STABLE AND MIGRATORY PLANT FORMATIONS 65 



Common to both is only one character — poverty in assimil- 

 able nitrogen " (28). Again, Schroter and Kirchner say : 

 " One character especially must be brought into prominence 

 which all forms of Pinus viontana have in common — namel}', 

 the poverty of assimilable nitrogen in the soil ; limestone 

 crags, limestone screes, and dune sands all show very little 

 nitrogen content, and in the humus of the 'Hochmoor' it 

 is present in a form available only with difficulty. Pinus 

 tnontana is, however, specially well equipped against nitrogen 

 poverty in the soil (according to P. Muller's recent investi- 

 gations), through its capacity to assimilate nitrogen with the 

 aid of endotrophic mycorhiza " (29). 



Nitrogen is the necessary element of plant food which for 

 its existence in an assimilable form depends most on the 

 local relations of the soil to plant life. The supply, and form 

 of occurrence of nitrogen in the soil, would therefore appear 

 to be of great importance in defining the type of vegetation 

 that can persist over any particular area of the land surface. 



Apart from aquatic habitats and those liable to flooding, 

 the relation between the humus content and the nitrogen 

 content in natural soils is probably a close one. Where the 

 soil is strongh' acid the humus and nitrogen content may be 

 high, but the latter in a condition physically unavailable for 

 plant life. In soils where acidity is prevented by abundance 

 of calcium carbonate the humus content might appear to be 

 an index to the available nitrogen ; but since the nitrogen 

 chiefly becomes available through the destruction of humus, 

 and may perhaps further result directly from plant decay 

 without the intervention of humus formation, this is by no 

 means necessarily true. 



In the case of very shallow, dry limestone soils plant decay 

 is rapid, apparently owing to the frequent aeration of the 

 soil and the prevention of acidity, and the accumulation of 

 humus is minimised. The available nitrogen in such a soil 

 would therefore depend on the balance from the loss of 

 ammonia that occurs during the processes of decay. In 

 cultivated soils excessive liming is believed to cause a 

 wastage of nitrogen, both as ammonia, and through too rapid 

 nitrification and leaching depleting the stock of humus (22). 

 In natural, shallow, dr}' limestone soils the loss of ammonia 



