66 Dilute Acids and Pliosphorus Compounds in the Soil 



Hall and Plymen^, however, argued from their numerous analyses that 

 there was no evidence of two definite groups, but that all the facts could 

 be explained on the view that a considerable number of compounds 

 occur of different degrees of solubility which, however, merge gradually 

 one into the other. 



Dyer used 1 % citric acid as a solvent, and found that it brought 

 out amounts of phosphorus comparable with those which might be 

 expected from a knowledge of the crop producing properties of the soil. 

 This particular solvent had already been recommended by ToUens^, 

 and used by Stutzer^ for the examination of phosphatic manures, but 

 it was finally selected by Dyer on theoretical grounds. At that time 

 plant roots were supposed to excrete acids that dissolved the soil phos- 

 phates and other mineral substances which then passed into the roots 

 by osmosis. Now Dyer found that 1 % citric acid had approximately 

 the same degree of acidity as cell sap, and argued that it must therefore 

 exert approximately the same solvent action on the soil phosphates as 

 the plant excretions, and would therefore give the most faithful picture 

 of the phosphatic material available for plant nutrition. 



The conception of the plant root as a special excreting and dissolving 

 agent is now generally abandoned, as no satisfactory evidence can be 

 obtained that any acid other than carbonic acid is excreted, or that 

 any action beyond respiration is concerned. With this conception 

 the theoretical basis underlying the selection of 1 % citric acid has gone 

 too, and the method becomes purely empirical, and justifiable only by 

 the extent to which its results are of value in soil analysis. 



Judged by this empirical standard 1 % citric acid has proved fully 

 satisfactory, and in Great Britain it is very generally adopted. Wood* 

 and also Hall and Plymen^ found that it gave results which accorded with 

 the agricultural history of the soil. But it has not found acceptance 

 elsewhere. N/200 hydrochloric acid has been recommended in 'the 

 United States, and 2 % HCl (about N/1-82) in Sweden. Mitscherliche 

 adopts a saturated solution of carbonic acid, again, in order to simulate 

 the action of the plant roots, and has carried out extended observations 



1 Trans. Chem. Soc. 1902, 81, 117^4. 



2 A. Grupe and B. Tollens, Ber. d. deutsch. Chem. Gesell. 1880, 13, 1267 ; v Ollech and 

 Tollens, Journ. f. Landiv. 1882, 30, 519. 



3 Chem. Ind. 1884, 7, 37. 



* Wood, T. B., Trans. Chem. Soc. 1896, 69, 287, also Wood and Berry, This Journal, 

 1905, 1, 114-21. 



6 Trans. Chem. Soc. 1902, 81, 117-44. 

 « Landw. Jahrb. 1907, 36, 309-369. 



