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Sterilization of Soil by Irradiation and Some Further Observations on Soil Enzyme Activity¹

ABSTRACT

As an introduction to the study of radiation-sterilized soil as a medium for microbial and plant growth, survival curves for microorganisms have been obtained with 5 and 9 Mev. electrons, hard X-ray and gamma radiation. For a given soil, bacterial survival curves with these radiations are almost superimposable, but they differ among soils. Bacterial numbers in soils approach zero at 2 Mrep. doses, but 4 Mrep. doses are necessary to insure complete sterility for larger soil volumes.

Soil sterilized by radiation still manifests enzyme activity (phosphatase, urease) in the presence of suitable substrates. If this is a general situation, studies of the uptake of some organic nutrients from sterile soil by sterile root systems will not be free from the problem of hydrolysis in situ. Sterilized soil is not toxic toward tomato plants nor does it provide extra nutrient to the plants as a result of radiation. It does, therefore, provide the plant physiologist with a medium for the study of uptake of inorganic nutrients by sterile plants which possesses all the purely chemical and structural features of natural soil.

¹ Contribution from the College of Agriculture, University of California, Berkeley. Supported in part by the U. S. Atomic Energy Commission, Contract No. AT(11-1)-34, Project 50.

² Professor of Soil Biochemistry and Research Assistants, respectively. The present address of R. A. Luse is the Puerto Rico Nuclear Center, Rio Piedras.

Received for publication August 7, 1961. Accepted for publication October 10, 1961.