40 



DATE VAKIETIES AND DATE CULTURE IN TUNIS. 



percentage of carbonic acid. In respect to potash and sulphuric acid 

 the difference is not noteworthy. 



It remains to consider the composition of the drainage water. A 

 sample was taken from the " Khandak-el-Kebir," the main drainage 

 ditch of the oasis of Tozer (see p. 33), at a point some distance beyond 

 the last garden toward the shott. This sample, like that of the irri- 

 gation water described on page 30, after having been kept for two 

 months in a rubber-stoppered bottle, remained perfectly sweet and 

 good. Tested with phenolphthalein as soon as it Avas collected, the 

 water showed a very faint pinkish opalescence, indicating a . slight 

 trace of free carbonates. An analysis of this sample in the labora- 

 tories of the Bureau of Soils showed the presence of 530.4 parts of 

 salts per 100,000 of water, which is a little more than two and one-half 

 times the amount found in the irrigating water (see p. 30). Water 

 of this concentration could be safely used in irrigating many crops, 

 and considering the fact that the drains penetrate everywhere, and 

 that the sample was collected at a season (February 5) when compara- 

 tively little irrigation is going on, its relatively small salt content 

 indicates that the soils of the oasis generally are not highly saline. 



TABLE 13. Chemical composition of drainage water, Tozer oasis. 



Comparing the percentage composition of the saline matters in 

 the drainage water with that of the irrigation water, we find a 

 remarkably close agreement. As compared with the soil samples 

 (Nos. 1 to 6 in Table 12), the drainage water contains relatively 

 much more magnesium, sodium, and potassium and very much less 

 calcium. In respect to acid constituents there is relatively much less 

 sulphuric acid and much more hydrochloric and carbonic acids. In 

 comparison with the salt crust, of which a sample was taken near the 

 spot where the water sample was collected (sample 9 in Table 12), 

 the drainage water contains relatively much more calcium, potassium, 

 and carbonic acid and considerably more hydrochloric acid. On the 

 other hand, it is much poorer in sodium and is considerably poorer in 

 sulphuric acid than is the crust. 



a See T. H. Means, Circ. 10, Bureau of Soils ; and Bui. 80, Bureau of Plant 

 Industry, p. 38. 

 92 



