1916] Sharp: Soluble Salts and Soil Colloids 311 



the other two soils, which were much less affected in both these 

 particulars. Evidently the extent of decomposition of the salt 

 by the soil is a factor in determining the final physical condition 

 thereof, and would certainly seem to indicate that the changes of 

 the physical condition of the soil resulting from salt treatments 

 are considerably more than a mere shifting of the soil particles 

 to new positions, as some investigators would lead us to believe. 

 Moreover, it appears proper to infer that these after-effects of salt 

 treatments can be more properly referred to the salt as a chemical 

 agent than as a physical agent. 



If it be true that the chemical reaction between soils and salt 

 solutions results in a chemically equivalent exchange of bases, as 

 Sullivan^" states, then it seems proper to assume that the calcium 

 and magnesium in the leachings from soils treated as described 

 above represent approximately the amount of absorbed sodium. 

 At least it would appear that the calcium and magnesium in the 

 solutions, less the quantity normally present in distilled-water 

 extractions, is an index of the absorbed sodium. This statement 

 has received further justification in some solubility studies, the 

 data of which have not been published. Undoubtedly consider- 

 able quantities of sodium have been removed from the salt solu- 

 tions by the soil, which in return has given up calcium and mag- 

 nesium. The absorbed sodium has become so firmly fixed in the 

 soil that no amount of washing can dissolve it ; otherwise the 

 wash Mater after passing through the soil would be slightly alka- 

 line. On the contrary, the first portions of the wash water com- 

 ing through appear to be slightly acid, which is in accord with 

 the work of Sullivan, ^^ Parker," and others, but if the washing 

 is continued the leachings eventually become neutral or just per- 

 ceptibly alkaline, as is the case when distilled water is in contact 

 with the normal Davis soil. These remarks, supported with 

 stronger evidence from the rapidly accumulating literature on 

 chemical exchanges between salt solutions and soils or silicates, 

 point to the formation, under the conditions herein reported, of 

 a sodium alumino-silicate compound, or possibly a series of such 



12 Loc. cit., p. 27. 



13 Loc. cit., p. 8. 



1* Journ. Agric. Research, vol. 1, no. 3, p. 1, 1913. 



