234 SOIL CONDITIONS AND PLANT GROWTH 



" unavailable ".^.' Here, however, the agreement ends, for no 

 two dilute acids give the same results and no two Associations 

 of Agricultural Chemists recommend the same dilute acid. 

 Dyer's i per cent, citric acid (91) is adopted in Great Britain, 

 and its use has been justified by Wood's investigations (319) 

 and by those of Hall and Flymen (120^).^ N/200 hydro- 

 chloric acid has been recommended in the United States, 2 per 

 cent, hydrochloric acid by Nilson in Sweden, aspartic acid in 

 Hawaii, and so on. Mitscherlich (201c) uses a saturated solu- 

 tion of CO2 as being the nearest approach to natural con- 

 ditions. Some of the younger workers in the United States 

 are using the water extract as the nearest feasible approach to 

 the soil solution.^ A 2 per cent, citric acid solution has been 

 suggested by Bergu.* Bogdanow ^ in his investigations of 

 Russian soils used 2 per cent, acetic acid. The German 

 " Verband landwirtschaftlicher Versuchsstationen " recom- 

 mends both 25 per cent, and 10 per cent, hydrochloric acid 

 (see 224/). Lastly Ramann proceeds in a different manner 

 altogether and adopts a method depending on the interchange 

 of bases. The results obtained by different acids are shown in 

 Table LXI, Similar results have been obtained by Engels 

 (95) (see also p. 344). 



An empirical method can be retained only so long as it 

 justifies itself by results. Few agricultural chemists of repute 

 would be prepared to draw up a scheme of manuring on the 

 basis of soil analysis alone, though probably most would like 

 to have some analytical data before them. 



Even strong hydrochloric acid dissolves only a part of the 

 potash and phosphoric acid, the remainder not coming out till 

 after treatment with hydrofluoric acid. 



The different action of the various dilute acids has been 



1 Daubeny (78) originated this distinction, using the terms " active " and 

 *' dormant ". 



2 Also those of O. Lemmermann, A. Einecke, and L. Fresenius in Germany 

 (Landw. Versuchs-Stat., 1916, 89, 81-195). 



3 J. S. Burd (62a). 



* Landw. Versuchs-Stat., 1901, 55, 19 ; also by Engels (95). 

 ^ Expt. Station Record, 1900, 11, 130; 1901, 12, 725. 



