242 PLANT SOCIOLOGY 



nitrates shall predominate. Very acid soils without herbaceous cover, 

 in conifer forests, have a decided tendency toward denitrification. 



2. ORGANISMS OF WEATHERING AND SOIL FORMATION 



Microorganisms are directly concerned in the formation of soils by 

 the disintegrating action which they exert upon inorganic mineral 

 substances and by the separation of mineral particles. Their indirect 

 action lies in the formation of humus. 



Biogenic Weathering of Rocks. — The physical and chemical weath- 

 ering of rocks is furthered not only by the higher plants but also by 

 algae, hchens, and bacteria. The bacteria which cause weathering of 

 silicates are probably the most important of all biological weathering 

 factors. Bachmann (1915, 1916) has investigated algae and fungi 

 which dissolve Ume. He proved that algal filaments of Trentepohlia 

 actually penetrate calcite crystals. 



As early as 1890 Muntz called attention to the corrosion of rocks by 

 nitrobacteria. In the bituminous schists whole rock layers are found 

 permeated with and crumbled by nitrobacteria. From the decomposi- 

 tion of these calcareous slates an exceedingly fertile loam results which 

 in high mountains is usually occupied by Saxifraga hiflora, Campanula 

 cenisia, Leontodon montanus, and in lower altitudes by such Leguminosae 

 as Hedysarum, Oxytropis montana, and Onobrychis, long before any sign 

 of humus formation is visible. 



Kiirsteiner (1923) found considerable quantities of Bacterium 

 m£sentericus and other schizomycetes in primary erosion layers free of 

 humus and in fine rock crevices of high alpine summits even at heights 

 of 4,000 to 4,200 m. These minute organisms probably hasten chem- 

 ical erosion by production of carbonic acid, H2CO3. Carbonates dis- 

 solve in the presence of carbonic acid, especially the widely distributed 

 carbonates of calcium, magnesium and iron. Silicates of the alkahes 

 and alkaline earths are decomposed by carbonic acid (Ramann, 1911, 

 p. 28). Very succinct data on the importance of microorganisms 

 in weathering and soil formation are found in the work of Wollny 

 (1898). 



Humification. — Physicochemical weathering of rock furnishes the 

 raw material for soil formation. Higher plants derive their elementary 

 food materials from the residual soil and after death return them to the 

 earth in complex compounds. 



The dead plant material forms the starting point in the chemical 

 transformations which are summed up in the term "humification" — 

 the transformation of organic substances into humus. In this process 

 the active forces are the communities of soil organisms, especially 



