38 DATE VAKIETIES AND DATE CULTURE IN TUNIS. 



Upon examination of the data given in Table 11, a striking dis- 

 crepancy is seen to exist between the percentages of water-soluble 

 components to dry weight of soil, as stated in columns 4 and 5, re- 

 spectively. By reference to Table 12, however, the explanation is 

 found to be simple. In taking the electrical resistance of the sample, 

 the soil was mixed with just a sufficient quantity of distilled water 

 to bring it to the point of saturation, which in soils of this texture 

 was found to be about 30 per cent of the dry weight of the soil ; and 

 the reading is taken as soon as the soil and water are thoroughly 

 mixed. In making the chemical analyses, on the other hand, one 

 part of soil is digested in 20 parts of water, and the mixture is 

 allowed to stand twenty-four hours to bring it to an equilibrium 

 before it is filtered. By the second method the soil is in contact 

 with from 60 to 100 times as much water as by the first method, and 

 for nearly 288 times as long. Consequently a great amount of cal- 

 cium sulphate goes into solution when the soil is prepared for chem- 

 ical analysis that remains in the solid state and is hence unaccounted 

 for in the samples as made up for a test of electrical resistance. By 

 reference to column 2 of Table 12 it is seen that (except in sample 

 9) calcium sulphate amounts to from 60 to 85 per cent of the total 

 water-soluble matter, of which only an insignificant fraction can be 

 dissolved in the amount of water and with the limited time allowed 

 for taking the electrical resistance. 



It should be said in this connection that experiments have shown 

 a saturated, pure solution of calcium sulphate to be harmless and even 

 beneficial to plants, and that an excess of calcium sulphate in the pres- 

 ence of magnesium and sodium salts has an extraordinary influence 

 in modifying the toxic action of the latter. 6 The calculations of 

 water-soluble contents of the soil based upon the electrical resist- 

 ance, although confessedly only rough approximations, are of some 

 value as indicating the amount of the readily soluble and actually 

 harmful saline contents of the soil. On the other hand, they do 

 not include the gypsum or calcium sulphate that is present, and 

 thus do not tell the whole story. If the latter were merely neutral, 

 it could simply be deducted from the total salts as ascertained by 

 chemical analysis; but as it plays an important part in counter 

 acting the effect of the sodium and magnesium salts, its presence can 

 not be ignored. 



a For a description of the methods used, see Cameron in Bui. 18, Bureau of 

 Soils, U. S. Dept. of Agriculture, pp. GG and 67. For a discussion at some length 

 of the significance of the high gypsum content of these soils, see W. T. 

 Swingle, Bui. 53, Bureau of Plant Industry, pp. 73 and 74. 



& Compare Kearney and Cameron in Report No. 71, U. S. Dept. of Agriculture, 

 p. 39. 

 92 



