120 THE DATE PALM. 



to decompose the soluble carbonates present in the upper layers of the 

 soil. It is not impossible that the obvious injury which results to the 

 date palm from imperfect drainage may be caused by soluble carbon- 

 ates, which can form under such conditions, even in the presence of 

 gypsum. a It is a matter of much importance to determine the limits 

 of resistance of the date palm to black alkali, as to which it is now 

 impossible to speak with any certainty. 6 Even if the date palm proves 

 to be sensitive to the soluble carbonates it will nevertheless still be 

 possible to engage in date culture on black alkali lands by treating 

 them with gypsum and providing for good drainage. 



As yet no data are available for a study of the comparative alkali 

 resistance of the different varieties of the date palm, but doubtless a 

 careful investigation would show that there exists a considerable vari- 

 ation in this important character. Marked differences are known to 

 exist among the diverse sorts of date palms in their ability to endure 

 cold (see footnote, p. 61), and, as shown in the chapter on heat require- 

 ments, there are enormous differences in the amounts of heat required 

 to ripen early and late varieties; it is reasonable to expect similar lack 

 of uniformity in their ability to withstand alkali. The great impor- 

 tance of date culture, constituting as it does the only profitable industry 

 that can be followed on very alkaline lands, would warrant a careful 

 search in the date plantations in the most alkaline regions of the Old 

 World deserts, in the hope of securing varieties still more resistant to 

 alkali than those we now possess. 



The high degree of alkali resistance of the date palm permits 

 brackish water to be used in irrigating. Commercial date plantations 

 of large extent exist at Ourlana and at Chegga in the Algerian Sahara, 



Color is given to this supposition by the observation of Masselot (Bui. Direc. 

 Agric. et Comm., Tunis, vol. 6 (1901), No. 19, p. 135) that a disease common among 

 young palms, known as "Merd el Ghram," in the Tunisian Sahara, caused by exces- 

 sive irrigation in badly drained soils, is accompanied by a blackening of the soil about 

 the plant. The palms suffering from this disease cease to grow, sicken, and turn yel- 

 low; they may be cured by drainage and by replacing at the same time the black- 

 ened soil about the foot of the tree with fresh earth. These symptoms seem to 

 indicate the formation of black alkali, and that it has a very injurious action on the 

 date palm. 



& It can not be assumed that because the date palm is enormously resistant to 

 white alkali it must necessarily be able to support large amounts of black alkali, for 

 the soluble carbonates have a decidedly alkaline reaction, whereas white alkali, in 

 spite of its misleading name, may be nearly neutral in reaction. It is well known, 

 especially from the interesting experiments of Prof. H. J. Wheeler, of the Rhode 

 Island Experiment Station, that plants differ enormously in their requirements as to 

 soil reaction. Lupines, for instance, are injured by soils having an alkaline reaction, 

 whereas clover, soy beans, and most ordinary crop plants of humid regions are greatly 

 harmed by soils having an acid reaction. It is possible that the date palm is injured 

 by soils having a decidedly alkaline reaction, even if the amount of salts in solution 

 in the soil water be small. 



