Studies onAzotobacter species in soil

Summary Methods for countingAzotobacter species in soil have been examined. The highest counts were obtained from soil suspensions shaken in sterile distilled water containing 10-g glass beads and plated on to glucose agar. Mannitol has been rejected as a suitable substrate in agar media because it gives lower counts of Azotobacter than glucose, an effect which is further enhanced by drying the agar plates. A clear medium free from precipitated phosphate and CaCO₃ is recommended for the agar-plate method; the Azotobacter count is affected by the phosphate concentration.The agar-plate and dilution-tube methods were compared; the latter is less accurate but more convenient when many soil samples have to be examined.