ALKALI CONDITIONS AT BISKRA. 



77 



oasis, either indirectly by growing crops needing abundant irrigation between the 

 palms, or directly in soaking the ground about the trees. It is doubtless because of 

 the very low alkali content of the irrigation water in winter and the only moderate 

 content in summer that the alkali is not troublesome in this oasis, although surface 

 flooding is never practiced, water being applied in excavations called "dahir," hold- 

 ing a barrel or more (PI. XVII), which are made about the base of the tree (see 

 p. -17). Biskra has clay soils of great depth (as much as 40 feet) and this doubt- 

 less constitutes an additional reason for irrigating by means of dahir, since such 

 soils are difficultly permeable for water and have a great water capacity, so that if 

 irrigation were practiced by flooding the whole surface the water would largely be 

 evaporated or absorbed by the surface layers of the soil, and only a small proportion 

 would ever percolate to the roots of the date palm, especially in summer, when the 

 supply of water is scanty. 



Station No. 1, where soil samples were secured, was in a garden belonging to the 

 Victoria Hotel, some 25 feet away froni a century-old date palin (see PI. XIII), and 

 near a vigorous young Deglet Noor palm (see fig. 1, p. 16). Alfalfa and burr clover 

 (Mfdicago denticidata) were growing where the sample was obtained. The subsoil 

 was a stiff c]ay. 



The percentage of the weight of the soil soluble in 20 times its weight of water was 

 0.42 for the surface foot and 0.40 for the subsoil. The following salts were found by 

 Mr. Seidell: 



TABLE 12. Amount and nature of salts soluble in excess of water in soil from date garden 

 at Biskra (expressed in percentages of the total weight of the soil) - 1 



'TMs table is identical with Mr. Seidell's original analysis. 



Disregarding the very slightly soluble calcium carbonate, the following would rep- 

 resent approximately the alkali content of the soil water: 



TABLE 13. Per cent of alkali in soil of palm garden at Biskra, Algeria. 



The amount of alkali present in this soil is insignificant and in no way affected the 

 growth of alfalfa. This sample is also interesting as being a heavy clay soil of the 

 sort which largely composes the oasis of Biskra, but which does not occur in the 

 other oases studied. Such soils are not considered as favorable for date culture 

 as loamy or sandy loam soils; nevertheless date palms grow very well at Biskra, 

 although the late sorts do not ripen their fruits properly because the summer and 

 autumn are not hot enough. 



Of the area surveyed by Messrs. Holmes and Means, of the Bureau of Soils, in the 

 Salton Basin, California, 23,120 acres, or 30 per cent, is a heavy clay comparable to 

 this sample, and half this area contains less alkali than the Biskra garden, and a 

 quarter more contains only slightly greater quantities (0.4 to 0.6 per cent), where 



Such soils are common in the Salton Basin in California. (See PI. Ill, pp. 106 

 and 108.) 



