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Counting viable azotobacter chroococcum in vertisols i. methods for preparation of soil suspensions



Counting viable azotobacter chroococcum in vertisols i. methods for preparation of soil suspensions



Plant and Soil 117(1): 9-16



Methods for preparing soil suspensions for counting Azotobacter chroococcum in vertisols by soil dilution and plating were investigated. Mechanical methods to promote disaggregation of soil and Azotobacter microcolonies by shaking soil suspensions with glass beads (10% w/v) or coarse and (1-2 mm fraction) increased soil dispersion and Azotobacter colony counts. Chemical methods for disaggregation were unsatisfactory. The non-ionic detergent 'Agral' (0.004, 0.002, 0.1, 0.5 and 2.5% w/v) had no significant effect on soil dispersion and Azotobacter count. Both sodium pyrophosphate (0.03, 0.1, 0.3 and 0.9% w/v) and sodium metaphosphate as 'Calgon' (0.022, 0.066, 0.2, 0.6 and 1.8% w/v) increased soil dispersion but were toxic to Azotobacter. Increasing time of shaking soil:distilled water suspensions increased deflocculation of the clay and Azotobacter counts to a maximum after 6-23 hours shaking. Comparable results were obtained within 30-60 minutes of shaking with coarse sand, but shaking with sand beyond 2 hours reduced counts through mechanical damage to cells. Counts from suspensions in physiological saline (0.75% NaCl) and in distilled water were similar. Counts from suspensions in Jensen's mineral base shaken for < 3 hrs were lower than from distilled water due to flocculation of the soil by Ca2+ ions, but were higher on extended shaking up to 23 hours due to better cell protection. Shaking soil suspensions in distilled water with 10% w/v coarse sand for 30 minutes is recommended when counting Azotobacter in vertisols.

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