The soil replica plating technique was used to evaluate the influence of clay
minerals and pH on antagonistic interactions between fungi and bacteria in soil In general, the antagonistic activity of bacteria towards filamentous fungi was
greater in soil than on agar. The spread of Aspergillus niger through soil was
inhibited by Serratia marcescens when the organisms were inoculated into
separate sites in soil, and this antagonistic effect was maintained when the soil
was amended with 3, 6, 9, or 12% (vol/vol) montmorillonite, whereas the addition
of kaolinite at a concentration of 3% reduced the antagonism and at 6, 9, or 12%
totally eliminated it. Similar results were obtained with the inhibition of A. niger
by Agrobacterium radiobacter and of Penicillium vermiculatum by either S.
marcescens or Nocardia paraffinae. When A. niger and S. marcescens were
inoculated into the same soil site, A. niger was inhibited in all soils, regardless of
clay content, although the extent of inhibition was greater as the concentration
of montmorillonite, but not of kaolinite, increased. A. niger was inhibited more
when inoculated as spores than as mycelial fragments and when inoculated 96 h
after S. marcescens, but a 1% glucose solution reduced the amount of inhibition
when the fungus was inoculated 96 h after the bacterium When the pH of the
soil-clay mixtures was altered, the amount of antagonism usually increased as the
pH increased. Antagonism appeared to be related to the cation-exchange capacity
and the pH of the soil-clay mixtures Bacillus cereus and another species of
Bacillus showed no activity in soil towards A. niger under any of the environ mental conditions tested, even though the Bacillus sp. significantly inhibited A.
niger and seven other fungi on agar