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] QUANTITATIVE STUDY.