160 RECLAIMING ALKALI LANDS 



The reclamation of land by flooding is used extensively 

 in the lower Nile Valley in Egypt. Details of the methods 

 used are described by Means (12). After land has been 

 reclaimed by flooding it is desirable to raise a crop that 

 can endure alkali and water till the soil is in a proper 

 condition for other crops. Rolet (12) recommends rice 

 for climates in which it will grow. White sweet clover 

 (Melilotus alba) is also an excellent crop for this purpose. 



Neutralizing Sodium Carbonate. The methods used in 

 removing most of the salts are not entirely satisfactory 

 for sodium carbonate, or black alkali. This salt dissolves 

 organic matter from the soil and deflocculates the particles, 

 thereby injuring the soil structure and making the pene- 

 tration of water very slow. The high direct toxicity of 

 this salt also renders it much more harmful than the 

 sulphates. Hilgard and his associates (8), working in 

 California, found that under suitable conditions sodium 

 carbonate can be made to react with gypsum to form 

 sodium sulphate and calcium carbonate. The reaction is 

 as follows: 



Na 2 C0 3 + CaS0 4 = Na 2 S0 4 + CaCO 3 . 



This changes the alkali from a very injurious to a much 

 less harmful salt. 



Shinn and Hilgard (15) used 3000 pounds of gypsum 

 to the acre in Tulare, California, with good results. The 

 best results were secured on plats treated with gypsum in 

 connection with drainage. Later reports of the experi- 

 ments made by Hilgard and Loughridge (8) and by 

 Shinn (14) show that the treatment continued to be suc- 

 cessful. In some cases gypsum was used at the rate of 

 7.7 tons to the acre annually for thirteen years with a 

 gradual amelioration of the alkali spots. In the four 



