360 .MICROBIOLOGY OF SOIL 



BIOLOGICAL FACTORS 



MOLDS. Distribution. While the study of the lower bacteria in the 

 soil has attracted the attention of many investigators, that of fungi and 

 actinomyces received, until recently, but scant consideration. Fungi oc- 

 cur in all soils, cultivated as well as uncultivated, rich or poor in organic 

 matter, heavy or light in texture. Most of them are obligate sapro- 

 phytes, although facultative parasites are found in large numbers in 

 the soil, especially where single-cropping or short rotations favor the 

 survival of the particular organisms. The isolation of soil fungi has 

 been accomplished either by the dilution method, where- a sample of 

 soil was shaken with water, and only a certain dilution was used for 

 inoculation; and by the direct method, where a clump of soil was inocu- 

 lated into a sterile medium, and the fungi developing on it were isolated. 

 About 150 different species of fungi have been isolated from different 

 soils, and the data accumulated by investigators in this country and 

 in Europe seem to point to the fact that many of these fungi are 

 universal in their habitat, since the same species are recorded to have 

 been isolated from different soil types and in different localities. Most 

 of the work done refers to the classification of the organisms isolated. 

 The largest group of soil fungi belong to the following genera: Mucor, 

 Zygorrhynchus, Rhizopus, Aspergillus, Penicillium, Fusarium, Tri- 

 choderma, Cephalosporium, Monilia, Cladosporium, Alternaria, and 

 Acrostagmus. Many other genera have been isolated, but to a more 

 limited extent. As to the individual species occurring in the soil, 

 Hagem, having isolated about 30 mucors from the soil, states that 

 certain Aspergitti occur in larger numbers than all the mucors taken 

 together. As to quantitative relations, no exact data are available. 

 Some investigators report only several hundred fungi per g. of soil, 

 while others record as many as 1,000,000 per g. of soil; that is the 

 total number of spores and pieces of mycelium that develop on suitable 

 media. As to the numbers and types in relation to depth, Goddard 

 concluded that there does not seem to be an appreciable variation in 

 numbers at the different soil depths. There are very few fungi in the 

 soil below 8 inches and one of the most common forms at these depths 

 is Zygorrhynchus vuilleminii. It was formerly thought that soil fungi 

 are abundant only in acid soils, but recent investigations make it 

 appear that also limed and well-cultivated soils have an abundant 

 fungus flora. 



