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Research Bulletin (Iowa Agriculture and Home Economics Experiment Station)

Abstract

1. By means of the" modified synthetic" agar plate method bacteria are shown to be present in large numbers in a typical Wisconsin drift soil when it is completely frozen and the temperature is below zero degrees Centigrade; furthermore, increases and decreases in numbers of organisms occur during this period and large numbers are found after the soil has been frozen for a considerable period than before it begins to freeze.

2. During the fall season, the number of bacteria present in the soil diminishes gradually with the lowering of the temperature.

3. Frozen soils possess a much greater ammonifying power than non-frozen soils whether they are tested by the peptone solution method or by the dried blood or cottonseed meal method.

4. During the fall season, the ammonifying power of the soil increases until the temperature of the soil almost reaches zero, when a decrease occurs, and this is followed by a gradual increase and the ammonifying power of the soil reaches a maximum at the end of the frozen period.

5. The nitrifying power of frozen soils is weak and shows no tendency to increase with extension of the frozen period.

6. Frozen soils possess a decided denitrifying power which seems to diminish with the continuance of the frozen period.

7. During the fall season, the denitrifying power of the soil increases until the soil freezes, after which a decrease occurs.

8. Frozen soils possess a nitrogen-fixing power which increases with the continuance of the frozen period, being independent of moderate changes in the moisture conditions but restricted by large decreases in moisture.

9. In the fall, the nitrogen-fixing power of the soil increases until the soil becomes frozen, when it almost ceases, after which a smaller nitrogen-fixing power is established.

10. These results confirm Conn's conclusion that bacteria are alive and multiply in frozen soils. The results of the physiological determinations lend support to his theory of the existence of specific groups of bacteria in the winter which are adapted to growth at low temperatures.

11. The theory is advanced that because of the surface tension exerted by the soil particles on the films of water, the presence of salts in this water, and the concentration in salts which may occur in it when the main body of soil water begins to freeze, it seems justifiable to assume that under average winter conditions, when the soil temperature is not depressed far below zero, the hygroscopic water in soils remains uncongealed and consequently bacteria may live in it and multiply sometimes to a comparatively large extent.

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