74 THE DATE PALM. 



altogether, or to include the very much larger amounts reported in the 

 original analyses. 01 



The solubility of gypsum in the soil moisture is difficult to estimate, 

 especially in the presence of large amounts of other salts in varying 

 proportions^ The researches of Doctor Cameron and Mr. Seidell, 6 of 

 the Bureau of Soils, show that in pure water at 25 C. the solubility of 

 calcium sulphate is about 0.21 per cent, or 2.1 grams of calcium sulphate 

 per liter of water, which would equal 0.27 per cent of gypsum. In a 

 1 per cent solution of common salt 0.44 per cent of gypsum is dissolved, 

 and in a 4.9 per cent salt solution 0.75 per cent of gypsum. In mag- 

 nesium chloride an even greater solubility was observed and in a 10.5 

 per cent solution of this salt 11.13 per cent of gypsum dissolves. On 

 the other hand, salts which yield either calcium or sulphuric acid ions 

 on solution decrease the solubility of gypsum. In a 1.54 per cent solu- 

 tion of sodium sulphate only 0. 16 per cent of gypsum is dissolved, though 

 in a stronger solution more is taken up until, in a 22.2 per cent solu- 

 tion of sodium sulphate, 0.26 per cent of gypsum is dissolved. Calcium 

 chlorid in solution depresses even more the solvent power of water 

 for gypsum. 



Estimating the water content of the Saharan soil, mostly sandy loam, 

 at 10 per cent on the average, and the solubility of calcium sulphate at 

 0.5 per cent (equal to 0.6 per cent of gypsum) on the average in the 

 salts such as occur in the Fougala and Oued Rirh region of the Sahara, 

 the amount of calcium sulphate to be counted as alkali would be 0.05 

 per cent of the weight of the soil. When there -were large amounts 

 of sodium sulphate present, as at Chegga, the amounts of calcium 

 sulphate would be much less, probably about 0.02 per cent (equal to 

 0.025 per cent of gypsum) of the weight of the soil. c 



This method of expressing the amount of alkali is the one most 

 easily applied where the analyses are made by extracting the alkali 

 with an excess of water, but it is very doubtful whether it gives a cor- 



a In most alkaline soils the presence of gypsum is advantageous by preventing the 

 formation of the very harmful carbonates of sodium and potassium (see pp. 101 and 

 119) by neutralizing the poisonous effect of the salts of magnesium (see p. 89). 

 The physical action of alkali in rendering the soil water too concentrated to support 

 the roots of plants is, however, exerted as much by gypsum as by any other salt in 

 solution in equal amounts. 



& Cameron, Dr. Frank, and Seidell, Atherton. Bui. No. 18, Bureau of Soils, U. S. 

 Department of Agriculture, pp. 39, 40, and 46-57. 



c Mr. Seidell's original analyses are given in every case as a footnote in order to 

 facilitate any comparisons which students of alkali conditions may desire to make 

 with analyses reported in other ways than has been done by the writer. As a result 

 of this slight emendation the analyses are brought into such shape that the results 

 may be compared, without danger of serious error, with the determinations of alkali 

 made by the electrical method, on which data all the newest and best maps of the 

 alkali lands of the Southwest which have been issued by the Bureau of Soils have 

 been prepared. 



