Soil Organisms and Their Role in Soil Fertility

Soil Organisms

• Soil organisms encompass all living organisms found in the soil.

• Microorganisms dominate soil biological activity, contributing to 60-80% of total activity.

• The research area studying these organisms is termed Soil Biology.

• Essential for nutrient transformations, nutrient cycles would halt without them.

• Soil contains diverse microorganisms, contributing significantly to soil processes and structures.

Classification of Soil Organisms

• Soil organisms can be broadly classified into two groups:

  • Soil Flora (Plant kingdom)

  • Macro-flora: Plant roots, Macro-algae

  • Micro-flora: Bacteria, Actinomycetes, Fungi, Algae

  • Soil Fauna (Animal kingdom)

  • Micro fauna: Protozoa, Nematodes

  • Macro-fauna: Earthworms, Termites, Ants, Grubs

Classification of Bacteria

• Heterotrophic Bacteria:

  • Nitrogen fixing:

  • Symbiotic (e.g., Rhizobium)

  • Non-Symbiotic:

    • Aerobic (e.g., Azotobacter)

    • Anaerobic (e.g., Clostridium)

  • Associative Symbiotic (e.g., Azospirillum)

  • Non-nitrogen fixing

  • Aerobic - Ammonifiers

  • Anaerobic - Denitrifiers

• Autotrophic Bacteria:

  • Chemoautotrophic (e.g., Thiobacillus)

  • Photoautotrophic (e.g., Chlorobium)

Soil Flora

Macroflora

• Major role in soil:

  • Decomposed plant materials contribute to soil organic matter, which supports both microorganisms and higher plants.

  • Roots enhance soil structure by exerting pressure that opens up pore spaces, improving air and water movement in soil.

Microflora

Bacteria

• Unicellular, minute organisms (0.5 - 1 micron in diameter).

• Abundant in soil (300 to 3000 kg of live weight/ha).

• Shapes vary: cocci (round), bacilli (rod-like), and spirilla (spiral).

• Types based on temperature tolerance:

  • Psychrophiles: <10 °C

  • Mesophiles: 20 °C-40 °C

  • Thermophiles: >40 °C

• Abundant genera: Pseudomonas, Azotobacter, Bacillus, etc.

Functions of Bacteria

• Nitrogen fixation and decomposition processes.

• Vital transformations of organic matter, carbon, nitrogen, phosphorus, and other elements.

Fungi

• Aerobic and heterotrophic; includes molds and mushrooms.

• Important for organic matter decomposition.

• Symbiotic relationships (e.g., Mycorrhizae) enhance nutrient absorption in plants.

Actinomycetes

• Filamentous bacteria, sharing characteristics with fungi.

• Found more abundantly in dry, tropical soils and degrade various organic materials.

Algae

• Photosynthetic organisms contributing to soil fertility.

• Blue-green algae (cyanobacteria) fix atmospheric nitrogen and enhance soil fertility.

Nitrogen Fixing Systems

• Various organisms participate in nitrogen fixation:

  • Symbiotic: Rhizobium in legumes, Frankia (Actinomycetes).

  • Associative: Non-legumes host Azotobacter, Beijerinckia.

  • Non-Symbiotic: Including free-living cyanobacteria and anaerobes.

• Nitrogen fixation rates in kg N/ha/year:

  • Symbiotic: 50-100

  • Associative: 5-30

Soil Fauna

Macrofauna

• Earthworms:

  • Feed on organic materials, enriching soil with nutrients through digestion.

Microfauna

• Protozoa and Nematodes:

  • Protozoa mainly feed on bacteria.

  • Nematodes include saprophytic, predatory, and parasitic types; some can harm plants.

Soil Viruses

• Obligate parasites that require a host for multiplication, can affect vital agricultural bacteria.

Interaction Among Soil Organisms

• Organisms affect each other's population dynamics:

  • Symbiotic relationships can enhance growth and nutrient cycling, while predation can suppress certain populations.

Role of Organisms in Soil Fertility

• Microbes:

  • Decompose organic matter

  • Recycle and fix nutrients

  • Enhance soil structure

  • Control pests and diseases

Soil Fertility Evaluation by Biological Tests

• Techniques include:

  • Neubauer method

  • Standard and Demont technique

  • Various microbiological methods

Harmful Activities of Soil Organisms

• Some organisms can be damaging: rodents, nematodes, and certain fungi may cause plant diseases or reduce plant vigor.

• Anaerobic conditions can lead to nutrient unavailability and toxicities.

Conclusion

• The interdependence of soil life is summarized as: "There can be no life without soil and no soil without life; they have evolved together." - Charles E. Kellogg