A METHOD FOR COUNTING THE NUMBER OF FUNGI 



IN THE SOIL' 



SELMAN A. WAKSMAN 

 Received for publication November 2, 1921 



The numbers of fungi in the soil are usually determined by 

 the plate method used for the determination of the number of 

 bacteria. In view of the fact that the dilution used for the 

 determination of bacteria is necessarily high, due to the large 

 numbers of bacteria in the soil, the fungi are so diluted that very 

 few appear on the plate: of a dozen plates prepared from the 

 same soil, using the same dilution, three or four may be free 

 from fungi entirely, three or four may have only one or two fungus 

 colonies, while three or four may have several colonies, particu- 

 larly in the case of humus-rich and acid soils. It has been 

 pointed out by the author (1922 a) that the probable error in- 

 volved in the determination of the numbers of fungi by this 

 method is so great, as to make the results absolutely worthless. 



To reduce the variabiUty of the numbers of fungi on the plate 

 and thus obtain a low probable error, low dilutions have to be 

 used, so as to have 30 to 100 fungus colonies developing on the 

 plate; this would necessitate a dilution of only 500 to 2000 for an 

 ordinary fertile soil. But, if the common plate used for the 

 determination of bacterial numbers is employed, so many bac- 

 teria will develop on the plate, as to prevent the development of 

 most of the fungi. 



To obviate this difficulty, use was made of the fact that fungi 

 can grow readily at a much higher acidity than the bacteria and 

 actinomycetes. 



The author and others have long made use of the fact, that, 

 when a culture of a fungus is wanted free from bacteria raisin 



'Technical Paper No. (61) of the New Jersey Agricultural Experiment Station, 

 Department of Soil Chemistry and Bacteriology. 



339 



