SOIL MICROBIOLOGICAL EQUILIBRIUM 743 



of repeated wetting and drying of soil in increasing plant growth is 

 believed to be due to this phenomenon. Air drying of soil very markedly 

 increases the amount of water soluble substances especially the inor- 

 ganic soil constituents 11 - 12 (table 82). 



The cultivation of the soil has in itself a stimulating effect upon the 

 bacterial activities; this is, however, negligible in comparison with 

 the effect of drjnng. Proper aeration supplies the oxygen necessary 

 for bacterial activities, and brings about more fundamental changes 

 influencing these activities. This is particularly true when cultivation 

 of soil is accompanied by air drying. 



Influence of caustic lime upon soil processes. Caustic lime (about 

 0.5 per cent) has had a recognized value as an antiseptic. When 

 applied to the soil, even in the presence of large quantities of CaC0 3 , 

 CaO was found to disturb or even destroy the state of equilibrium 

 normally existing between the micro-flora and micro-fauna of the 

 soil. 13 The action of CaO is intermediate between the action of anti- 

 septics and changes induced by high temperatures. Its action seems 

 to consist in bringing about a greater decomposition of the organic 

 nitrogen constituents of the soil. The numbers of bacteria are at 

 first depressed but later on they rise. 14 ' 15 Many bacteria and the 

 larger protozoa are destroyed. The numbers of bacteria remain 

 depressed until the excess of the calcium oxide is transformed into 

 carbonate when active bacterial multiplication takes place. The 

 action of lime depends, of course, upon the character of the soil, each 

 soil neutralizing a definite amount before the phenomenon of partial 

 sterilization becomes evident; when there is no further absorption 

 of lime, the free alkali begins to function as a disinfectant. This is 

 the reason why CaC0 3 does not produce the same effect. 



Partial sterilization of soil. Russell and Hutchinson 16 demonstrated 

 that partial sterilization of soil (heating to 60°C. or treatment with 



11 Kelley, W. P. Ammonification and nitrification in Hawaiian soils. Hawaii 

 Agr. Exp. Sta. Bui. 37, 1915. 



12 Gustafson, A. F. The effect of drying soils on the water-soluble constit- 

 uents. Soil Sci., 13: 173-213. 1922. 



13 Hutchinson, H. B. The partial sterilization of the soil by means of caustic 

 lime. Jour. Agr. Sci., 6: 320. 1913; 6: 302. 1914. 



14 Fischer, H. Uber den Einfluss des Kalkes auf die Bakterien eines Bodens. 

 Landw. Versuchsta., 70: 335-342. 1909. 



16 Miller, F. Uber den Einflusz des Kalkes auf die Bodenbakterien. Ztschr. 

 Garungsphysiol., 4: 194-206. 1914. 



16 Russell and Hutchinson, 1909-1913 (p. 311). 



