158 RECLAIMING ALKALI LANDS 



Harris and Robinson (5) have shown that shade is very 

 effective in reducing evaporation. This suggests the 

 desirability of keeping alkali land constantly shaded, 

 preferably by a crop, which not only shades the soil but 

 also causes the water to pass into the air through the 

 plants without coming to the surface. A growing crop 

 may therefore be considered as one of the most important 

 agencies in the reclamation of land containing small 

 quantities of alkali. 



A water-table near the surface is the chief cause of 

 harmful . evaporation. It is difficult to prevent the pas- 

 sage of large quantities of water to the surface when there 

 is an unlimited supply 2 or 3 feet below. The prevention 

 of alkali accumulation calls for a lowering of the water- 

 table to several feet from the surface. The growing of 

 green manure crops instead of leaving the land uncropped 

 is one way of reducing the surface accumulation of 

 alkali. 



Plowing Under of Surface Alkali. Hilgard (9) has 

 shown at the Tulare Substation, California, that the injury 

 caused by alkali was reduced by plowing the surface ac- 

 cumulation under. Part of a very bad alkali spot was 

 trenched to a depth of two feet and the surface soil thrown 

 to the bottom. The spot thus treated produced good 

 wheat crops for two years, which was the time required 

 for the alkali to return to the surface. Ordinary plowing 

 is to some extent similar to the above treatment; hence 

 the tendency of salts to accumulate at the surface by 

 evaporation of water is in part overcome by ordinary 

 field practices. 



In order that this operation may be effective, the plow- 

 ing should be as deep as possible, since salt turned under 

 only 3 or 4 inches deep would return rapidly to the sur- 



