E. J. Russell and J. A. Prescott 67 



to show that it gives results in agreement with those of pot experiments. 

 Aspartic acid, acetic acid, and others have also been used. 



From the fact that so many dilute solvents remain in use by 

 analysts, it may be inferred that almost any acid can be made 

 to give satisfactory results provided sufficient trouble is taken to ascer- 

 tain suitable conditions of extraction. Direct experimental verification 

 of this view was obtained by Hall and Plymen, who found that all the 

 dilute acids gave the same kind of results, although there were con- 

 siderable differences in the amount of phosphoric acid brought out; 

 but citric acid proved to be on the whole at least as convenient as any 

 other, and, as it had already been in use for some years, there was no 

 advantage in giving it up. It is on this basis that the analytical side 

 of the problem has solved itself, and in any country where considerable 

 experience has been gained with a particular solvent there is probably 

 little to be said in favour of making any change. The extensive litera- 

 ture that has grown up round this branch of soil analysis is largely 

 concerned with the accumulation and record of experience of the parti- 

 cular solvent adopted, and need not therefore be discussed by us at this 

 stage. 



There is, however, a wider problem of more fundamental significance. 

 By studying the reaction under definite conditions in the light of the 

 well established laws of chemical dynamics it ought to be possible to 

 discover the type of the reaction, and thus to obtain information as to 

 the nature of the phosphorus compounds in the soil. The first attempts 

 in this direction were made by Hall and Amos^. Soil was extracted 

 with successive doses of citric acid and the P2O5 determined: the 

 results were plotted in the usual way. The curves, however, could 

 not be fitted by any of the ordinary equations. Similar negative 

 results were obtained by de Sigmund^ in an interesting series of investi- 

 gations with nitric acid. Our own experiments also yield the same 

 results. It may be taken as established that the reaction is not of the 

 simple type presented by the familiar mono-, di-, or tri-molecular 

 reactions of the text-books. 



Hall and Amos considered that their results could be explained on 

 the supposition that soil contains several phosphorus compounds of 

 varying degrees of solubility. This may be so, but it cannot be the 

 only factor. We shall show that citric acid extracts considerably more 

 phosphorus from the soil than equivalent concentrations of nitric acid, 



1 Trans. Chem. Soc. 1906, 89, 205-22. 



2 J. Amer. Chem. Soc. 1907, 29, 929-36. 



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