OVER-IRRIGATION AND ALKALI 399 



farms, that calcium sulfate, or gypsum, added to a soil 

 containing sodium carbonate, changed the carbonate to 

 a sulfate; that is, gypsum changed black alkali to white 

 alkali. Twice as much gypsum as there is sodium car- 

 bonate in the soil, should in time be worked into the soil 

 thoroughly. To add 200 to 400 pounds of gypsum an 

 acre annually, is better than to attempt to add the full 

 quantity all at once. After each treatment the soil should 

 be irrigated. This is an excellent corrective for black 

 alkali; and it is, indeed, the only known chemical cor- 

 rective for alkali. 



238. Scraping the surface. Another method of com- 

 bating alkali is to allow evaporation to go on until the 

 alkali has crusted the soil surface, and then to scrape off 

 this crust and to remove it permanently from the soil. By 

 this method, hundreds of pounds of alkali per acre may 

 be removed from the soil; but not enough is carried off 

 really to improve the soil, and the labor involved is so 

 large as to make the whole process of doubtful value. 



239. Tillage against alkali. Alkali is most injurious 

 if concentrated near the surface. If distributed evenly 

 throughout the soil relatively large quantities of alkali 

 may be endured by plants. This condition may be secured, 

 measurably, by reducing evaporation and thereby pre- 

 venting the rise of alkali. This is a very effective method 

 of preventing damage from alkali. Orchards seriously 

 injured by alkali have frequently been restored to a 

 profitable condition by thorough cultivation. The com- 

 mon custom is to plow under the crust, irrigate thoroughly, 

 and follow this by a thorough cultivation as often as 

 needs be throughout the season. Alkali lands should also 

 be cultivated in the spring, when evaporation is likely to 

 go on rapidly. 



