54 Microbiological Population of the Soil 



Distribution of Fungi and Other Nonbacterial 

 Microorganisms in the Soil 



Fungi are important constituent groups of the soil population. 

 They are widely distributed, certain forms being characteristic of 

 one type of soil as a natural medium for their development, and 

 others of other soils. Fungi exist in the soil in the form of vegetative 

 mycelium and of spores. A colony of a fungus developing on the 

 agar plate may thus represent either a spore or a piece of mycelium. 

 The latter produces a fine network around the soil particles. The 

 mycelium is sensitive to drying of soil, as a result of which dry soil 

 contains fewer fungi. 



The mycelium of some fungi does not break up readily into fine 

 particles, so that each of these would develop into a colony on the 

 plate. Because of this, the plate method does not give a fair idea 

 of the abundance or distribution of various fungi in the soil. A high 

 plate count of certain fungi may merely indicate a high sporulating 

 capacity of these organisms. This is true particularly of species of 

 Aspergillus and Pcnicillium, the mycelium of which also breaks up 

 readily. On the other hand, the mycelium of the mucorales does 

 not break up readily, which accounts for their relatively low num- 

 bers as determined by the plate method. Although it has been def- 

 initely demonstrated that the normal fungus population in the soil 

 is present extensively in the mycelial state, the question is still raised: 

 to what extent does the plate count represent the actual abundance 

 of fungi in the soil? 



Brierley found that the plate count of fungi is open to certain 

 criticisms: (1) The slow-growing Basidiomycetes are almost all 

 eliminated in plating the soil and are not found among the colonies 

 developing on the plate. (2) The same is true of some of the slow- 

 growing Ascomycetes and Fungi Imperfecti. (3) Some of the Phy- 

 comycetes require special techniques for their isolation and do not 

 develop on the plate at all. Most of the published lists of fungi 

 found in the soil, especially when determined by the plate method, 

 thus represent only a fraction of the total fungus population. 



There is no basis for comparing the relative abundance of the 

 bacterial and fungus flora of the soil with their potential activity in 

 the soil, especially when it is not known whether the fungi represent 

 mycelium or spores. The presence in a given amount of soil, as 

 measured by the plate method, of a thousand fungi may indicate a 



