Friday 29 January 2016

Soil Organisms and Organic Matter

  1. Macrofauna

    1. Vertebrates(gophers, moles, rodents etc.)

    2. Annelids

      1. Earthworms

        1. process upto 15 tons/A/Y

        2. casts weigh up to 16,000 lbs/A

        3. improve aeration and drainage

        4. plow in organic matter

        5. prefer moist, aerated, finer textured soils with pH around 6.5

        6. biomass of 100 - 1000 lbs/A
        Giant Gipsland Earthworm Megascolides australis
    3. Arthropods (insects, arachnids, crustaceans)

      1. ants, termites, beetles, grubs, centipedes

      2. early breakdown of organic matter

      3. add to total soil organic matter

    4. Mollusks(snails,slugs)

  2. Microfauna

    1. Nematodes(threadworms, eelworms)

    2. Rotifers

    3. Protozoa(Amoeba)

  3. Microflora

    1. Bacteria

      1. Simple, single celled microbes, most are heterotrophic although there are many that derive energy from autotrophically from chemical reactions.

      2. most numerous microbes in the soil -- 107 to 108 per gram of soil

      3. have a very diverse physiology-- anaerobic and aerobic

      4. populations are dependent on food source and environment.

    2. Actinomycetes

      1. intermediate between bacteria and fungi-- now classified as bacteria.

      2. filamentous bacteria

      3. 2nd most abundant organism in the soil (over 1 million / gm)

      4. decompose resistent organic compounds(cellulose and chitin)

      5. like pH 6-7.5

      6. some fix Nitrogen

      7. give soil its rich earthy smell

      8. some produce antibiotics(streptomyces)

    3. Fungi

      1. no chlorophyll -- heterotrophic organisms(energy from organic compounds)
      2. fungi are fewer in number than bacteria but make up the largest biomass of any microbe in soils. A mass of hyphae is called mycelium. Mushrooms are the fruiting bodies of fungi.
      3. yeasts (not very important in soils)
      4. mushrooms
      5. Molds (widest pH range)
      6. dominate at low pH due to reduced competition
      7. often have the greatest biomass of any soil microbe
      8. decompose cellulose, lignins and complex organic compounds
      9. mycorrhizal fungi- colonize plant roots in a symbiotic relationship utilizing carbon from the plant while benefitting the plants by increasing uptake of phosphorus and possibly other nutrients and water.
    4. Algae
      1. contain chlorophyll and are photosynthetic. They are autotrophic orgaisms

      2. primary producers

      3. live near the soil surface

      4. found in moist to wet soils at or near a neutral pH.

  4. Environmental Factors affecting Microorganism populations

    1. Moisture Content: Moist soils around -1 bar are ideal for microorganisms. Soil moisture conditions too dry or too wet inhibit growth.

    2. Adequate Temperature Range: Activity of microorganisms is limited below 5 degrees C (45 degrees F).

    3. Adequate Carbon: Organic matter is a food source. Different organisms use different types of organic matter

    4. Adequae Soil pH: Different microorganisms work in specific pH ranges.

    5. Proper Competing Organisms: Microbes are particular to the types of orgnisms that are around them.-- by planting certain types of plants different types of organisms may be controlled.

    6. Aeration: There are aerobic and anaerobic organisms

      1. Heterotrophs

        1. get energy and carbon from organic compounds

        2. most numerous types

        3. includes nitrogen fixing bacteria

      2. Autotrophs: obtains energy comletely from the oxidation of inorganic elements or compunds such as iron, sulfur, hdrogen, ammonium and nitrates or from radiant energy

        1. don't require organic source for energy
        2. oxidize S and NH4+ for energy (inorganic material)
        3. obtain C from CO2
        4. includes nitrifying bacteria

  5. Organic Matter

    1. Definition: living or dead plant or animal material in soil. Ranges from large undecomposed material to fine, highly decomposed material.

      1. Organic matter content of soil - Highly decomposed material we call humus. This is last stage of decomposition and it is a colliodal complex of materials that is resistant to further decomposition.

