316 THE PRINCIPLES OF SOIL MANAGEMENT 



from the soil and drain it off through the tile. When 

 used for the amelioration of alkali spots in a semi-arid 

 region, the natural rainfall will in time effect the removal. 



In laying tiles, it is necessary to have them at such 

 a depth that soluble salts in the soil beneath them will 

 not readily rise to the surface. This will depend upon 

 those properties of the soil governing the capillary move- 

 ment of water. Three or four feet frequently suffices, 

 but the capillary movement should first be determined. 



After drains have been placed, the land is flooded with 

 water to a depth of three or four inches. This is allowed 

 to soak into the soil and pass off through the drains, 

 leaching out part of the alkali in the process. Before the 

 soil has time to become very dry the flooding is repeated 

 and the operation kept up until the land is brought into 

 a satisfactory condition. 



Crops that will stand flooding may be grown during 

 this treatment, and they will serve to keep the soil from 

 puddling, as it is likely to do if allowed to dry on the 

 surface. If crops are not grown, the soil should be har- 

 rowed between floodings. 



The operation should not be carried to a point where 

 the soluble salts are reduced below the needs of the 

 crop, or to lose entirely their effect upon the retention of 

 moisture. 



181. Correction of black alkali. The use of gypsum 

 on black-alkali land has sometimes been practiced for 

 the purpose of converting the alkali carbonates into sul- 

 fates, thus ameliorating the injurious properties of the 

 alkali without decreasing the amount. The quantity 

 of gypsum required may be calculated from the amount 



