148 MOVEMENT OE SOEUBLE SALTS 



the alkali. Drainage of a soil in California (22) removed 

 about 85 per cent of the sodium chloride, 83 per cent of 

 the sodium sulphate; drainage and conversion to sulphate 

 reduced the sodium carbonate content to 65 per cent of 

 the original quantity. 



Rate of Alkali Movement. - Theoretically, the alkali 

 salts are so soluble that their removal from the soil by 

 drainage should take only a short time, but in practice it 

 often takes several years to reduce the salt content of 

 seriously affected alkali lands sufficiently to produce 

 crops. Dorsey (5) attempts to explain the difficult move- 

 ment by the theory that the salts from the descending 

 free-water solution are drawn into the s capillary spaces 

 of the soil where rapid downward movement is prevented. 

 Subsequent downward percolation is attributed to dif- 

 fusion of the salts outward into the free-water spaces. 



Warington (32) states that the first water percolating 

 through land containing soluble salts at the surface was 

 much more concentrated than subsequent leachings but 

 that where the chloride was first incorporated in the soil 

 and then leached its concentration in successive leachings 

 gradually increased. He explains this by assuming that 

 the first water that comes from a drain passes through 

 cracks and burrows of insects and comes direct from the 

 surface, while that passing through the soil spaces alone 

 does not arrive until later. 



To explain the extremely slow movement of soil solu- 

 tions through alkali soils, especially those under laboratory 

 or other conditions where the alkali is added to the soil as 

 a single salt, Sharp (29) offers the theory that the alkali 

 salts react with the colloids of the soil causing diffusion. 

 He found that where solutions of sodium chloride or sodium 

 sulphate were in constant contact with the soil the rate of 



