As it is not possible to count the soil fungi _in situ_, any estimation of the numbers present in a soil must be arrived at by indirect means. The method adopted is to make as fine a suspension as possible of a known quantity of soil sample in a known amount of water, dilute this to 1/5000, 1/10000, and so forth by regular gradations, incubate cubic centimetres of the final dilution on artificial media in petri dishes, and count the colonies of fungi developing in each plate. Using the average figures from a series of duplicate plates, the number of “individual” fungi in a gram of the original soil sample may then be calculated. The very few students who have made quantitative estimations have obtained very unsatisfactory results. In bacterial or protozoal estimations, the shaking of the soil suspension separates the unicellular individuals, so that in the final platings each individual from the soil theoretically gives rise to one colony on the medium. In the case of fungi, the organisms may be in the form of unicellular or multicellular spores or larger or smaller masses of unicellular or multicellular mycelium differing for each particular species or phase of development within the single species. The organisms may be sterile in the soil or form fruiting bodies, consisting of few or myriads of locally or widely distributed spores. In the process of shaking the soil-suspension fungi of different organisation or of differing developmental stages may be broken up and moieties fragmented in totally different ways or to very different degrees. With protozoa and bacteria the relation of soil individual to plate colony is direct; with fungi we do not know what is the soil “individual” nor whether it is the same for different fungi; nor can we yet profitably discuss any significant numerical relationship of plate colonies to soil organisms. Thus Conn[4] has pointed out that the plate count of a fungus indicates only the ability to produce reproductive bodies and found that the spores of one colony of _Aspergillus_, if distributed evenly through a kilogram of soil, could produce the average plate counts obtained by Waksman. Abundant vegetative growth may, in some species, reduce or inhibit spore formation, so that of two species the one giving a lower count might really be much the more important and plentiful in the soil. Further, the colonies developing in the final plates represent only a selected few of the fungi present in the soil sample, the _Basidiomycetes_, and no doubt many other forms, being absent. In addition, different media differ among themselves in the average number of colonies developing on the plates, each medium giving, as it were, its own point of view. Thus, in one experiment carried out at Rothamsted by Miss Jewson, using the same soil suspension, twenty plates of Coon’s Agar gave 357 colonies, of Cook’s Agar 246, of Czapek’s Agar 215, and of Prune Agar 366. Thus if one only used