1916] Sharp : Soluble Salts and Soil Colloids 329 



which the same strength of different salt solutions operates to 

 clarify suspensions from different soils. Moreover, the addition of 

 these salts seems to bring about a decided increase in the colloidal 

 content of the soil, which is obviously manifested when the 

 soluble salts are removed by distilled water. On the other hand, 

 the introduction of calcium salts to a soil does not apparently 

 bring about the production of a colloidal complex, although some 

 interchange of ions occurs. In fact, the addition of CaCl, fol- 

 lowed with washing seems to have the opposite effect on the Davis 

 clay-loam soil. For these reasons it seems evident that the in- 

 soluble as well as the soluble products formed by the interaction 

 of salts on soils must be considered as important factors in such 

 practices as prescribe the use of soluble salts on soils. 



It is apparent that at least the Na, K and NH^ ions form a 

 colloidal substance upon their introduction into the silicate com- 

 plexes of the Davis soil. The Ca-ion, and probably others, form, 

 on the contrary, non-colloidal substances when introduced into 

 the complex silicates of the Davis soil. It is also made clear 

 that the acid ion is of little importance in the phenomena follow- 

 ing the washing out of salts from the soil, by the fact that NaNOg, 

 NaCl and NaoSO^ all operate to produce approximately the same 

 results. 



Some General Observations on Other Peculiar Appearances 



Accompanying the Washing Out of Certain 



Salts from Soils 



Stewart^^ in a recent paper attributes the color of the nitre 

 spots to the action of sodium or potassium nitrates on the organic 

 matter of the soil. Bearing somewhat upon this subject, we have 

 noted that the addition of NaCl or Na.S04 to a soil materially 

 increases the organic matter in the supernatant solution if the 

 depth of color of such a solution can be relied upon as an index 

 of the quantity of organic matter contained therein. But the 

 most striking results in this connection appear when these neutral 

 salts are removed from the soil by washing. The soil itself as- 

 sumes the typical gray color of alkali soils in the field, and the 



51 Journ. Amer. Soc. Agron., vol. 6, no. 6, p. 247, 1914. 



