SYMBIOSIS 81 



It will thus be evident that most of the valuable mineral 

 ash is determined to the leaves, and that the timber itself 

 contains only a little. Hence the rapid soil deterioration, 

 as far as mineral plant food is concerned, that takes place 

 when leaves are removed from woodlands. The removal 

 of leaves is, moreover, also accompanied by a loss of moisture 

 and also a loss of nitrogen, for decaying leaves and humus 

 afford the chief source of nitrogen in the form of nitric acid 

 for the growing trees. 



Now, experiments have shown that the leaves of many 

 trees, especially Beech, Hornbeam, and Poplars, will, as they 

 decay, become associated with conditions under which the 

 free nitrogen of the air is rendered available for plant food. 

 The free nitrogen of the air is also utilised by leguminous 

 trees such as Acacia (Robinia) by means of micro-organisms 

 contained in nodules on their roots. There are also trees 

 which are not leguminous, such as Alder, which have nodules 

 on their roots containing a fungus, and which, by a process 

 of symbiosis, manufacture and absorb nitrogenous substances, 

 which are, in their turn, utilised by the tree. 



A somewhat similar condition prevails in the case of 

 Oak, Spanish Chestnut, Beech, and some other trees, on 

 whose roots are small mico-rhizas, the mycelial filaments of 

 which absorb nitrogenous substances for the tree, and perhaps 

 also manufacture them, thus utilising the free nitrogen of 

 the air. 



So also, a symbiotic growth is witnessed when mixtures 

 of Pinus montana and Spruce are made. In this case, it 

 would seem that a fungus on the roots of the former aids in 

 the provision of nitrogenous food material for the latter ; 

 especially is this the case on poor sandy soils. 



The amount of nitrogen required by timber crops is very 

 small when compared to the requirements of agricultural 

 crops. The perpetual removal of mature timber from well- 

 managed woodlands should never result in a loss of nitrogen 

 to the soil. There will, on the other hand, often be an actual 

 gain of soil nitrogen. 



The average amount of lime, potash, and phosphoric 



F 



