258 BIOLOGICAL ACTIVITIES OF MOLDS 



make the soil an excellent source of organisms for study. Many of 

 the molds used for monographic treatments of certain groups have 

 come from soils. That molds exist in the soil as growing cells rather 

 than exclusively as chance spores brought in by wind was finally 

 proved by Conn and others who, by direct microscopic methods, 

 demonstrated mycelium below the surface of the soil, and by Waks- 

 man and McClennan by less direct methods. On the basis of work 

 at Rothamsted Experimental Station in England, RusselP^ esti- 

 mated that in fertilized soil there was 1700 pounds of living fungus 

 cell material per acre (2,000,000 pounds of soil), about twice as 

 much as the material from bacteria, algae, and protozoa combined. 

 It is probable that the source of most of our common molds is the 

 soil, or the decaying vegetation on the surface of the soil. The 

 taxonomy and distribution of the various molds found in soils are 

 discussed in a number of contributions. The student is referred 

 especially to a book by Oilman-^ published in 1945 which gives 

 brief descriptions of all species of molds that have been reported in 

 soil, together with keys to genera and species, and rather poor 

 sketches t)f each genus. 



Well over 250 species of molds have been isolated from soils. The 

 most common genera are Penicillium, Zygorrhynchus, Trichoderma, 

 Fusarium, Aspergillus, Mucor, Rhizopus, Alternaria, and Clado- 

 sporium, but many others are frequently found. Recently a con- 

 siderable flora of Oomycetes (and of Myxomycetes!) has been dem- 

 onstrated by special techniques. The relative abundance of differ- 

 ent molds varies considerably with climate and with season. Here 

 in Minnesota with our continental climate of cold winters and hot 

 summers we find a decided difference between our early spring and 

 our late summer flora. In the spring we find that Penicillium will 

 produce almost as many colonies as all the other genera combined 

 but in the heat of the summer we get many more Aspergillus, Alter- 

 naria, Fusarium, and Cladosporium. This may be partly due to 

 organisms brought in from the southwest by prevailing winds but 

 controlled experiments indicate that this is only part of the explana- 

 tion. It has been stated that in warm climates the soil Aspergilli 

 outnumber the Penicillia and in cold climates the reverse. 



Soil fungi play two important roles in maintaining soil fertility. 

 They readily decompose complex organic substances, especially 

 starch, cellulose, chitin, and proteins, thus rendering the elements 

 contained in dead plant and animal tissues available to plants as 

 food. They assimilate soluble inorganic nitrogen compounds and 

 minerals, thus removing them temporarily from soil solution, and 



