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The Soil Flora
(or Garden)
- Macroflora: Vascular plants, Mosses, etc.
- Microflora: Bacteria, Actinomycetes, Fungi, Algae
Influences on Microbial Activity
- Temperature (70°-100°F most
active microbes)
- Moisture (Field capacity is optimal)
- Aeration (want a nice mix of pores filled with
water and air)
- pH (optimal pH is 6-7)
- Soil organic matter
Soil Microorganisms: Bacteria
- Most numerous in soil
- Most diverse metabolism
- Can be aerobic or anaerobic
- Optimal growth at pH 6-8
- Examples: Nitrosomonas and Nitrobacter in nitrification
processes, N2 fixers, fire blight is caused by a bacterium
Soil Microorganisms: Actinomycetes
- Transitional group between bacteria and fungi
- Active in degrading more resistant organic compounds
- Optimal growth at alkaline pH
- 2 important products:
– produce antibodies (streptomycin is produced by an actino)
– produce geosmin
- Negative impact - potato scab (Streptomyces
scabies)
 Soil Microorganisms: Fungi
- Dominate the soil biomass
- Obligate aerobes
- Can survive desiccation
- Dominate in acid soils
- Negative impacts:
– Apple replant disease (Rhizoctonia, Pythium, Fusarium,
and Phytophtora)
– Powdery mildew is caused by a fungus
- Beneficials:– Penicillium
Nematode-trapping Fungus
Plant root / Soil / Microbial Interactions
Beneficial
- Symbiotic associations such as that found with Rhizobia (N2 fixing bacteria, ex. legumes)
- Fungi-mycorrhizal associations: important for water
and P uptake; also improves soil structure
- Earthworm channels: improve permeability and aeration
Deleterious
- Agrobacterium (bacteria) cause gall formation in
plants
- Fungi causing soil-borne plant rot diseases
- Rhizoctonia and Pythium (involved with replant disease)
Next page: Nutrient cycling
and composting
Updated
July 15, 2004
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