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Counting viable azotobacter in vertisols iii. the non proportionality phenomenon

Counting viable azotobacter in vertisols iii. the non proportionality phenomenon

Plant and Soil 117(1): 31-40

In counting Azotobacter in vertisols by the soil dilution and spread-plating method, mean colony counts/plate did not decrease in proportion to the dilution factor and consequently derived counts of Azobacter cells/g soil decreased with increasing dilution of the soil suspension. This non-proportionality phenomenon was analysed in several experiments with six soils. Conformity to the Poisson distribution for counts on parallel plates was measured by Fisher's index of dispersion (.chi.2), which proved too high (P < .05) at lower dilutions, indicating inaccurately low mean colony counts. At highest dilutions, proportional errors increased resulting in less precise estimates of means, because with a Poisson distribution the standard deviation is equal to the square root of the mean, and the multiplication factor for derived counts/g soil is greatest at highest dilutions. By varying both plate surface area and dilution factor, results indicated that the non-proportionality phenomenon is caused by crowding at lower dilutions increasing the probability of colony coincidence on the plates. Graphical analysis of results of several dilution series indicated that derived counts are best based on dilutions giving 10-40 colonies/9 cm diameter plate, which is best achieved from two-fold dilution series.

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