XIV APPENDIX. 



mixture of this gas (one containing 35 to GO per cent, of oxygen) 

 for nitrification. Further " when the supply of oxygen becomes 

 limited and anaerobic conditions are produced, denitrification sets in 

 and this continues until practically all the nitrates are destroyed. " 

 Therefore it is clear that all these factors of the soil are inter- 

 dependent one on the other. Without a proper supply of oxygen 

 active nitrification will not occur and where the latter condition 

 is not fulfilled adequate regeneration will not take place. We have 

 already seen the conditions under which nitrification can be 

 increased in coniferous woods by ordinary forest operations. 



The Dehra Dun experiments of Hole have further shown that 

 when rain water rich in oxygen is held in contact with orHinary 

 forest soil, the oxygen is rapidly exhausted and the supply of Co 2 

 is increased. An increase in the Co 2 dissolved in the soil water 

 necessarily means an increase in soil acidity detrimental to active 

 nitrification. He has also proved that a water culture solution 

 containing an excess of Co 2 and a deficiency of oxygen has a 

 directly toxic effect on plant roots. The advocates of the soil 

 toxin theory also have shown that such toxins cannot exist under 

 conditions which bring air, and therefore oxygen, into contact 

 with toxic solutions. 



The extent to which nitrification is going on in the soil is 

 frequently apparent from the vegetation found on the ground. 

 In Sweden Hesselman states : " Where the humus nitrogen is 

 transformed into nitrate, there appear nitratophilous plants such as 

 raspberry. Epilobium angusti folium, Arenaria trinervia, etc. " 

 In Kulu, Trevor writes : " There can further be no doubt that the 

 presence of shrubs and plants of the natural order Leguminosse is 

 especially desirable and the presence of Indigofera Gerardiana 

 has been observed to improve the vigour of young deodar 

 growing in its vicinity. " In Kulu a growth of raspberry and 

 Indigofera follows a fire, indicajbing active nitrification, and profuse 

 regeneration of pine and spruce is frequently a feature of such old 

 burns. 



