soils science test 3

  1. Sources of Acidity in soil
    CO2 --> H2CO3 carbonic acid--> HCO3 and H+

    Organic matter accumulation

    Oxidation reactions

    Plant uptake of cations
  2. how does pH effect the soil
    controls soil chemistry (nutrient availability and toxicity)

    soil biology (microbial activity and diversity)

    Soil physics (determines aggregate stability, Al=good aggre. Na =bad aggre)
  3. Ph can are raised by ______ reactions
    ph can be lowered by ______ reactions
    • raised by reduction rxn
    • lowered by oxidation rxn
  4. define buffering capacity
    the ability of soil to resist change in pH. It is increased in soils with high CEC
  5. describe how carbonic acid is a source of acidity
    CO2 reacts with water to form H2CO3 which breaks down to HCO3 and H+

    CO2 + H20--> H2CO3 --> HCO3 + H+

    at equalibrium at pH 5.6
  6. Describe how organic matter accumulation is a source of acidity
    organic material forms soluable complexes with non acid nutrient cations (CA, MG, ect) increasing their loss by leaching

    Organic acids form H+ ion
  7. describe how oxidation reactions are a source of Acidity
    strong acids are produced which increase soil acidity
  8. describe how plant up take is a source of acidiy
    plants take up neg charged anion or exude a positive charge as a different cation. when they take up more Cation than anions , plants usually exude H+ ions into the soil solution to retain charge balance in the soil
  9. Factors affecting soil acidity
    • leaching
    • Al hydrolysis -
  10. explain Al hydrolysis
    Al 3+ splits water molecule into H+ and O- ions. Al combines with the OH-, leaving the H+ to lower the pH of the soil solution. Can create 3 H+ ions in solution. Then these H+ join together forming large polymers with many + charges. Once bound to the colliods negative surface, polymers don't exchange and masks potential cation exchange capacity
  11. How does leaching effect soil as an acidifying factor
    Leaching removes HCO3, CO2 which produce OH

    They accumulate Al and H
  12. Sources of Alkalinity
    • Carbonates & bicarbonates
    • Reduction reactions of N S Fe
    • Weathering reactions to certain salts
  13. Describe carbonate and bicarbonate reactions with H20 and OH-
    • -CO3 2- (carbonate) from Ca minerals like calcite
    • -HCO3 (bicarbonate) and from carbonic acids

    • React with H20 and OH- ions
    • CaCO3== Ca +CO3
    • CO3 +H20 == HCO3 + OH-
    • HCO3 +H20 == H2CO3 + OH-
    • H2CO3 == H20 + CO2 gas into air
  14. Describe how weathering reactions raise pH
    Weathering creates calcarous (calcite laden) and sodic (sodium -carbonate laden) horizons

    Calcite and carbonates engage in reactions w/ H20 and OH- to create more OH-
  15. Name the three pools of acidity
    • Pool of :
    • Active Acidity
    • Exchangeable Acidity
    • Residual Acidity
  16. Define active acidity
    • H+ in soil solution
    • -smallest of the pools
    • -determines the solubility of many substances
    • - environment around plant roots
    • -easy to adjust or fix pH
  17. Define exchangeable acidity
    exchangeable Al and H on te colloid surface

    Once the Al3+ is released through CEC and the solution hydrolyzes H20, H+ is released into the soil solution lowering pH

    • to nuetralize exchangeable acidity it takes 100 times more liming material than active acidity pool
    • Smectites> vermiculite> kaolinite
  18. define residual acidity
    H, Al, and AlOH molecules that are bound in inexchangeable form

    -once the pH increases the bound H dissociates and the Al ions are released. This Al can be hydrolysed and will add more H+ to the soil solution so the exchange rate can be 1000-100000 times greater than active acidity
  19. how does buffering capacity work and why is buffering important
    Once the soil solution is nuetralized, it is relensihed with H+ ions from the exhangeable pool

