644 CAUSES OF FERTILITY AND STERILITY [cha*. 



excess of magnesium over calcium in certain soils 

 results in sterility. With this may be correlated the 

 fact that the soils resting upon the serpentine, which 

 is a compound containing magnesia, are notoriously 

 poor, also that certain very impoverished clays on the 

 Wealden formation contain a high proportion of 

 magnesia. 



Sterility caused by salt is sometimes to be seen in this 

 country in the marshes near the sea : more often a " salt- 

 ing," even where the sea has regular access, is clothed 

 with vegetation which is able to endure very consider- 

 able proportions of salt. Most farm crops will grow in 

 soil containing 0-25 per cent, of salt, and in the reclaiming 

 of the old sea lake of Aboukir in Egypt, it was found 

 that grasses would grow freely when there was still as 

 much as 1 per cent, of salt in the soil, and a scrubby 

 winter crop of barley was grown on soil containing more 

 than 1 J per cent. " With 2 per cent, of salt in the soil, a 

 fair crop of dineba (grass), 2 feet high, can be grown ; 

 with 1 per cent, it attains its full height of 4 feet, and 

 sells as a standing crop at from 20s. to 25s. per acre. 

 For l berseem,' or clover, the percentage of salt should 

 not exceed |, and about the same for ' sabaini (quick- 

 growing or seventy-day) rice/ " Much, however, depends 

 upon the relations between water supply and evapora- 

 tion, as to the amount of salt in a soil which would be 

 tolerable to vegetation. From time to time cases occur 

 in this country of crops being destroyed, and land 

 rendered sterile by the incursion of sea water ; the effect 

 is not always apparent at first, though sea water contains 

 as much as 2-7 per cent, of sodium chloride and 0-5 per 

 cent, of other soluble salts, but the permanent pasture 

 becomes seriously injured, and for two or three years 

 even the arable land yields very indifferent crops. 

 Dymond has attributed this after effect to the injurious 



