NATURAL WATERS 69 



treated with the flood-water in order to increase the thickness 

 of the soiL . The process is known as " warping," and the 

 " warp *' soils are extremely rich and fertile. The Nile in 

 Egypt affords a still better example of a river used in this 

 manner. 



In countries of limited or unevenly distributed rainfall 

 irrigation is often practised. In this case, since there is very 

 little drainage, the composition of the water used is of im- 

 portance. If the water be chai'ged with common salt, sodium 

 sulphate, or sodium carbonate, there is a gi^ave danger of the 

 surface soil, by the prolonged concentration of the water, 

 becoming charged with the soluble matter to such an extent 

 as to seriously interfere with plant growth. The soil is then 

 said to become " brak " or " alkali." The usual causes of this 

 sterile condition are sodium sulphate and chloride ("white 

 alkali ") or sodium carbonate ('* black alkali ") derived either 

 from the soil itself, or partly from the water used for irriga- 

 tion. 



Different crops are possessed of different resistant powers to 

 these salts. As a rule, sodium carbonate is the most effective 

 in causing injury to plants, and sodium sulphate the least. 

 Fortunately, however, " black alkali " — i.e., sodium carbonate 

 — can be rendered almost innocuous by the application of 

 gypsum to the soil, when by double decomposition calcium 

 carbonate and sodium sulphate are formed : 



CaSO, + Na.COg = Na^SO, + CaCOj. 



If ** white alkali " be due to common salt it cannot be cured, 

 except by drainage. 



According to American results (and in various parts of the 

 United States large areas of alkali soils exist), the following 

 table gives the highest proportions of sodium chloride, sodium 

 sulphate, and sodium carbonate which may be present in soils 

 without injury to the plants named. The figures represent 

 the amounts in pounds of the various constituents present in 

 the upper four feet of soil per acre ; 



