282 CAUSES OF FERTILITY AND STERILITY [chap. 



per cent, of iron pyrites and 074 per cent, of ferrous 

 sulphate, as well as some insoluble basic sulphate of 

 iron. Another example of land reclaimed from the 

 sea contained 078 per cent, of pyrites and 1-39 per 

 cent, of ferrous sulphate. Under anaerobic conditions 

 such as are set up in a completely waterlogged soil, 

 the sulphates present are reduced to sulphides, and, by 

 further reactions with the compounds of iron present, 

 black ferrous sulphide is produced. Cultivation with 

 a free use of lime and chalk is the best means of 

 ameliorating such soils, which always show an acid 

 reaction. 



Kearney and Cameron in America have shown that 

 salts of magnesia possess, even in solutions of great 

 dilution, a toxic action upon plant roots, which is much 

 diminished if calcium salts be present at the same time. 

 Loew at the same time has indicated that a comparative 

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



