ALKALI SALTS 401 



gypsum should be harrowed into the surface, not plowed 

 under. The reaction is as follows : — 



Na 2 C0 3 + CaS0 4 = CaC0 3 + Na 2 S0 4 



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

 it is recommended that the land shall first be treated 

 with gypsum, as the substitution of alkali sulfates for 

 carbonates causes the soil to assume a much less compact 

 condition and thus facilitates drainage. It also prevents 

 the loss of organic matter dissolved by the carbonate of 

 soda and the soluble phosphates, both of which are pre- 

 cipitated by the change. 



314. Scraping. — Removal of the alkali incrustation 

 that has accumulated at the surface is sometimes re- 

 sorted 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 method of alkali eradication is never very effi- 

 cient, and is often dangerous, as it encourages the 

 presence of very large amounts of alkali salts in the sur- 

 face soil. 



315. Flushing. — Often alkali accumulations may be 

 washed from the soil surface by turning on a rapidly 

 moving stream of water. The texture of the soil, as well 

 as the slope of the land, must be just right for such a 

 procedure. Generally so much water enters the soil 

 that the land remains heavily impregnated with alkali 

 salts. Both this method and the previous one, even 

 if successful, are only temporary. Moreover, lands 

 carrying so much alkali as to admit of either one of these 

 procedures may be so heavily charged as never to yield 

 to any form of either eradication or control. 



2d 



