SOIL FUNGI 261 



soil in the form of residues or green manures, which contain only a very 

 small amount of nitrogen (0.3 to 2.0 per cent). Fungi rapidly decom- 

 pose practically all the constituents of the organic matter added to the 

 soil, with the possible exception of the lignins. The fungi are very 

 economical in this process, assimilating as much as 30 to 50 per cent of 

 the carbon for the synthesis of cell substance. 81 The latter contains 

 3.5 to 8.0 per cent of nitrogen. In other words the minimum nitrogen 

 content of the fungus mycelium is twice as much as the maximum nitro- 

 gen content of green manure. If one part of fungus mycelium is synthe- 

 sized for every three parts of green manure and plant stubble 

 decomposed, and if the former contains three or more times as much 

 nitrogen as the latter, this element will be completely reassimilated by 

 the fungi; they may even assimilate, under certain conditions (when the 



plant material is low in nitrogen), the available nitrogen compounds in 

 the soil (p. 515). 



The role of fungi in the growth of higher plants may thus be both 

 beneficial and injurious, depending upon conditions. This can be 

 illustrated well by the phenomenon of formation of "fairy rings." 

 When the spores of Agaricus germinate in the soil, small circular areas 

 are formed and the native grasses are stimulated. The mycelium 

 begins to spread in all directions as fungi usually do in culture media. 

 The outward growth is slow — about 12 cm. a year. The sod is at first 

 stimulated by an increase in the available nitrogen resulting from the 

 decomposition of the organic matter in the soil, then killed by insufficient 

 soil moisture in the area of dense mycelium. When the mycelium in its 



81 Waksman and Heukelekian, 1924 (p. 443) ; Waksman and Skinner, 1926 

 (p. 190). 



