1916] Sharp : Soluble Salts and Soil Colloids 315 



A solution of N/10 NaCl was allowed to act upon successive 

 portions of Davis soil until practically no more calcium or mag- 

 nesium was taken up by the solution, so that a solution of 

 chlorides was formed which was quite inert toward this soil. A 

 solution secured in this manner appears to approach the satura- 

 tion point with respect to calcium and magnesium compounds 

 of the soil. This solution remained practically N/10 with respect 

 to chlorine throughout its successive periods of contact with the 

 soil, and at no time was the solution entirely depleted of its 

 sodium content hy the interchange with calcium and magnesium. 

 The application to the Davis soil of a solution secured in the 

 fashion described above and followed by washing with water did 

 not result in such pronounced diffusion as was observed upon 

 the application of N/10 NaCl solution followed by washing with 

 water. It may therefore be inferred that the chemical exchange 

 of bases plays a significant role in the mechanism by which dif- 

 fusion is produced in soils which have been washed after an 

 addition of NaCl. 



By the various lines of reasoning outlined above, the majority 

 of the more obvious possible causes for the extreme diffusion of 

 the soils under discussion have been completely eliminated or at 

 least reduced to factors of little significance. For this reason, 

 more mature thought on the subject has directed our attention 

 to the absorption of sodium, which has been previously men- 

 tioned, as the keynote to the deteriorated physical condition of 

 the salt-treated, water-washed soils. The substitution of sodium 

 for the calcium, magnesium, or other bases in the silicate complex, 

 or the direct addition of sodium to such a complex, results, ac- 

 cording to our proposed hypothesis, in the formation of new 

 jelly-like colloids, capable of becoming highly diffused when in 

 contact with water. The fact that leaching NaoCOg, NaOH, 

 NaHCOg, NaCl, Na^SOi, and NaNOg from the soil brings about 

 the same appearance of the soil and the same physical manifesta- 

 tions of deflocculation, becomes more comprehensible if the view 

 is accepted that the absorption of sodium and the new compounds 

 formed thereby are, in the main, the factors responsible for the 

 deflocculated condition of the soils so treated. Moreover, these 

 various treatments are accompanied by the appearance of widely 



