242 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



PHOSPHORIC ACID DISSOLVED. l 



( i gram substance to 400 cc. water. ) 



Parts per million 



From calcium phosphate 43 



From ferric phosphate 42 



From basic ferric phosphate 33 



From aluminium phosphate 58 



From basic aluminium phosphate 4 



Decaying organic matter, by reducing ferric to ferrous phos- 

 phate, and perhaps by combination with the phosphoric acid, tends 

 to render phosphoric acid available. 



The slow process of weathering breaks up the complex silicates 

 of the soil grains and releases the compounds of phosphoric acid 

 therein. At the Rothamsted Experiment Station, where wheat 

 has been grown with various fertilizers since 1852, the amount of 

 phosphoric acid soluble in citric acid was as follows, on plots 

 that did, and did not, receive phosphoric acid. 



Pounds per acre 



The first plot produced on an average 12^4 bushels wheat con- 

 tinuously, but the weathering was sufficient to maintain nearly 

 constant the quantity of phosphoric acid soluble in citric acid. In 

 the other case (producing 24 bushels wheat) the addition of 

 fertilizers made a considerable increase in the soluble phosphates 

 present. The weathering of the minerals constantly releases the 

 phosphoric acid inclosed therein. In the presence of decomposable 

 compounds of iron, and aluminium, there is a constant tendency 

 for the phosphoric acid to change to less soluble forms. Calcium 

 phosphate is dissolved by the soil water, brought in contact with 

 iron and aluminium oxides and unites with these to form phos- 

 1 Bureau of Soils, Bulletin, 41. 



