118 JOURNAL 0?' FORESTRY 



In natural forest soils the factor described under B plays the greatest 

 role. Losses of nitrogen from the soil occur in the following way : 



A. Through the carrying away of soluble nitrates in the ground 

 water. 



B. Through the activities of denitrifying agents. 



C. Through the removal of timber from the forest. 



Previous researches on the formation of nitrates in forest soils have 

 followed three rather distinct lines of investigation : ( i ) Earth samples 

 have been examined to determine the occurrence of nitrate-forming 

 bacteria; (2) the nitrate-forming capacities of the earth samples have 

 been determined, and (3) the nitrogen content of trees and plants on 

 different sites has been investigated. 



Hesselman has made a number of detailed studies in different forest 

 types, making use of all methods of investigation previously proposed, 

 and from these studies he reaches the following conclusions : 



The mild humus of beech forests contains, distributed throughout its 

 entire portion, nitrate-forming bacteria, as well as denitrifying bacteria. 

 Herbs and grass on such soil also contribute a considerable supply of 

 nitrates. Earth samples on such soils can by storing form considerable 

 amounts of nitrates. 



Coniferous forests with a mossy ground covering are characterized 

 by the absence in the humus of bacteria that can change ammonium 

 sulphate into nitrates. Denitrifying agents are normally absent. Po- 

 tassium nitrate is not found in the tissues of plants constituting the 

 ground cover. The nitrate requirements of such forests must be satis- 

 fied by some means other than the assimilation of nitrates. 



Nitrogen is changed into nitrates in several different kinds of natural 

 soils. It is characteristic of the nitrate-forming soils that the forma- 

 tion of the humus takes place under the influence of electrolytes or 

 mineral salt solutions. In such soils the formation of mild humus is 

 hastened by insects and earthworms, which mix the humus with the 

 ground water. The formation of humus on sites where the mineral- 

 containing ground water is rapidly drained away gives rise to humus 

 forms in which the nitrogen is not changed to nitrates. On account of 

 its physical formation, mild humus becomes nitrated, while raw humus 

 does not. In many situations the nitrification is so rapid that a consid- 

 erable supply of nitrogen becomes stored up in the tissues of the plants 

 composing the ground cover. This is especially true in close stands of 

 beech, elm, oak, ash, alder, and especially in situations where the soil is 

 penetrated by rapidly running water. Even in the highest mountain 

 regions the ground cover in such situations contains a considerable 



