Soils in the green-house are exposed to conditions that are admittedly more or less artificial. The range of temperature and moisture is not the same as that in field soils, while the aeration of green-house soils is, if anything, even more artificial. The artificial conditions are emphasized still more strongly when the soil is kept in small pots. The operations incident to the filling of the pots and the applications of fertilizers involve a more intimate contact of the soil particles with atmospheric oxygen than is possible under field or garden conditions. This leads to an abnormal multiplication of the soil bacteria and to a consequent abnormally rapid oxidation of the organic matter. In the course of time the more readily decomposable portions of the organic matter in the soil become depleted and this is followed, in turn, by a decline in the numbers of bacteria that will grow on agar plates. It is possible that the rapid falling off in numbers is due not merely to the depletion of the readily decomposable organic matter, but also to the accumulation of certain cleavage products injurious to the bacteria. Of the latter factor we have no direct knowledge and it is referred to in this place only as a possible explanation of the facts recorded below. It may be added here, also, that the greatly decreased number of bacteria appearing on agar plates should not be accepted as absolute proof that the total number of microorganisms in the soil had diminished. There is a possibility that a compensating increase had occurred of bacteria that do not grow on agar plates, as for instance, the nitrous and nitric ferments, etc.….