Biological Flashcards Preview

IPNI Soil and Water Management > Biological > Flashcards

Flashcards in Biological Deck (18):

List Sources of Organic Matter (6)

  • plant / crop residues
  • green manures / cover crops
  • animal manures
  • Composts from ag, industry, municipalities
  • Biosolids
  • Soil animals (earthworms)


Pools of organic matter in soils

  • organic materials decomposing
    • roots, residues, manures
  • Biomass
    • microorganisms
  • Decomposed Organic materials


Humus is the most stable form of these pools

  • accounts for most of the organic matter in soil



When Organic Materials undergo decomp they also ________

  • contribute to biomass and humus pools
    • 80% of added materials converted to CO2 and lost from soil
    • 15% becomes humus
    • 5% is biomass
  • difficult to build up humus


Soil Organic Matter can retain Nutrients

  • retain nutrients by adsorption
    • Cations held by CEC
  • ALL negative charges on soil organic matter are pH dependent 
    • as pH increases, the negative charge on organic matter increases


Soil Organic Matter and Buffering 

  • soil organic matter contributes to buffering against changes in pH


Beneficial Effects of Soil Organic Matter

  • as SOM increases, soil aggregates become more stable
  • SOM source of nutrients (primarily N, also P, S) when decomposes
    • humus contributes to CEC (retains cations)
  • as SOM increases, water holding of soil increases 
    • SOM can hold large amounts of water
    • avail to plants so plant avail water increased
      • surface crusting reduced , infiltration increased
  • stable agregates also help with drainage
    • soil aeration improves


Crop Rotation, Cover Crops, and tillage / residue

management affect Carbon in soil

  • maintaining / increasing organic matter content of soil requires additions to offset losses through decomposition
    • Grow cover crops
      • increasing cover crop complexity can increase carbon
    • manures 
    • returning crop residues
    • recycling biosolids and other organic wastes
  • Most effective way to sequester Carbon is to:
    • reduce or eliminate tillage


Tillage Affect on Soil Organic matter

  • Takes only 5 years or less to reduce organic soil matter by tilling
    • tilling speeds up decomposition
  • takes decades to build Organic Matter back up
  • Only a fraction of organic matter additions are converted to humus
    • Takes 10 tons / acre of humus to increase soil organic matter by 1% 


How following Factor Impacts Soil Microbial Activity


  • as temp increases above freezing, increase in microbial activity up to optimum temp, then decrease
    • around two fold increase in activity for every 10 EC increase in temp from 15 C to 35C 
      • above 35, decline usually occurs


How following Factor Impacts Soil Microbial Activity


  • aerobic microbial activity usually optimum at 40 to 60 percent of a soils water holding capacity (drier than field capacity)
  • as soil dries, small declines occur, followed by rapid declines at critical soil water content
    • critical level varies by microbial 
  • anerobic microbial acitivity occurs in saturated soil, ceases in presence of 02
    • facultative anaerobes can function in presence or absence of 02


How following Factor Impacts Soil Microbial Activity

Soil pH

  • Fungi are more active in acid soils
  • bacteria more active in nuetral to basic soils
  • Rates of reactions carried out by specific microorganisms change as pH changes
    • minimum, optimum, and maximum value


How following Factor Impacts Soil Microbial Activity

Organic Matter

  • additions of organic C to soil stimulate microbial activity
    • the more decomp the C is, the more rapid the increase in activity
      • green manure has high decomp C, Sawdust low


How following Factor Impacts Soil Microbial Activity


  • as soil salinity increases, microbial activity declines
    • amount depends on microbes and salinity
    • microbes spend more energy to absorb water as salinity increases
      • enzymatic processes within microbial cell can be impaired by soluble salts


How following Factor Impacts Soil Microbial Activity

Nitrogen Application

  • Wide range of impacts 
    • in general, better soil fertility increases microbial activity


How following Factor Impacts Soil Microbial Activity


  • tillage increases aerobic soil microbial activity
    • lasts days, weeks
    • long term can decrease organic matter content in soil


C:N Ratio affects Organic Matter Decomp

  • N rich (low C/N ratio) organic materials release large amounts of inorganic N as NH4+ to the soil when they are decomposed by microorganisms
    •  organic materials such as green manures, animal manures, etc
    • microorganisms do not need the organic N to build new cells
      • the released NH4+ is avail to plants (mineralization)
  • N poor (high C:N ratio) cause microorganisms to remove large amounts of inorganic N from soil, decreasing N avail
    • Immobilization


Soil Health

  • holistic approach to evaluating how well soil performs at supporting various finctions such as
    • crop production,
    • animal production,
    • water quality.
    • air quality,
    • carbon sequestration


Physical, Chemical, Bio properties of soil health

  • major factor is organic matter content of a soil, and efforts to increase organic matter levels
  • organic matter important component of 
    • physical
      • structure, aeration, infiltration, drainage, avail water
    • chemical
      • CEC, retention of nutrients
    • biological
      • hebicide degradation, N fixation, N and P release during decomp