    2. Composition

      1. Green tissue - 85 to 90% water and 10 to 15% dry matter.

      2. Dry Matter

        1. C, H. and O - 90%.

        2. All nutrients but important reservoir of N, P, K, S.

        3. Rapidly decomposing materials - cellulose, starches, sugars, amino acids and proteins.

        4. Slowly decomposing materials - fats, oils, resins, and lignin.

      3. Decomposition

        1. Due to enzymatic reactions of many different soil microrganisms. An oxidation reaction similar to burning.

          CH2O + O2 ++> CO2 + CH2O + energy.

        2. All stages of decomposition are present in soils.

        3. Decomposition process - microorganisms feed on organic matter to attain nutrients.

          1. Assume soil is low in fresh organic matter.

            1. Find low microorganism population and activity.

          2. Incorporate fresh plant material.

            1. Microorganism population multiplies many-fold.

            2. Tremendous evolution of CO2 and heat.

            3. Many of the nutrients in organic matter are incorporated into microbial tissue initially.

            4. As energy source diminishes, microbial activity lessens and microbes die, decompose, and some of the nutrients are released. Return to low microbial population and activity.

            5. Result - conversion of organic material to humus with release of nutrients and energy.

  6. VI. Soil Microorganisms - bacteria, fungi, atinomycetes, etc.

    1. Some decompose organic material, others responsible for other beneficial processes.

    2. Kinds and populations - affected by same things that affect plant growth: aeration, moisture, temperature, food supply, pH(p.157).

    3. Beneficial Reactions

      1. Ammonification - breakdown of organic matter and release of N as NH4+ .

        1. R-NH2 + H2O --> R-OH + NH3 + energy

          NH3 + H2O --> NH4+ + OH-.
        2. All plant nutrients are present in organic matter, and it is a particularly important reservoir of N, P, K, S.

        3. Ammonification is enhanced by well-drained, high base status soils.

        4. Fate of released NH4+

          1. Used by microrganisms.

          2. Plant uptake..

          3. Remain on exchange complex or fixed by illite.

          4. Undergoes conversion to NO3-.

      2. Nitrification - conversion of NH4+ to NO3- by two special purpose bacteria.

        1. Two-step process

          2 NH4+ + O2 --> 2 NO2- + 2 H2O + 4 H+ + energy

          2NO2- + O2 --> 2 NO3- + energy

          1. Second reaction is usually rapid and NO2- although very toxic, is seldom a problem.

          2. Acid forming process. Reason organic matter decomposition and NH4- containing fertilizers are acid forming.

        2. Responds to aeration, good moisture, 80 to 90 F temperature, high base status soil, and supply of NH4+.

      3. Nonsymbiotic N fixation - accomplished by free-living organisms (Azotobacter, blue-green algae) that need only organic matter and O2.

        1. If N content of organic residues is low and insufficient, these organisms can remove N from air to meet their needs.

        2. When they die, N is released and can be used by plants.

        3. Fix 10 to 20 lb N/acre/year.

      4. Symbiotic N fixation - accomplished by Rhizobium bacteria in association with legumes.

        1. Live in root infections called nodules.

        2. Obtain energy from host plant. Obtain N from air and fix enough for themselves and the host plant.

        3. Amount N fixed depends on how much is in the soil - high soil N. low N fixation.

        4. Respond to aeration, good moisture, 80 to 90 F temperature, high base status soil (pH 6 to 6.5), and supply of NH4+.

        5. Alfalfa - 250 lb N/acre/year. Soybean - 100 lb N/acre/year. Field pea - 50 lb N/acre/year.

  7. Relation of C/N ratio in organic materials to decomposition and soil N.

    1. High ratio (> 30) - material low in N so microbes lack N and decomposition is slow. If any soil N is present in the soil, microbes will compete with plants for it.

    2. Have to apply additional N to hasten decomposition of high C/N material and prevent N deficiency on crop.

    3. Intermediate ratio (15 to 30) - balanced N content. Meets microbes needs and does not cause N deficiency.

    4. Low ratio (< 15) - contains excess N. N will be released into the soil with decomposition. Basis for legumes in rotations or green manure crops.

  8. Organic matter management

    1. Try to conserve in soil. Increases structure, water infiltration, CEC, and filth.

    2. Use soil for waste management - take advantage of decomposition for waste disposal.

      1. Convert it to stable organic residues and nutrients.
      2. Use plants to remove nutrients.
      3. Can lead to nutrient toxicities (Cd, Ni, Cu. Zn) and pollution of ground water by leaching (N) or erosion (N, P).
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