1916] Sharp: Soluble Salts and Soil Colloids 301 



particles does occur when salts are added to soils, but under the 

 conditions obtaining in many of the experiments described in the 

 literature it would seem highly improbable that enough of the 

 added salt remains in contact with the soil to bring about such 

 a movement of the particles. In our experience, the removal of 

 the salt creates among the soil particles a new adjustment which 

 seems of greater importance than that effected by the addition 

 of the salt. Furthermore, if our laboratory experience is properly 

 applicable to. field conditions, it would seem of greater advantage 

 to apply CaCOg rather than CaSO^ w^hen draining soils contain- 

 ing NaCl and NaoSO^, the reverse being apparently true in case 

 of soils impregnated with NaoCOg. 



Laboratory Investigations of the Causes of Salt 

 Effects on Soils 



The laboratory studies herein reported have been purposely 

 designed to throw some light on the possible causes contributing 

 to the above-noted effects of salts on soils, or at least to ascertain 

 if a relationship exists between the formation of these peculiar 

 physical conditions and the simultaneous occurrence of certain 

 other events. Certain well-known theoretical considerations, re- 

 inforced by concrete laboratory experience, have directed the 

 attempts to locate these fundamental causes into three well- 

 defined channels. The first of these is based on an assumption 

 that the salt-and-water treatments have actually increased the 

 quantity of the colloidal matter of the soil. It involves neces- 

 sarily a study of the soil itself and of the amount and degree of 

 diffusion of the colloidal matter therein contained. The second 

 line of reasoning connects the increase of calcium and magnesium 

 in the percolate from the salt-treated soils, and the absorption of 

 sodium, with the appearance of conditions suitable for the 

 formation of new colloidal matter, as well as with favoring the 

 extreme diffusion of that already present. The third considera- 

 tion ascribes the diffused condition of the soil colloids to the 

 presence of a small quantity of OH-ions in the soil solution. It 

 requires the theoretical assumption that these ions are associated 

 in some manner with the absorption of sodium. Although these 



