Partial segregation of bacteria and isolation of Pythium from the coarser soil fractions.

Abstract

Soil collected in the Australian Capital Territory was divided into fractions by sieving and subsequent deposition from aqueous suspensions. The greater proportion of the Pythium, and indeed the fungal population, was found, by plating, to be concentrated in the coarser portion of the soil, most of the bacterial population being in the colloidal part. After deposition of the colloids for 4 weeks, and with them the bacteria, a mean of less than one bacterium was plated per ml of supernatant liquid. The separation of bacteria and fungi facilitated the isolation of Pythium. The Pythium population of the surface soil was about 1000 per g. The mean number of isolates of Pythium in 11 other surface soil samples was 1.7 per mg using the fraction sedimenting from an aqueous suspension in 5 min. In similar fractions of soil taken at 18 and 24 in. below the surface the number of Pythium isolates was 3 per 10 mg.