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Radioactive Phosphorus and the Growth and Metabolic Activities of Soil Microorganisms¹

ABSTRACT

Experiments were conducted in quartz sand and in a silt loam to determine effect of differing levels of P³² on the growth and metabolic activity of soil microorganisms. Determinations were made of carbon dioxide evolved and of numbers of microorganisms, and in some experiments, of amounts of soluble phosphate, nitrate, and ammonia.

In quartz sand treated with phosphorus, dextrose, urea, and mineral salts, and with differing amounts of P³², activity levels of 0.005 and 0.05 mc gave no appreciable differences. At 0.5 mc P³² per mg of P³¹, the rate of CO₂ production was reduced during the first week, and differences in the soil flora were encountered. Recovery in CO₂ production then followed, but the soil flora differences persisted throughout 3 weeks of incubation.

In Seymour silt loam treated with 0.5 mc of P³² per mg of P³¹, there was found no influence on microbial numbers or on carbon dioxide evolution, but the amounts of soluble phosphate, ammonia, and nitrate at the end of 3 weeks were lower than for 0.05-, 0.05-, or 0-mc levels of treatment. When the phosphorus treatment was supplemented with dextrose and urea, no differences in the P or N status of the aliquots could be demonstrated. With dextrose and urea added and a 5.0-mc level of activity, there was only slight reduction in CO₂ evolution and only slight differences in microbial numbers.

¹ Soil Phosphorus Research Project of the North Central Region. Contribution from the B.P.I.S.A.E., and the Iowa Agricultural Experiment Station. Journal paper no. J-1383, Project 965, of the Iowa Agricultural Experiment Station, Ames, Iowa. Received for publication August 31, 1951.

² Soil Scientist and Bacteriologist, respectively. The authors are indebted to Dr. L. A. Dean for many helpful suggestions in planning this work.