1916] Sharp : Soluble Salts and Soil Colloids 293 



in the permeability of a soil when NaOH is washed from it. A 

 similar experiment performed by Mayer with sodium phosphate 

 was not accompanied with the sudden reduction in permeability 

 of the soil as was noted when NaOH was washed from the soil. 

 Likewise the washing out of lime-water produced no marked 

 effect on the rate of percolation, but the leaching out of added 

 NaCl, either with pure water or with lime-water, reduced percola- 

 tion to a minimum. Mayer ascribes the poor physical condition 

 of drained sea-shore lands to the washing out of the salt, and 

 significantly remarks that this effect, which frequently appears 

 in the second year, is probably more injurious to crop plants 

 than the toxicity of the salt itself. 



Van Bemmelen- in his cla.ssical researches on colloids has also 

 observed a similar decrease in the rate of percolation when loosely 

 bound salts are washed from clays or from the hydrated oxides 

 of tin, silica, and manganese. Moreover, he noted that these 

 colloids, when subjected to salt treatments followed by leaching 

 with water, invariably exhibited a high degree of diffusion upon 

 suspension in water for a second time. He further asserts that 

 this process can be indefinitely repeated by alternately adding 

 to and washing salt from the colloids. The colloidal particles, 

 as remarked by Van Bemmelen, become so exceedingly small, 

 during the process of washing salts from them, as to pass through 

 the filter paper. 



Warington^ also refers in a general way to the appearance 

 of somewhat similar phenomena when soils, previously treated 

 with acids, are washed with water. 



It would appear, with the above exceptions, that those who 

 have studied absorption, adsorption, or other physico-chemical 

 effects of salts on soils, have failed to recognize the existence of 

 any relation between the washing out of salts and the subsequent 

 condition of the soil. In fact. Hall and Morison* assert that the 

 flocculating effect of salt solutions on kaolin are reversible, that 

 is to say, upon the removal of the added salt the kaolin resumes 

 the original condition of diffusion. 



2 Journ. prakt. Chem., 2nd ser., vol. 23, p. 388, 1881. 



3 Physical Properties of Soil (Cambridge Univ. Press, 1900), p. 30. 



4 Journ. Agri. Sci., vol. 2, p. 244, 1907. 



