130 RELATION TO PHYSICAL CONDITIONS 



sink and leave typical holes. Sodium and magnesium 

 chlorides and sulphates have less marked, but very dis- 

 tinct, effects on moisture movements. 



Evaporation of Moisture. - - The vapor tension of water 

 is reduced by the presence of dissolved salts; hence the 

 presence of alkali reduces the rate of evaporation. The 

 rate of decrease of evaporation produced by the various 

 salts is shown by Briggs (3) and by Harris and Robin- 

 son (n). It is not equal to the reduction in the vapor 

 tension of the solution since the air at all times contains 

 some moisture. The results of Harris and Robinson 

 showed an evaporation of 190 grams from distilled water 

 and only 100 grams from an equal surface of water in which 

 had been dissolved 30 per cent of sodium chloride. Sand 

 moistened with distilled water had a loss of 80 grams, 

 whereas that with a 2-normal solution of sodium nitrate 

 evaporated but 53 grams of water. 



In an experiment by the author a loam soil, to which 

 had been added various quantities of the sodium chloride, 

 sodium sulphate, and sodium carbonate, was placed in 

 petri dishes in a closed chamber in which the air was kept 

 saturated. The soils all took moisture from the air, the 

 rate of absorption depending on the salt and the concen- 

 tration. In the higher concentrations so much moisture 

 was absorbed that free water covered the surface of the 

 soil. A condition similar to this is often found in nature 

 where the soil of an alkali spot is wet constantly during 

 the season even though the surrounding soil is dry. 



REFERENCES 



1. Bemmeln, J. M. von. On the Plasticity of Clay Soils. Chem. 



Weekbl. 7 (1910), pp. 793-805. 



2. Breazeale, J. F. Formation of Black Alkali (Sodium Carbonate) 



in Calcareous Soils. Jour. Agr. Rsch. 10 (191 7), pp. 541-589. 



