REDUCING EVAPORATION 155 



adapted to the conditions. Where an irrigation canal 

 passes through a formation that is high in soluble salts 

 the water becomes alkaline and carries the soluble material 

 to the land where the water is applied. A canal in a forma- 

 tion of this kind becomes porous when the salts are dis- 

 solved. This allows seepage water to percolate more 

 readily from the canal, increasing the quantity of water 

 which comes out on land below; this in turn causes water- 

 logging together with deposition of alkali salts. Lining 

 the canal with cement over the salt-bearing formation 

 will do more toward permanent reclamation than any 

 number of temporary devices on the land itself which do 

 not remove the source of the trouble. 



Often a large area becomes water-logged from a single 

 source, and in arid soils water-logging is generally fol- 

 lowed by alkali accumulation. A ditch across the head 

 of the land to cut off the water in cases of this kind will 

 often prevent or overcome the difficulty without applying 

 methods of reclamation on the land itself. 



Some soils contain a layer several feet below the surface 

 in which the salt is very concentrated. Where this is the 

 case, every effort should be made to prevent a rise of the 

 salt to the surface where it will hinder crop growth. If 

 it remains at considerable depth, it may be entirely harm- 

 less, whereas it might entirely prevent plant growth if it 

 rose to the root zone. These examples show the relation 

 of reclamation methods to the source of alkali. 



Reducing Evaporation. The chief method by which 

 alkali accumulates at the surface of the soil is through 

 evaporation. The author (4) has shown the ease with 

 which salts move with moisture through the soil. When- 

 ever water evaporates from the soil surface more water is 

 moved to the surface by capillarity and the process re- 



