SOIL ORGANISMS 239 



exceedingly simple. As Raunkiaer (1926, p. 9) showed, it is best to use 

 plant sap which is pressed out of the plants. 



To a fraction, of a drop of plant sap in a watch glass a drop of sulphuric acid 

 and a crystal of diphenylamine are added. A deep blue coloration takes place in 

 the case of a high nitrate content. According to the intensity of the blue color- 

 ation, several degrees of nitrate content may be distinguished. This method is 

 not applicable to woody plants, for in the presence of lignin the reaction does not 

 occur. 



Nitrophilous Plant Communities. — Although at present very few 

 reliable investigations on nitrification in natural plant communities are 

 on record, these are sufficient to indicate the range of influence of this 

 factor. Intensive nitrification occurs in most meadow soils and in 

 many forest soils with an herb layer. Individual communities such as 

 Alnus woods with numerous herbs, and deciduous forests in Sweden, 

 also the communities around springs are characterized by a high nitrate 

 content. Snow-land communities composed of Catabrosa algida, Poa 

 alpma, Cerastinm cerastioides, Saxifraga stellaris, and other species were 

 found rich in nitrates by Hesselman (1917, p. 40). Generally, flowing 

 oxygenated water seems especially favorable to nitrification (see p. 

 172). Species of the halophilous shore vegetation, such as Atriplex 

 and Honkenya peploides, also give a strong nitrate reaction. On the 

 contrary, all plant communities which form raw humus are poor in 

 nitrates. This has some connection with the lack of bases. Even 

 though nitrate bacteria can endure strongly acid reactions, their 

 activity is, nevertheless, greatly decreased in acid media. Soils of 

 forests and heaths rich in Calluna, Vaccinium, and Empetrum all have 

 a low nitrifying power. This seems to hold true also for high moors, 

 while water-logged Carex flat moors and probably all communities of 

 the Molinion nitrify abundantly. 



In forest the best nitrification is shown after cutting when nitri- 

 fication and decomposition of litter are favored by light. Rubus, 

 Galeopsis, Fragaria, Epilobimn angustifolium, and Mercurialis perennis 

 form a group of nitrate indicators. According to Nemec and Kvapil 

 (1927), mixed conifer and deciduous forests are more favorable for 

 nitrification than pure stands of either sort. In the beech forest with 

 Rumex acetosella nitrification is less than in that characterized by the 

 presence of Asperula, Mercurialis, and Paris. 



In the mild humus of deciduous forests the formation of nitrates 

 proceeds normally. Anemone nemorosa, according to Raunkiaer, has 

 its highest nitrate capacity in the beech woods. It is surpassed there, 

 as in oak and alder woods, by M. perennis. Plants store more nitrates 

 in the shade than in the sunlight. 



