342 NATURE AND PROPERTIES OP SOILS 



as it is likely to do if allowed to become dry at the surface. 

 If crops are not grown, the soil should be harrowed between 

 floodings. The operation should not be carried to a point 

 where the soluble salts are reduced below the needs of the 

 crop.^ The use of gypsum on black alkali land has sometimes 

 been practiced for the purpose of converting the alkali carbon- 

 ates into sulfates, thus ameliorating the injurious properties 

 of the alkali without decreasing the amount. The quantity 

 of gypsum required may be estimated from the amount and 

 composition of the alkali. The soil must be kept moist, in 

 order to bring about the reaction, and the gypsum should be 

 harrowed into the surface, not plowed under. The reaction 

 is as follows : 



Na^COs + CaSO, = CaC03 + Na^SO^ 



When soil containing black alkali is to be tile-drained, it 

 is recommended that the land should first be treated with gyp- 

 sum, as the substitution of alkali sulfates or carbonates causes 

 the soil to assume a much less compact condition and thus fa- 

 cilitates drainage. It also prevents the loss of organic matter 

 dissolved by the carbonate of soda and the soluble phosphates, 

 both of which are precipitated by the change. 



Removal of the alkali incrustation that has accumulated at 

 the surface is sometimes resorted to. Very often the rise of 

 alkali is encouraged by applications of irrigation water, which 

 is allowed to evaporate unretarded. The salts are thus carried 

 upward by the capillary movement of the soil-water. This 



^ It has been suggested that elemental sulfur could be used to advan- 

 tage in alkali land, especially where carbonates and bicarbonates abound. 

 Sulfur generally oxidizes in the soil quite readily, producing an acid 

 [see par. 221]. Instead of trying to remove all of the alkalinity by 

 leaching, it might be more practicable to add sulfur. 



Lipman, J. G., Sulfur on Alkali Lands; Soil Sci., Vol. II, No. 3, 

 p. 205, 1916. 



Hibbard, P. L., Sulfur for Neutralising Alkali Soil; Soil ScLj Vol. 

 XL, No. 5, pp. 385-387, 1921. 



