288 CAUSES OF FERTILITY AND STERILITY [chap. 



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 

 Nile banks, was protected by insignificant dykes, which 

 dykes were burst every very high flood, and thus allowed 

 to be swept over* by the Nile and washed once every 

 seven or eight years. All this is at an end now in the 

 tracts under perennial cultivation, and other remedies 

 have to be found." 



The only remedy for the evils attending irrigation 

 is the introduction of drainage channels at a lower 

 level than the canals bearing the irrigation water ; in 

 this way the percolation through the soil, which in 

 humid climates naturally removes the salts not taken 

 up by the crops, is effected artificially ; there is some 

 apparent loss of water, but this is absolutely necessary 

 to maintain the land free from injurious salts. As 

 an example, the following passage may be quoted 



