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Tuesday: Give a presentation on your soil Order

Tuesday: Give a presentation on your soil Order. About 15 minutes Provide references Include all of the following:. Maps (global, US, MN if applicable) General characteristics Typical environments Photos (of profiles) Suborders State soils Diagnostic horizons if present

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Tuesday: Give a presentation on your soil Order

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  1. Tuesday: Give a presentation on your soil Order • About 15 minutes • Provide references • Include all of the following:

  2. Maps (global, US, MN if applicable) • General characteristics • Typical environments • Photos (of profiles) • Suborders • State soils • Diagnostic horizons if present • % ice-free land surface covered • Field trip

  3. Suggested sources: • Textbook • http://soils.cals.uidaho.edu/SoilORDERS • NRCS website

  4. Soil Aeration

  5. Why is soil aeration important? 1. Ventilated soil allows gases to be exchanged with atmosphere (important for photosynthesis and respiration) by: • Mass flow: air forced in by wind or pressure • Diffusion: gas moves back and forth from soil to atmosphere acc. to pressure

  6. 2. Aeration allows water to move through soil. Nutrients are carried to plants in water. 3. Allows roots to penetrate soil. 4. State of aeration determines types of decomposition (aerobic or anaerobic). 5. Determines form of nutrient elements (reduced or oxidized).

  7. Opposite of well-aerated soil is compacted soil. Compacted soils are not well-aerated. High bulk density

  8. Can be corrected by a soil aerator.

  9. Aerator sandals!

  10. Saturated soils are also not well-aerated. How can we express / measure how aerated or saturated a soil is?

  11. Can express how well-aerated a soil is by: • REDOX POTENTIAL (Eh) • Reduction-Oxidation potential • Tendency of a substance to accept or donate electrons.

  12. Oxidation • Loss of electrons • Fe+2 Fe+3 e- -26 -25 +28 +28 Fe+2 Fe+3

  13. Reduction • Gain of electrons • Fe+3 Fe+2 e- -26 -25 +28 +28 Fe+2 Fe+3

  14. Iron Fe+2(ferrous) R Fe+3 (ferric) O Nitrogen N+3 in NH+4(ammonium) R N+5 in NO3- (nitrate) O Manganese Mn+2 (manganous) R Mn+4 (manganic) O Oxidized/Reduced forms of…

  15. Sulfur S-2 (sulfide) SO4-2(sulfate) Carbon CH4(methane) CO2 R O R O

  16. ethylene ethanol Hydrogen sulfide

  17. Oxidation reaction(loss of electrons) electrons that could potentially be transferred to others 2FeO + 2H2O 2FeOOH + 2H+ + 2 e- Fe+2Fe+3 H+ ions formed

  18. Redox potential • Tendency of a substance to accept or donate electrons • Measured in volts or millivolts • Depends on pH and presence of electron acceptors (oxidizing agents) • Used to quantify the degree of reduction in a wetland soil

  19. Oxidizing agent • Substance accepts electrons easily • Oxygen is very strong electron acceptor, but in the absence of oxygen, other substances act as electron acceptors

  20. Reducing agent • Substance donates electrons easily

  21. Aerobic Respiration • Oxygen is electron acceptor for organic carbon, to release energy. • As oxygen oxidizes carbon, oxygen in turn is reduced (H2O) O2 + C6H12O6 CO2 + H2O Electron acceptor Electron donor

  22. To determine Eh (See graph) • Insert electrode in soil solution: • free dissolved oxygen present : Eh stays same • oxygen disappears, reduction (electron gain) takes place and probe measures degree of reduction ( mv) • As organic substances are oxidized (in respiration) Eh drops as sequence of reductions (electron gains) takes place:

  23. Graph shows: • sequence of reductions that take place when well aerated soil becomes saturated with water • Once oxygen is gone, the only active microorganisms are those that can use substances other than oxygen as electron acceptors (anaerobic) • Eh drops • Shows Eh levels at which these reactions take place • Poorly aerated soil contain partially oxidized products: • Ethylene gas, methane, alcohols, organic acids

  24. organic substrate oxidized (decomposed) by various electron acceptors: • O2 • NO3- • Mn+4 • Fe+3 • SO4-2 • rates of decomposition are most rapid in presence of oxygen

  25. Aeration affects microbial breakdown: • Poor aeration slows decay • Anaerobic organisms • Poorly aerated soils may contain toxic, not oxidized products of decomposition: alcohols, organic acids • Organic matter accumulates • Allows Histosol development

  26. Significance of aeration: • Forms/mobility • Redox colors • Nutrient elements • Roots • Decomposition

  27. Some conclusions about aeration: 1. Forms and Mobility Soil aeration determines which forms of chemicals are present and how mobile they are

  28. 1. Forms and Mobility: A) Poorly aerated soils • reduced forms of iron and manganese Fe+2, Mn+2 • Reduced iron is soluble; moves through soil, removing red, leaving gray, low chroma colors (redox depletions) • Reduced manganese : hard black concretions

  29. Manganese concretions

  30. 1. Forms and Mobility B) Well-aerated soils: • Oxidized forms of iron and manganese Fe+3 Mn+4 • Fe precipitates as Fe+3 in aerobic zones or during dry periods • Reddish brown to orange (redox concentrations)

  31. Plate 26  Redox concentrations (red) and depletions (gray) in a Btg horizon from an Aquic Paleudalf.

  32. Plate 16  A soil catena or toposequence in central Zimbabwe. Redder colors indicate better internal drainage. Inset: B-horizon clods from each soil in the catena.

  33. 1. Forms and Mobility C. Nutrient Elements • Plants can use oxidized forms of nitrogen and sulfur • Reduced iron, manganese • Soluble in alkaline soils • More soluble in acid soils; can reach toxic levels

  34. 2. Root respiration • Good aeration promotes root respiration • Poor aeration: water-filled pores block oxygen diffusion into soil to replace what is used up in respiration

  35. 3. Decomposition In aerated soils, aerobic organisms rapidly oxidize organic material and decomposition is rapid In poor aeration, anaerobic decomposers take over and decomposition is slower

  36. Hydric Soils

  37. Wetland criteria : • Hydrology • Hydric soils • Hydrophytic plants

  38. Hydric soil • soil that is saturated, flooded, or ponded long enough during the growing seasonto develop anaerobic conditions in the upper part. • Oxygen is removed from groundwater by respiration of microbes, roots, soil fauna • Biological zero = 5°C

  39. Why is “during growing season” important part of definition? • If wet period is during COLD time of year (too cold for microbial growth and plant root respiration), might not have anaerobic conditions. • It is anaerobic conditions that cause a soil to be hydric, not just saturation!!!

  40. How can a saturated soil be aerobic? • If water is flowing • If microbes and plant roots are not active

  41. Hydric soils support growth and regeneration of hydrophytic plants.

  42. Hydric soil indicators: • Color • Chroma 1or 2 or gley (Fe++2 grey or green) • May have redox concentrations or concretions • Sulfidic materials (odor of rotten eggs) • Sulfate reduction

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