Qualitative Study.
With very rare exceptions soil fungi cannot be examined in situ, and the necessary basis of any qualitative research is the isolation of the organisms in pure culture. Most soil forms belong to the Fungi imperfecti, and often show considerable plasticity on artificial media. This makes it very difficult to determine them by comparison with type herbarium specimens or published morphological diagnoses. In consequence many soil fungi have not infrequently been given new specific names, as humicola, terricola, and so forth, which is very unsatisfactory, and means that the determinations have little significance.
Furthermore, most artificial media are slight variations on a few common and simple themes, and are very selective, permitting the growth of a moiety only of the fungi present. In addition, many fungi grow so slowly that they are overwhelmed by the more rapidly germinating or spreading forms, or on the other hand, they may be eliminated by the metabolic products of different adjacent colonies. The extremely selective nature of the technique commonly used is shown if one tabulates systematically all the fungi which have been recorded or described in soil investigations. Of Phycomycetes there are fifty-six species of eleven genera; of Ascomycetes twelve species of eight genera; and of Fungi imperfecti, including Actinomycetes but not sterile Mycelia, 197 species of sixty-two genera. Rusts and Smuts one might not expect, but that of the multitudes of Basidiomycetes growing in wood and meadow not one should have been recorded is indeed startling. It was at first thought that many imperfect fungi might be conidial stages of Basidiomycetes, but much search among forms isolated at Rothamsted has, up to the present, failed to reveal clamp connections in the hyphæ.
Since various species of soil fungi have different optimum temperature, humidity and other conditions[3] one would not expect to find an even geographic distribution. Very little is yet known of this aspect, but Rhizopus nigricans, Mucor racemosus, Zygorrhynchus vuilleminii, Aspergillus niger, Trichoderma koningi, Cladosporium herbarum, and many species of Aspergillus, Penicillium, Fusarium, Alternaria, and Cephalosporium have been commonly found throughout North America and Europe wherever soils have been examined. Species of Aspergillus, however, would appear to be more common in the soils of south temperate regions and species of Penicillium, Mucor, Trichoderma, and Fusarium more abundant in northern soils.
It is well known that in many plant and animal communities there occurs a definite rhythm, various species following each other in a regular sequence as dominants in the population. Although it is not yet possible to make any definite statement there would seem indications that this may also be true of the soil fungi.
Much work has been done on the distribution of species at different depths in the soil, but the results are still confusing. Thus, examining eighteen species, Goddard[7] found no difference in relative distribution down to 51⁄2 inches. Werkenthin[26] found identical species from 1-4 inches, and then an absence of fungi from 5-7 inches, which latter was the greatest depth he examined. Waksman[25] found little difference in the first six inches, but very few species below 8 inches except Zygorrhynchus vuilleminii, which extended down to 30 inches and was often the only species occurring below 12 inches. Taylor[23] has reported species of Fusarium at practically every depth to 24 inches. Rathbun[19] found Aspergillus niger, Rhizopus nigricans, and species of Fusarium and Mucor down to 34 inches, and Oospora lactis, Trichoderma koningi, Zygorrhynchus vuilleminii and species of Penicillium, Spicaria and Saccharomyces as deep as 44 inches. Eleven species were isolated from the alimentary canal of grubs and worms, and Rathbun concluded that soil fungi may be spread by these organisms.
On an unmanured grass plot at Rothamsted twenty species were isolated from a depth of 1 inch, nineteen from 6 inches, and eleven from 12 inches, whereas on the unmanured plot of Broadbalk wheat field twenty-six species were obtained from 1 inch, seven from 6 inches, and five from 12 inches. There appeared to be no conspicuous differences between the floras of the two plots save that in the Broadbalk plot there were fewer Mucorales, and Zygorrhynchus mœlleri and Absidia cylindrospora were absent. In the grass plot samples about one-half the forms occurring at the lower levels were isolated also from the upper levels, but in the Broadbalk sample the five forms isolated from 12 inches, and five out of seven of those at 6 inches occurred only at those levels, i.e. each of the three levels appeared to have a specific flora. The difference in depth distribution in these two cases may be due to the fact that in the Broadbalk plot the stiff clay subsoil occurs at 5-7 inches, whereas in the grass plot the depth of soil is greater than 12 inches. Much further work needs to be done on this aspect before any definite conclusion can be reached.
Much scattered information is available concerning the effect of soil type, manuring, treatment, cropping, and so forth upon the fungus content, but no clear issue as yet emerges from the results. Hagem[8] found that cultivated soils vary greatly from forest soils in the species of Mucor present, and that certain species seem to be associated in similar environments. Thus in pinewoods Mucor ramannianus is usually found, together with M. strictus, M. flavus, and M. sylvaticus, and with this “M. Ramannianus Society,” M. racemosus, M. hiemalis, and Absidia orchidis, are frequently associated. The differences found by Hagem between the species of Mucor from forest and cultivated land could not, however, be confirmed by Werkenthin.[26]
Dale,[5] examining sandy, chalky, peaty and black earth soils, found specific differences, although many of the species were common to all. A soil which had been manured continuously for thirty-eight years with ammonium sulphate alone, contained twenty-two species, whereas the same soil with the addition of lime only had thirteen species. Both Goddard[7] and Werkenthin,[26] in their investigations, found a constant and characteristic fungus flora regardless of soil type, tillage, or manuring. Waksman’s[25] studies of forest soils showed few species of Mucor but many of Penicillium and Trichoderma[2]; orchard soil contained no species of Trichoderma, very few of Penicillium, but a large number of species of Mucor; species of Trichoderma were common in acid soils, whilst cultivated garden soil contained all forms. The examination of very differently manured plots on the Broadbalk wheat field at Rothamsted has not shown any striking differences in the fungus flora, all the more important groups of species being represented in every plot, but significant minor differences are present. Thus, plot 13, manured with double ammonium salts, superphosphate and sulphate of potash, is especially rich in “species” of Trichoderma, whereas the unmanured plot contains large numbers of species of green Penicillium, Trichoderma, and a species of Botrytis (pyramidalis?).
The effect of the crop upon the fungus flora is seen in cases where the same crop is grown year after year as in certain flax areas, where species of Fusarium accumulate in the soil and tend to produce “flax sickness.”[13]