1 66 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



Ammonia-Soluble Phosphoric Acid. The ammonia-soluble phos- 

 phoric acid of the soil is assumed by some chemists to be in com- 

 bination with the organic matter which accompanies it, and to be 

 of considerable value. It comes in part, however, from inorganic 

 phosphates which occur in the soil and are decomposed by am- 

 monia, allowing a portion of their phosphoric acid to go into solu- 

 tion. 1 The humic acids precipitated with acids contain a small 

 portion of the ammonia-soluble phosphoric acid, and this is prob- 

 ably in organic combination. The quantity of phosphoric acid held 

 in this way is small, even in soils richly supplied with humus. It 

 cannot be taken up until released by oxidation of the humus, and 

 can only be regarded as a reserve store of phosphoric acid, not 

 nearly so important as the nitrogen held in the humus in much 

 larger proportion. 



1 Fraps, Am. Chem. Jour., 1898, p. 574; Bulletin 135, Texas Exp. Sta. 



