248 CAUSES OF FERTILITY AND STERILITY [chap. 



extent that they are mere bogs and swamps and alkali 

 flats, and the once fertile lands are thrown out as ruined 

 and abandoned tracts." 



Nor is it necessary that the subsoil be charged with 

 salts for irrigation to produce alkali land; the mere 

 continual evaporation of ordinary river or spring water 

 may cause such an accumulation of saline matter at the 

 surface as is harmful to vegetation. This is well seen in 

 Egypt, where perennial irrigation is practised with the 

 Nile water, and the following quotation from Willcock's 

 Egyptian Irrigation will explain the action that takes 

 place :— 



" The introduction of perennial irrigation into any 

 tract in Egypt means a total change in crops, irrigation, 

 and indeed everything which affects the soil. Owing to 

 the absence of rain, the land is not washed as it is in 

 other tropical countries, unless it is put under basin 

 irrigation. 



" An acre of land may receive as many as twenty 

 waterings of about 9 cm. in depth each, i.e., a depth 

 of water of i-8o metre per annum, which is allowed 

 to stand over the soil, sink about half a metre into 

 the soil, and then be evaporated. Since the Nile 

 water, especially in summer, has salts in excess, these 

 salts accumulate at the surface, and if not eaten down 

 by suitable crops, soon appear as a white efflorescence. 

 While the spring level is low, capillary attraction 

 cannot bring up to the surface the spring water, which 

 generally contains a fair proportion of salts, but where 

 the spring level is high the salt-carrying water comes 

 to the surface, is there evaporated, and tends to further 

 destroy the soil. In old times the greater part of 

 the cultivation land was under basin irrigation, and 

 was thoroughly washed for some fifty days per annum ; 

 while the rest, consisting of the light sandy soils near the 



