MICROBIOLOGICAL STUDIES OF THE ACTION OF SOME SELECTED SOIL FUMIGANTS

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Abstract
Microbiological studies involving the use of three soil fumigants, methyl bromide, a propane–propene mixture, and ethylene dibromide, established a biological basis for the evaluation of soil fumigants as fungicides, bactericides, and actinomycides and provided quantitative and qualitative data from which the efficacy of each fumigant was rated as high, medium, low, and poor in each capacity and permitted a study of the significance of population trends. Due emphasis was placed on the two factors, concentration of the fumigant and the period of confined exposure, by determining the value of each factor required to produce, under controlled conditions of temperature and soil moisture, a 90% reduction in numbers of each microbiological group. Methyl bromide proved to be the most effective fumigant in all three capacities while ethylene dibromide formulations were least effective. The concentrations of, and exposure periods to, ethylene dibromide and the propane–propene mixture required to give satisfactory reduction of the three microbiological populations are considered high and neither fumigant is recommended as a fungicide, bactericide, or actinomycide. However, ethylene dibromide proved to be more effective than methyl bromide against root-knot nematodes and is recommended for use in this capacity. Soil fumigation effectively altered the microbiological balance in favor of fungi that were normally suppressed by a predominance of aspergilli and penicillia. Of four physiological groups of bacteria studied, nitrifiers and cellulose decomposers were most severely reduced in numbers by methyl bromide fumigation. The actinomycetes showed a greater tolerance of fumigation than either the fungi or bacteria.