THE COLLOIDAL PROPERTIES OF SOIL 



169 



differ from one going on in aqueous solution such as most 

 chemists are accustomed to think about. We must therefore 

 be prepared for unexpected results : it is possible that some 

 of the changes formerly attributed to bacteria may in part be 

 due to colloids (see p. 215). 



Further, the colloids influence both micro-organisms and 

 plants (p. 239) and therefore indirectly affect the reactions in 

 the soil. 



Again, the remarkable changes observed in heated soils ^ and 

 soils stored in a dry condition, such as the increase in amount 

 of soluble matter,^ in the rate of nitrification,^ and in the pro- 

 ductiveness, are probably much influenced by changes in col- 

 loids,* Gedroiz's^ experiments with oats grown in soils kept 

 dry for a number of years are given in Table XLV. 



Table XLV. — Effect of Storage in a Dry State on the Productive- 

 ness OF Soils. (Oats : Gedroix, 1908.) 



The Estimation of Soil Colloids. 



Efforts have been made to determine the amount of col- 

 loidal material in the soil by studying the absorption of dye- 

 stuffs or of water vapour. So far the results are not easy to 

 interpret nor are they always related to the other colloidal 

 phenomena. It would therefore be premature to attempt any 



' S. U. Pickering, Journ. Agric. Sci,, 1910, 3, 32 and 258. 



* U.S. Department of Agric. Bureau of Soils Bull. 8, p. 13 ; Bull. 22, p. 41 ; 

 also yoiim. Agric. Res., 1918, 12, 383. 



3 Buddin, yourn. Agric. Sci., 1914, 6, 452-455. 



* For study of the biological changes in soil during storage see F. E. Allison, 

 Soil Sci., 1917, 3, 37, and W. Giltner and H. V. Langworthy, yourn. Ag. Res., 

 1916, 5, 927. 



* Gedroiz, Bull. Internal. Instit. Agric. Rome, 1915, p. 37. 



