SOIL FUNGI 239 



for example, has shown that cultivated soils have a distinctly different 

 population of Mucorales than pine forest soils. The influence of reac- 

 tion on the fungus population of the soil can be seen from the following 

 example : 



A soil receiving manure year after year, in addition to minerals (pH 5.5), 

 had 79,000 fungi; the same soil receiving lime in addition to manure (pH 6.7) 

 had only 10,000 fungi per gram. The soil receiving no manure or fertilizer (pH 

 5.1), had 87,000 fungi; the same soil limed (pH 7.0) had only 16,000 fungi. The 

 soil receiving ammonium sulfate and mineral (pH 4.2) had 129,000 fungi; the same 

 soil limed (pH 5.2) had 32,000. 



Methods of demonstrating the occurrence and abundance of fungi in the 

 soil. The methods of studying the occurrence of fungi in the soil can 

 be divided into two groups; (1) Those methods which demonstrate the 

 presence of particular fungi in the soil, without any reference to the 

 question whether these occur there only in the form of spores or also 

 in the form of vegetative mycelium. (2) Those methods which tend 

 to demonstrate the occurrence of fungi in the soil in the form of vege- 

 tative mycelium. The first method is usually carried out as follows: 



Soil samples are taken under aseptic conditions into sterile containers. There 

 is greater danger of exposing the soil to air contamination, in the study of fungi 

 than of bacteria. Various fungus spores are very abundant in the ordinary bac- 

 teriological laboratory, and because of the smaller number of fungi than bacteria 

 in the soil (or cells developing into colonies) this error introduced will be greater 

 in the case of fungi. The presence of dust fungi will lead also to misstatements 

 in reference to the types of fungi present in the soil. 



A definite amount of soil is diluted with a definite amount of sterile tap water, 

 and sufficiently stirred to separate the spores and pieces of mycelium from the 

 soil particles. One-cubic-centimeter portions of the desired dilutions are then 

 plated out with agar of definite composition and the plates are allowed to incu- 

 bate for 48 to 72 hours at 25° to 30°C. Ordinary bacteriological media can be 

 used for this purpose but acid media, having a reaction of pH = 4.0, are preferable 

 for this first step of the isolation of fungi. The acidity prevents the bacteria 

 from developing and the fungi can be isolated free from bacterial contamina- 

 tions. A lower dilution can be employed, than would be the case with media 

 upon which bacteria are able to develop; this allows the development of greater 

 numbers and, therefore, of a greater variety of fungi. The medium described 

 above (p. 19) can be used for this purpose. Any other sugar medium well 

 adapted for the growth of fungi, to which some citric acid is added (about 1 per 

 cent) can be used. 25 For the isolation of yeast, a medium containing saccharose 

 and 1.2 to 1.5 per cent citric acid is often recommended. Lactic acid can also be 



26 Piettre, M., and de Souza, G. Milieux acides pour l'isolement des champig- 

 nons. Compt. Rend. Soc. Biol. 86: 336-337. 1922; Isolement des levures en 

 milieux acides. Ibid., 338-340. 



