226 MOVEMENTS OF SALTS IN THE SOIL 



irrigated for four years, but was now without a crop. An 

 inspection of the table shows at once that the irrigation has 

 brought about a veritable rise in alkali. The soil, as before, 

 contains a well- marked band of salt, but this band has moved 

 to the surface. The first foot of soil now contains 19760 Ib. 

 of alkali salts per acre, an amount sufficient to make the 

 growth of ordinary crops impossible. 



No information is given as to the mode of irrigation adopted 

 in this instance, whether flooding of the land, or the supply 

 of water without flooding by means of channels or shallow 

 ditches ; both modes of irrigation are apt however to pro- 

 duce the same final result, unless special precautions are taken. 



In the case of land irrigated by the soakage of water from 

 channels running through it, it is clear that a rise in the 

 water level in the subsoil must occur ; and as a fact it does 

 occur to a large extent. In the Yellowstone Valley the 

 natural water level in the subsoil is at least 20 feet below 

 the surface. Irrigation from a high-level canal has raised 

 this water level in many places to within 3 feet of the surface, 

 while the lower parts of the irrigated district have been con- 

 verted into a swamp. It is evident that a rise of the subsoil 

 water must bring the salts in the soil to the surface. We 

 have here in fact another instance of the action already 

 several times described in the course of this chapter, only 

 in the present case the movement of the water is upward 

 instead of downward. 



Although a concentrated salt solution may be brought to 

 the surface by the displacing action of fresh water, the salts 

 would not remain accumulated here were it not for another 

 action, which has in fact a preponderating influence on the 

 question before us. The large amount of evaporation which 



