ABSORPTION OF SALTS BY SOILS 109 



substance could be made to change into sodium carbonate 

 in the presence of precipitated calcium carbonate through 

 which carbon dioxide was being forced, but that the action 

 was most vigorous when the strength of sodium sulphate 

 was only 750 parts per million. This is the probable ex- 

 planation of the fact discovered by certain investigators (5) 

 that black alkali was formed about the roots of plants 

 growing on white alkali. The carbon dioxide given off 

 by the roots of the plants made the calcium carbonate 

 soluble so that it would react with the white alkali to form 

 the black. 



Salts concentrated in some part of the soil by former 

 reactions might be acted upon by solutions borne from dif- 

 ferent sections containing other types of salts making 

 possible incessant and complete exchanges of ions of the 

 different salts. Referring again to the table of solubil- 

 ities, it is seen that salts do not maintain the same relative 

 solubility at all temperatures. This disturbs the equi- 

 librium as the temperature of the soil changes. 



Absorption of Salts by Soils. The alkali problem would 

 be much simplified if the soluble salts were simply held 

 in the active part of the soil solution. With such a con- 

 dition it would take but a few leachings of the soil to free 

 it of excessive salts. Through absorption and adsorption, 

 however, the soil tends to hold part of the salts when it is 

 drained. With high concentrations the soil has little 

 power to hinder free movement of salts, but with lower 

 concentrations the soil retains a larger proportion of the 

 salts. 



Part of this difficult movement is thought by some to be 

 caused by a mechanical adherence of the salts immediately 

 in contact with the soil particles; others consider that an 

 actual chemical reaction takes place. If no chemical 



