38 CHAPTER III 



of sodium, together with smaller quantities of the correspond- 

 ing salts of potassium, and very frequently more or less sul- 

 phate and chloride of magnesium and calcium. When these 

 salts come to the surface of the soil they may frequently be 

 seen as a white incrustation. Little or no sodium phosphate 

 is found in white brak, but occasionally large amounts of 

 nitrates are present, up to 20 per cent, of the total salts. 



The black variety is far more mjurious than the white in 

 its effect upon both soil and plant. 



Origin of Brak Salts. — For the present purpose it is 

 unnecessary to consider in any detail the various natural 

 agencies responsible for the existence of brak in soils. 



Brak salts are, of course, ultimately derived from the 

 decomposition of soil-forming minerals. When leaching occurs 

 the soluble salts are carried away, but in comparatively dry 

 areas gradual accumulation during ages of decomposition may 

 account for the brak condition of the present. 



In other cases the underlying shales and sandstones have 

 been found to be richly impregnated with pre-formed soluble 

 salts, which readily account for the presence of these salts in 

 the soils. Such shales and sandstones are said to have been 

 deposited in past ages from inland seas and lakes containing 

 large quantities of soluble salts. 



In still other cases the existence of brak is attributed to 

 more recent evaporation of saline lakes and shut-off arms of the 

 ocean, the soluble salts of course being left in the soil. 



The occasional occurrence of considerable quantities of 

 nitrates in soils has been traced in some cases to pre-formed 

 nitrates found in the underlying shales and sandstones ; in 

 others it is attributed to intense local nitrogen fixation by 

 non-symbiotic bacteria. 



Secondary reactions are liable to occur in the soil with the 

 possible formation of carbonates of sodium and potassium from 

 other less injurious salts of these metals. 



Finally, of great significance in present soil management, 

 the use of impure water for irrigation may at least in part 

 account for the presence of harmful salts in the soil. Seepage 

 water from brak areas or from rock formations rich in soluble 

 salts should never be used for irrigation. The safety limit of 

 soluble salt content of irrigation water will vary according to 

 the soil, the crop, the amount of water applied, the drainage 

 and other factors. Hilgard states that 685 parts per million 

 should be the limit under average conditions, though as litt'le as 



