THE SOIL FUNGI The serious study of the fungus flora of the soil commenced with the work of Oudemans and Koning a quarter of a century ago and since then our knowledge has shown a rapid growth associated chiefly with the names of Hagem and Traaen in Nor way, Lendner in Switzerland, Miinter in Germany, Dale and the Rothamsted workers in England, Jensen, Conn and Waksman in the United States of America and Takahashi in Japan. Compared with other soil organisms the fungi have been little studied but their primary importance is being increasingly recognized. For a i time indeed there was doubt whether fungi lived vegetatively in the soil or were present merely as inert spores, but work at Roth amsted has shown that they may occur almost entirely in the active mycelial form.
In spite of the very great and in many cases, at present almost insuperable difficulties in the identification of soil fungi, a large number of kinds have been recorded including species of eleven genera of Phycomycetes, eight genera of Ascomycetes, and sixty two genera of Fungi Imperfecti. In addition Actinomycetes are present in great variety, Smut fungi occur and Hymenomycetes are present in very great abundance.
Investigations on the distribution of species of fungi in soil, although numerous, have presented no clear issues. In the work at Rothamsted no good evidence has been obtained of distinct fungal associations conditioned by soil factors. The numerical balance of different species varies considerably but no constant and clearly cut differences in kind have been found such as Hagem has reported for the Mucorineae of forest soils in Norway or as are so evident in the distribution of the larger fleshy forms of woods and meadows. Also the different species do not show any seasonal periodicity, the incidence of any particular form being apparently a matter of chance or determined by purely local con ditions. Certain species however seem to penetrate the soil to greater depths than others, in the Rothamsted work species of Penicillium, Fusarium, Saccharomyces, Zygorrhynchus and Tri choderma always being found at the greatest depths.
Owing to the enormous differences in structure and reproduction of various soil fungi such as seen for example in species of Saccharomyces, Mucor, Sordaria, Tuber, Coprinus, Phoma, Actin omyces, it is not possible to obtain an estimate of the number of "individuals" per gram of soil that may be compared with proto zoal or bacterial numbers. Yet by critical standardisation of technique, quantitative research is possible and results may be ob tained that can be replicated and that do give information as to relative amounts of fungi in different soil samples. At Rothamsted, numbers exceeding "one million fungi" per gram of soil have often been obtained and these represent a fraction only of the fungi present. Different soils vary considerably in their numerical fungal content, the more fertile soils containing the greater numbers. Most of the fungi are in the superficial layers, the numbers fall rapidly in all cases below the 6–io inch depth, and at greater depths than about three feet fungi are practically absent. Beyond a suggestion of lower numbers in winter and higher numbers in summer there is no good evidence of seasonal periodicity.
In the course of their metabolic activities fungi play an active part in the soil economy and, under aerobic conditions such as occur in most soils, they are probably the most important factor in the decomposition of the cellulosic matter of plant residues. At Rothamsted it has been found that under experimental conditions three thermophilic soil fungi, Coprinus (fimetarius?), Aspergillus (fumigatus?) and Acremoniella (velutina?) can, jointly, rot down cellulosic matter to a manurial condition (the "Adco process") as rapidly as the whole soil population and far more rapidly than a synthesized bacterial flora of the soil. A great deal of the plant residues decomposed by fungi is however utilised by them in the building up of their protoplasm which is an important part of the soil organic matter.