

 acid appears to be so combined, nor is such combination always 

 effected with with great rapidity. The complete fixation in 

 some cases does not take place for several months. 



IV. Mixed with calcium carbonate, in the presence of soil 

 water, phosphates of iron and aluminum undergo decomposition 

 with the formation of calcium phosphate. With certain soils, 

 calcium hydroxide also apparently has a solvent action on the 

 phosphates of iron. Conditions affecting the equilibrium between 

 these compounds have not been determined. 



V. Absorption may be influenced to some extent by the 

 presence of some of the alkali salts. The effect is not very well 

 defined. In some cases the solubility of the soil phosphoric acid 

 seems to be increased in the presence of these salts. 



VI. In some cases the humus in soils has the effect of ren- 

 dering insoluble phosphates more available to plants. The action 



