70 JOURNAL OF FORESTRY 



latter class belongs the bulk of the Swedish forest, namely, all the 

 coniferous forests bearing heavy growths of moss and lichen. In 

 forests of this type, clear cutting or even thinnings may bring about 

 lively nitrification. The same result may be accomplished by cultiva- 

 tion in which the humus is mixed with mineral soil, even in closed 

 stands. Decaying brush or logs also favor nitrification. If the layer 

 of raw humus is very thick, cutting alone may not suffice to bring about 

 nitrogen formation, although the production of ammonia is increased. 

 Under such conditions, nitrification is hastened by cultivation or burn- 

 ing. Where large openings are made, the increased nitrogen supply, 

 together with increased light, may favor herbaceous growth to the det- 

 riment of forest reproduction. 



Hesselman enters into a detailed discussion of various theories which 

 seek to explain the favorable action of the various treatments above 

 cited upon soil nitrification. His own theory, supported by previous 

 investigations, is that the bacteria which are most active in nitrification 

 require a certain amount of salts (electrolytes) for their development. 

 He also cites the investigations of the Danish soil bacteriologist, Chris- 

 tensen, which show that one of the most universal nitrifying bacteria, 

 aaotobacter, occurs only in soil which is rich in salts, especially calcium. 

 It never occurs in acid soils, rarely in neutral soils, but is nearly always 

 found in soils of alkaline reaction. Hesselman's opinion is that all 

 soils are continually being inoculated with nitrifying bacteria. Where 

 conditions are favorable the bacteria develop rapidly, with the result 

 that nitrification is active ; but where conditions are unfavorable — i. e.,. 

 in acid or neutral soils — the bacteria die or do not develop sufficiently 

 to become a factor in soil nitrification. 



The function of all the measures recommended to promote nitrifica- 

 tion and aid reproduction, namely, opening up the stand, cultivation, 

 and burning, is to supply the salts which are absent in the raw humus 

 and which are required by the nitrifying bacteria. 



When the stand is opened by cutting, the addition of green slash 

 may supply enough salts to start nitrification. After the process is 

 once under way, there is a further liberation of salts due to chemical 

 action. Opening up the stand also increases evaporation from the soil, 

 and thus carries up salts from the lower strata. Increased aeration, 

 which is commonly regarded as an aid to nitrification, is not here re- 

 garded as a potent factor, because under ordinary circumstances aera- 

 tion, even in dense stands, is sufficient for the needs of nitrifying 

 bacteria. 



