SOILSCI

• Kinds and population of soil organisms (in fertile soil)

  • Bacteria
  • Actinomycetes
  • Fungi
  • Algae
  • Protozoa and other Animals
• Soil microorganisms


  • Exist as mixed population
  • A kind or group of microbes may dominate depending on environmental condition
• Classification of microorganisms based on factors affecting growth and activities

  • Trophism
  • Free oxygen availability
  • Temperature
  • Water availability
  • pH and nutrient availability
• Trophism


  The energy source and carbon requirements of the micro-organisms
• Types of trophism

  • Heterotrophs
  • Autotrophs
• Heterotrophs


  Obtain energy and carbon by decomposing certain organic materials
• Autotrophs


  Their source of carbon is CO2 and their source of energy is either light or participating in chemical reaction
• Types of autotrophs

  • Photoautotrophs
  • Chemoautotrophs
• Photoautotrophs


  Source of energy is light
• Chemoautotrophs


  Generate energy by participating in chemical reaction
• Types of microbes based on free oxygen availability

  • Aerobic
  • Anaerobic
  • Facultative
• Aerobic


  Microbes that live under adequate amount of free oxygen (O2)
• Anaerobic


  Microbes that live in the absence or highly limited free oxygen
• Facultative


  Can shift from aerobic (if with O2) to anaerobic (if no O2) or vice-versa
• Thermophilic microbes


  Can withstand high temperature even at higher than 50 oC
• Water Availability

  Some bacteria can thrive even at very low moisture content
• pH and availability of nutrients

  If growth of microbes is plotted against any one of the factor of growth, the growth curve would be
• Beneficial activities of soil microorganisms

  • Decomposition of organic matter
• Decomposition of organic matter by heterotrophs

  Leads to the formation of humus, and the release of nutrient ions
• Importance of the humus fraction

  • Imparts dark color of the soil
  • Influences soil properties (high specific surface area, high CEC, high nutrient holding capacity, high buffering capacity, high water holding capacity, Principal reserve of plant nutrients (N, P, S)
  • Improves soil structure
• Nitrogen transformation

  1. Nitrogen fixation

  2. Mineralization

  3. Immobilization

  4. Denitrification

  5. Ammonia volatilization

  6. Ammonium exchange

  7. Ammonium fixation

  8. Nitrate leaching

  9. Plant uptake
• Nitrogen fixation

  The conversion of N2 (gas) into NH3, NH4+

  N2 + 6 H+ - NH3, NH4+
• Groups of N-fixing bacteria

  • Free living heterotrophic bacteria can produce about 10-20 kg N/ha/yr
  • Free living photosynthetic (photoautotroph) - Blue Green Algae (BGA)
  • Symbiotic Relationship (nitrogen fixing microbes/plant relationship)
• Symbiotic relationship

  The plant provides energy or food and right environment for the microbes and in return the microbes fix or convert N2 (gas) to available form (NH4+)
• Appropriate soil conditions for symbiotic nitrogen fixation

  • Soil pH is near neutral to slightly alkaline
  • Sufficient Ca, Mo
  • Low level or amount of NH4+, NO3- in the soil
• Mineralization


  Conversion of Organic Nitrogen to available forms (ammonium, nitrite and nitrate)

  Organic N - NH4+, NO2-, NO3-
• Stages of mineralization

  • Ammonification
  • Nitrification
• Ammonification


  Part of microbial (heterotrophs) decomposition of organic matter in which the available form NH4+ is released from its organic form

  Organic N - NH4+
• Nitrification


  The biological oxidation of ammonium to nitrite (NO2) then to nitrate (NO3)

  NH4+ +2O2 NO2- NO3-+2H + H2O
• Immobilization


  The process in which the available forms of N is converted to Organic N (unavailable form in microbial tissues)

  NH4+, NO3- (microbes) Organic N
• Denitrification


  The biological reduction of nitrate to nitrous oxide and nitrogen gas

  NO2, NO3 - N2O, N2 (gas)
• Ammonia volatilization


  The chemical reaction that converts ammonium to ammonia gas.

  NH4+ +OH- - NH3 (gas) + H2O Alkaline pH
• Ammonium exchange


  Ammonium enters into cation exchange reaction, allowing available N to be stored in the soil.

  NH4+ - NH4, clay exchange sites
• Ammonium fixation


  A physic-chemical reaction in which ammonium ions are trapped in between crystal lattices of 2:1 clay type like vermiculite and montmorillonite making it unavailable form
• Nitrate leaching


  Excessive downward movement of water during heavy rain in which the nitrate (NO3-) is leached to the ground water
• Plant uptake


  The available forms of nitrogen are absorbed by plant roots

  NO2-, NO3-, NH4+ - plants via the root
• The decomposition of organic materials in soil is largely due to the enzymes secreted by

  microbes
• Decomposability of different organic material decreases towards the right and downward
• Sugar is easily decomposed while Lignin (your wooden chair is practically lignin) is the most difficult to decompose.
• This process converts unavailable form of N to its available form

  nitrogen fixation