    Soils high in clay and OM are highly buffered

    • Important because:
    • ensures some stability in pH , preventing drastic fluctuations in pH

    buffering dictates how much liming ammendments toadd
  20. when discussing pH and nutrient availability, what should you understand about low ph and its affects on plants and soil organisms
    • -Fungi are still alive
    • -bacteria and actinomycetes are beginning to die off
    • -Al toxicity a problem
    • -Fe , Mn, Zn, Cu is radily available
    • -N, K, S, Ca, Mg, Mo not easily available
    • -pH < 5 bacteria in nitrogen fixation die off so N is unavailable
  21. when discussing pH and nutrient availability, what should you understand about high ph and its affects on plants and soil organisms
    • -fungi living
    • -bacteria and actinomycetes are alive
    • -N, K, S, Mg, Ca, Mo are available
    • -P Cu, Zn is not available
    • - B deficiancy is a problem
    • -as pH increases micronutrient solubility rate decreases
  22. Where is aluminium toxicity most common (soil Chemistry)
    most commonly a problem in serverly acid soils <5.2
  23. What are the soluble forms of Al found in Acid soils
    Al3+, Al(OH)2+, Al(OH)-
  24. Whengoing from pH 5 to pH 4 what will drastically increase within the soil
    Soluable Al
  25. how does high OM effect soluable Al concentration in the soil
    Al is strongly attracted to OM which will tie up soluable Al in the soil solution.
  26. What effect does Al have on plants
    al damages cell membranes and restricts cell wall expansion

    Al interfers with P metabolism

    levels can't be tested by leaf sample cuz only in roots
  27. what is used to determine the amount of liming material required to eliminate exchangeable Al
    • Al saturation
    • Al >15% problem soil
  28. how do you correct Al toxicity
    OM and liming to 5.5

    Al bound with OM
  29. symptoms of Mn toxicity
    crinkling and cupping of leaves

    intervenal patches of chlorotic tissue
  30. How do you manage for Mn toxicity
    Lime to pH above 6.0

    manage irrigation

    aerate soil
  31. describe a liming reation
    liming materials react with CO2 and H20and create bicarbonate

    the salt ions of the liming material replace Al and H+ ions on colloid surface which lowers the % acid saturation on colloid surface raising the pH

    • Liming material ==ions + carbonate
    • carbonate + H20 == bicarbonate + OH
    • bicarbonate +H20==H2CO3 + OH
    • H2CO3== H20 + CO2 gas
  32. If Al is present during liming reaction, what happens to the Al?
    Al will react with some of the OH- ions forming Al(OH) which is insoluable and no longer dangerous to plants
  33. define mineralization
    the process that releases elements from organic compounds to inorganic compounds through microbial assimilation.

    NH4+ --> NO2--> NO3--> N2 gas

    Its the last step in decomposition.
  34. Define immobilization
    the conversion of inorganic compounds to organic compounds through microbial assimilation
  35. Define Nitrification
    The microbially mediated conversion of NH4+ to NO2- to NO3- in aerobic conditions
  36. Define denitrification
    Microbially mediated conversion of NO3- to N2 gas in anaerobic conditions

    Can happen in aerobic conditions in the micropores where water is trapped
  37. 2 key points in understanding Ammonia Volatilization
    1. High pH in water or soils solution leads to NH3 producation

    2. Urea especially volnerable, bury it
  38. Define Annamox and explain the conversion
    anaerobic ammonium oxidation

    bacteria at the aerobic/anaerobic interface converting NH4+ to NO2 to N2

    skips from NH4+ to N2 through bacterial assimilation
  39. Name the pools of N in the soil (7)
    • Plants
    • OM
    • microbs
    • ECE (NH3+)
    • ACE
    • in soil solution
    • in atmosphere
  40. What is NH4+ fixation and where does it happen?
    entrapment of NH4+ between the interlayer spaces of vermiculite
  41. NO3 > NH4+ due to ____ in _____soils

    NH4+> NO3 due to ____ in ___soils
    • nitrification aerobic
    • denitrification in anaerobic
  42. Types of N fert
    • BNF
    • plant and animal residues
    • NH4+ (acidify)
  43. 2 types of NO3- fert
    • KNO3
    • CaNO3
  44. describe the carbon cycle
    • most carbon in lithosphere
    • Plants fix C in tissue and into soil as humus
    • decomposition release C
    • tilling mining disturbs protect surface soil and exposes microbs to more air which speeds demposition releasing more CO2 than without tilling
  45. Define decomposition
    microbially mediated breakdown of complex molecules into more simple components

