ALKALI IN SOILS. 39 



Turning to Table 12, let us compare the different samples as to their 

 chemical composition. We notice at once that sulphuric acid is the 

 acid and calcium the base which largely predominate in all of them, 

 excepting sample No. 9. The latter, consisting of the efflorescence 

 that is brought out on the surface of the soil by the combined action 

 of capillarity and evaporation, is necessarily deficient in the slightly 

 soluble calcium salts. In the crust, as well as in the soils, however, 

 sulphuric acid is by far the most important constituent. We may 

 therefore regard the " alkali " of Tozer oasis as of the sulphate type. b 



Free carbonates were not detected by the phenolphthalein test, either 

 in the field or in the laboratory, except in the crust (sample No. 9), 

 in which a very faint alkaline reaction was observed upon testing 

 the freshly collected sample. In all other cases the reaction was 

 neutral, not only at the time the samples were collected, but four 

 months afterwards when examined in the laboratory. " Black al- 

 kali," therefore, apparently does not exist in Tozer oasis. The high 

 percentages of calcium carbonate obtained by acid digestion of the 

 soils (see column 6 of Table 11) would suggest the possibility of its 

 formation under certain conditions were it not for the relatively 

 small amount of sodium and the great quantity of calcium and 

 sulphuric acid present. 



The percentages of potassium and of carbonic acid differ compara- 

 tively little in the different samples, while those of sodium and of 

 hydrochloric acid show very considerable variation. The relatively 

 small part played by hydrochloric acid is surprising, when we con- 

 sider that in the adjacent Shott Jerid mines of table salt exist. The 

 smooth crust of salt that covers the bottom of the shott has the 

 appearance of containing a higher percentage of sodium chlorid than 

 do the efflorescences upon the soils of the oases, the latter being of the 

 fluffy, powdery aspect which the presence of a large amount of sul- 

 phates generally produces. 



It is interesting to note the difference in composition between the 

 efflorescence (sample No. 9) and the soil solutions (samples Nos. 1 

 to 6). In addition to the very small percentage of calcium in the 

 crust, as compared with the soil samples, we remark in the crust a 

 considerably higher percentage of magnesium, a very much higher 

 percentage of sodium and of hydrochloric acid, and a much lower 



a In discussing " alkali " soils of Utah, Gardner and Jensen ( Field Operations 

 of the Division of Soils for 1900, p. 231) remark: "Lime is always present in 

 the soil alkali in appreciable amounts, whereas in the crusts it is usually 

 absent." 



& In the soils of the Oued Rirh oases, in Algeria, also, sulphates generally pre- 

 dominate over chlorids, although the converse is frequently true. (See Bui. 53, 

 Bureau of Plant Industry, p. 96; and Bui. 80, Bureau of Plant Industry, p. 46.) 

 92 



