Soil Structure & Composition

Soil Composition

Living Matter

  • Mostly in the top 4″ of the soil.
  • Good guys & bad guys…but large volume & diversity control the trouble makers by making it a competitive environment for resources.
  • 1 teaspoon of soil contains:
    • 1 billion bacteria
    • several yards of fungal hyphae
    • several thousand protozoa
    • few dozen nematodes

Bacteria

  • attracted by the root exudate (carbohydrates and proteins secreted from the plant roots).
  • the numbers and kinds of bacteria that are attracted are controlled by the plant, depending on season and conditions
  • most bacteria need carbon sources to live. Carbon sources include plants, waste products from insects & mammals, and bodies of these organisms. by consuming these sources, they immobilize nutrients and become little bags of fertilizer (think time-release). they release nutrients in plant-available form when they are in the rhizosphere.
  • bacteria use slime to stick to substrates and move around. this slime traps pathogens. this slime is also responsible for sticking soil particles together, giving soil its structure.
  • vitamins and antibiotics are produced by some bacteria & fungi that help the plants
  • bacteria also work in the phyllosphere (leaf surface)

Fungi

  • attracted by the root exudate (carbohydrates and proteins secreted from the plant roots).
  • the numbers and kinds of fungi that are attracted are controlled by the plant, depending on season and conditions
  • fungal hyphae kill nematodes, which are after the plant roots
  • plants attract fungus to their roots for protection by secreting exudate from their roots into the rhizosphere. The rhizosphere is the zone immediately surrounding a root by about 1 mm (1/25″).
  • mycorrhizal fungi provide water & phosphorous, as well as other nutrients to the plant in return for the plant’s exudate that it lives off of.
  • nets & webs formed by fungi around the roots act as physical barriers to pathogens
  • vitamins and antibiotics are produced by some bacteria & fungi that help the plants
  • immobilize nutrients and become little bags of fertilizer (think time-release). they release nutrients in plant-available form when they are in the rhizosphere.
  • chemical fertilizers (salts) kill these fungi
  • fungal hyphae travel through the soil particles, binding them together thread-like into aggregates, giving soil structure

Protozoa

  • include amoebae, paramecia, flagellates, and ciliates.
  • eat bacteria then excrete wastes for roots to uptake

Nematodes

Insects

centipedes, springtails, ants, ladybug larvae

earthworms

  • ~50 earthworms per square foot
  • burrow through soil, giving pathways for air & water to enter & leave the soil.

slugs & snails

Organic-nonliving Matter

Inorganic

Soil Structure

  • Bacterial slime binds soil particles together
  • fungal hyphae thread together particle aggregates
  • worms, insect larvae & moles move through the soil in search of food and protection which create pathways that allow air & water to enter & leave the soil

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