SOIL MANAGEMENT. 39 



342 parts per million of sodium carbonate alone may cause seri- 

 ous injury in three or four years, while as much as 2,740 parts 

 per million of the less toxic salts would not be harmful. Mackie 

 considers from 600 to 700 parts per million to be the limit when 

 the salts are mainly sodium bicarbonate and sodium chloride ; 

 when the salts are largely of the sodium sulphate type, larger 

 quantities are permissable. Harris considers that under aver- 

 age conditions the limit is 500 parts per million for sodium 

 carbonate, 1,000 parts for sodium chloride, 4,000 parts for 

 sodium sulphate, and for the mixed salts, 4,000 parts per mil- 

 lion ; on poorly drained soils the limits must be placed lower. 



Movement of Brak Salts in the Soil and the Effect 

 OF Irrigation. — The greatest problem in connection with the 

 utilisation of brak soils is to control the movement of the sol- 

 uble salts. If kept in the lower depths of the soil, brak salts 

 would be harmless, but it is their natural upward movement 

 and gradual accumulation in large quantity in the top foot or 

 two that finally ruins the land. 



Under natural conditions in arid regions, when the water- 

 table is far below the surface, the soluble salts tend to accumu- 

 late, not at the surface, but at some little distance beneath, 

 approximately at the lowest point to which the natural rainfall 

 penetrates, perhaps three or four feet deep. The actual point 

 of maximum accumulation will vary according to the rainfall 

 and the porosity of the soil. Any soluble material in the upper 

 soil is carried downward by percolation, and the subsequent 

 upward movement of the moisture by capillarity is greatly 

 restricted, because the rather deep-rooted vegetation extracts 

 most of the water rapidly at some depth below the surface, and 

 at the same time the rapid drying of the surface soil reduces the 

 upward capillary movement of the moisture. Thus the soluble 

 salts are generally not brought to the surface to any appreciable 

 extent. 



When such lands are brought under irrigation, however, 

 this condition of balance is upset. The' soil is kept so much 

 more moist that capillary action is greatly increased and con- 

 siderable evaporation takes place at the surface. The soluble 

 salts then move upwards and are left at or near the surface. 

 When drainage is poor the condition is still worse, for then the 

 water-table may be raised to within a few feet of the surface. 

 Soluble salts washed downwards in such cases go only as far as 

 the water-table, and are later readily drawn up to the surface 

 again. 



