200 SCIENCE PROGRESS. 



solution of citric acid is, on the other hand, a far better 

 solvent of calcium phosphate in all its forms, and a worse 

 solvent of ferric and aluminic phosphate. 



A solution of ammonium citrate has been employed by 

 Petermann for many years to indicate the proportion of 

 available phosphoric acid in a soil ; or, as he expresses it, 

 to distinguish between the phosphates belonging to the 

 original rock, and the phosphates belonging to the residues 

 of previous manuring. Petermann finds that in well-manured 

 sandy soils or loams, from twenty-five to eighty per cent, of 

 the total phosphoric acid is soluble in alkaline ammonium 

 citrate ; while in schists and greywackes, only a few per 

 cent, of the total phosphoric acid are thus soluble. Gene- 

 rally a larger proportion of the phosphoric acid is soluble in 

 ammonium citrate in the case of arable than of virgin soils. 



Dr. Dyer describes experiments of his own in which 

 two soils were treated with ammonium citrate solutions of 

 various strengths, one of the soils being also treated with 

 various strengths of a solution of free citric acid. The 

 results show that the amount of phosphoric acid dissolved 

 by ammonium citrate increases with each increase in the 

 strength of the reagent ; the rate of increase of the dissolved 

 phosphate diminishes, however, rapidly when considerable 

 concentrations of the solvent are reached. The amount of 

 potash dissolved remains, on the other hand, nearly constant 

 after a certain considerable strength of the ammonium citrate 

 has been attained. The amount of phosphate dissolved by 

 the free citric acid, in the case of the single soil experimented 

 on, increases largely with an increase in the strength of the 

 acid, and the amount dissolved by the greatest concentra- 

 tion of acid (five per cent.) was more than three times as 

 great as that dissolved by a fifty per cent, solution of ammo- 

 nium citrate. No experiments are recorded as to the effect 

 of an increase in the strength of the citric acid on the amount 

 of potash dissolved. 



Dr. Dyer has chosen a one per cent, solution of citric 

 acid as the reagent for extracting soils, with the view of 

 determining the amount of " available mineral plant food " 

 which they contain. That extraction with such a solution 



