ALKALI CONDITIONS AT OURLANA. 



95 



the soil by the evaporation of the water used for irrigation. Three acre-feet of such 

 water, the least amount needed per annum, would carry on to the land no less than 

 50,000 pounds of dissolved salts, and, subtracting the excess of gypsum, some 40,000 

 pounds of harmful alkali, or 0.1 per cent of the surface foot of soil and 0.025 per cent 

 of the 4 upper feet of soil. Of course, some of the water drains off directly, and 

 even leaches alkali out of the soil, but much remains in the soil, and on evaporating 

 leaves the alkali behind. 



After a number of years' irrigation with strongly alkaline water such as that of 

 Ourlana a condition of approximate equilibrium is reached between the amount of 

 alkali carried to the land and the amount leached out by the drainage water. The 

 composition of the alkali in a soil in such a condition doubtless depends much more 

 on the composition of the irrigation water than on the character of the alkali orig- 

 inally present in the soil before irrigation was practiced. The influence of the com- 

 position of the irrigation water on the nature of the alkali is naturally most clearly 

 marked on lands that are well leached out by means of irrigation for a long period of 

 time with an abundance of water, accompanied with thorough drainage. 



A comparison of the composition of the alkali at two such stations, one at Chegga 

 and one at Ourlana, is of interest, because the artesian waters used for irrigation at 

 these two localities contain almost identical amounts of dissolved salts, though of 

 very different composition. The following tabulation shows the proportions of the 

 principal salts in the water and in the surface soil: 



TABLE 32. Proportions of sulphates and chlorids present in alkali of irrigation water and 

 in well-drained long-irrigated surface soils at Chegga and Ourlana, Algeria. 



It is clear from this table that sulphates preponderate at Chegga, both in the irri- 

 gation water and in the alkali of well-drained surface soil after irrigation for a term 

 of years, while at Ourlana the preponderance of chlorids, though not so great as 

 that of the sulphates at Chegga, is nevertheless plainly marked. In both surface 

 soils the approximation in composition of the alkali of the surface soil to that in the 

 irrigation water is evident, and is rendered still more clear by a study of the bases. 

 Magnesium, for example, is decidedly more abundant in the artesian water at Our- 

 lana than at Chegga, and in consequence the surface soils at Ourlana likewise show 

 more magnesium than those of Chegga. 



All three Ourlana stations show amounts of alkali large enough to be dangerous to 

 ordinary crops, and, in fact, in this oasis no other cultures were observed such as were 

 followed at the other oases studied, and all three stations show a pronounced surface 

 crust in spite of long-continued irrigation, accompanied with drainage by open 

 ditches. The sandy nature of these soils and their consequent low water content 

 cause the concentration of the soil water to be much higher in proportion to the per- 

 centage of alkali present than in heavier soils having a greater water content, such 

 as those of Biskra, for example (see p. 77). There is then every evidence that the 

 date palm is unharmed by these quantities of alkali, even when irrigated by water 

 of very bad quality, full of harmful chlorids. 



