356 



SOILS. 



render even large supplies of phosphoric acid inert and use- 

 less, by the formation of the totally insoluble ferric phosphate. 

 Aluminic hydrate probably acts in a similar manner. The 

 following table gives examples in point, as regards ferric 

 hydrate. 



HAWAIIAN SOILS SHOWING HIGH CONTENTS OF FERRIC OXID. 

 (Kept. Cal. Exp. Sta. 1894-5, page 27.) 



Unavailability of Ferric Phosphate. It will be noted that in the soils 

 from Oahu with an overwhelming amount of ferric oxid (mostly in the 

 form of hydrate or rust) the citric acid has taken up only an insigni- 

 ficant amount of phosphoric acid ; nitric acid took up 40 to 50 times as 

 much, and chlorhydric doubled even this. In the much less ferruginous 

 Hawaiian soils, though containing more alumina, the citric acid ex- 

 tracted nearly ten times as much ; proving that it is chiefly ferric oxid, 

 and not the alumina as has been supposed, that causes the insolubility 



