IQ6 THE FIRST BOOK OF FARMING 



plants so it is treated with oil of vitriol or sulphuric 

 acid to make it available. The phosphoric acid in 

 the ground rock is combined with lime, forming a 

 phosphate of lime which is insoluble. When treated 

 with the oil of vitriol or sulphuric acid, the sul- 

 phuric acid takes lime from the phosphate and forms 

 sulphate of lime or gypsum. The phosphoric acid 

 is left combined with the smallest possible amount 

 of lime and is soluble in water. It is then called 

 soluble or water soluble phosphoric acid. 



Now if this soluble form remains unused it be- 

 gins to take on lime again and turns back toward 

 its original insoluble form. After a time it gets to 

 such a state that it is no longer soluble in water but 

 is soluble in weak acids. It is then said to be re- 

 verted phosphoric acid. Reverted phosphoric acid 

 is also called citrate soluble phosphoric acid, because 

 in testing fertilizers the chemists use ammonium ci- 

 trate to determine the amount of reverted phos- 

 phoric acid. 



This form still continues to take on lime and by 

 and by gets back to the original insoluble form 

 called insoluble phosphoric acid. 



The soluble phosphoric acid and reverted phos- 

 phoric acid are available to plant roots. The in- 

 soluble form is not. 



The rock phosphates contain from 26 to 35 per 

 cent, of insoluble phosphoric acid. The acid phos- 

 phates or dissolved rock phosphates contain from 12 

 to 16 per cent, of available phosphoric acid and from 

 i to 4 per cent, of insoluble. 