    some components are mineralized by microbs to make new compounds
  46. Products of decomposition
    • Energy
    • Water
    • CO2
    • humus
  47. Aerobic decomposition is mediated by:
    bacteria fungi and earthworms
  48. as temp goes up decomposition ____
  49. When discussing OM which compounds break down first---> last
    • 1) Sugars, Starch Simple Proteins
    • 2)Crude Protein
    • 3) Hemicellulose
    • 4) Cellulose
    • 5)Fats and waxes
    • 6) Lignins and phenolic compounds
  50. factors influencing decomposition/mineralization
    • Molecular structure
    • Fragmentation
    • Microbial metabolism
    • Temp
  51. define humus
    amorphous and colloidial mixture of 5 ring carbon substances
  52. Define humic substances
    complex molecules with high molecular weight and resistant to microbial degradation
  53. Define nonhumic substances
    microbially modified plant compounds with simple structure
  54. Physical properies of OM
    • dark
    • improves aggrigation
    • improves water retention
  55. Chemical properties of OM
    • Increases CEC (humus=CEC 200)
    • increases buffering capacity
    • Detoxifies Al
    • Increases microbs through chelates (organic compounds) with humic substances
  56. what do Chelates do in soil OM
    increases soluability of micronutrients
  57. biological properties of OM
    increase bioiversity and microbially mediates reactions like immobilization and mineralization
  58. Name pools of OM insoil
    • Active
    • Slow
    • Passive
  59. Define active OM
    rapid turn over (1-2 yrs)

    • -Non-humic substances
    • -Nutrient source for NPS
    • -Easily depleted through tillage
    • -important source of aggregation through carbohydates in OM
  60. Define slow OM
    • intermediate stability (decades)
    • -source of longterm NPS
  61. define passive OM
    complez humic substances (centuries)
  62. Factors influencing soil organic matter and explain why it is a factor 5
    Temp- high temp=faster decomp

    Moisture- more water more veg more litter, also anarobic slows decomp

    Veg- litter

    Soil texture-high clay and silt usually rich in OM cuz it produces more biomass.

    Tillage- destroys microbs slows decomp, also carbon is explosed weather
  63. Soil aggregate is the fundimental unit of _______ for meso and microorganisms
  64. biological diversity is used as an indicator for _______
    soil health
  65. define microbial stability
    ability to continue to perform functions in a wide variety of environments
  66. Name primary producers
    • plants
    • algae
    • lichens
    • bacteria
  67. bacteria dominate the ______ decomposed materials

    fungi dominate the _____ decomposed materials
    bacteria easily

    fungi more complex
  68. what air to water % ratio is optimal for microbial activity
    40%: 60%
  69. What is the importance of BNF
    converts N2 to reactive N
  70. types of BNF relationships
    • -symbiosis=mutual
    • -obligate= mediated by prokaryate organisms
    • -associate=actinomycetes
  71. what is the nitrogenese reaction equation
    N2 + 8H +8e --nitrogenese-->2NH3 + H2
  72. what is nitrogenese
    is the key enzyme in BNF
  73. what is the role of leghemoglobin?
    it protects the nitrogenese from O2
  74. what are Frankia? where are they found? what are their contrubution to the soil environment
    Actinomycetes that fix N on nonleguminus plants

    forest soils

    N fixation exceeds leguminous action
  75. microbs that fix N on their own?
    anabeana cyanobacteria
  76. What nutrients are taken up in the mycorrhyal relationship
    P and immobile nutrients
  77. micronutrients
    B Cu Fe Mg Zn Mo Cl Cobalt Nickle
  78. mobile nutrients
    N P K Mg
  79. define chlorosis
    • yellowing due to loss of chlorophyll
    • Uniform and interveinal
  80. define necrosis
    death of plant tissue
  81. what symptom is reddish coloring
    accumulation of anthocynan
  82. describe a nitrogen deficiancy symptoms
    yellowing on older leaves, stunting reduced yeilds
  83. describe P deficiancy symptoms
    • purple or redish color
    • necrotic spots on older leaves
    • overall stunting
  84. describe K deficiancy symptoms
    • chlorosis along leaf margins
    • weak stems
    • small fruits
  85. describe Mg deficiancy symptoms
    interveinal chlorosis on older leaves
  86. describe S deficiancy symptoms
    • younger leaves necrotic
    • poor growth w/ delayed maturity
Card Set
soils science test 3
uhm soils science