1. Soil Aeration

2. Ventilation of soil allowing gases to be exchanged with atmosphere
Gas is exchanged by:
Mass flow: air forced in by wind or pressure
Diffusion: gas moves back and forth from soil to atmosphere acc. to pressure

3. Redox potential Tendency of a substance to accept or donate electrons
Oxidation-reduction potential
a way to characterize aeration Eh

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

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

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

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

9. Oxidation reaction electrons that could
potentially be transferred to others
2FeO H2O FeOOH + 2H+ + 2 e-
Fe Fe+3
H+ ions formed

10. 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

11. Oxidizing agent Substance accepts electrons easily
Oxygen is very strong electron acceptor

12. Reducing agent
Substance donates electrons easily

13. Aerobic Respiration
Oxygen is electron acceptor for organic carbon, to release energy.
As oxygen oxidizes carbon, oxygen in turn is reduced (H2O)
O2 + C6H12O CO2 + H2O
Electron
acceptor
Electron donor

14. 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:

15. Oxidized form
Reduced form
Eh (v) O2
H2O
NO3-1 N2
Mn+4
Mn+2
Fe+3
Fe+2
SO4-2
S-2
CO2
CH4

16. Graph (handout) 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
After O is used, Eh drops; then nitrate becomes e acceptor, reduced to nitrite, Eh drops; then Mn will be reduced by organisms that can do tha, Eh drops
Before methane can be produced, Eh has to be zero

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

18. 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

19. Some conclusions about aeration:
Forms/mobility
Roots
Decomposition

20. Some conclusions about aeration:
1. Forms and Mobility
Soil aeration determines which forms of chemicals are present and how mobile they are
Redox colors in Poorly and Well-Aerated Soil
Nutrient elements

21. 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

23. 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)

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

30. Plate 16 A soil catena or toposequence in central Zimbabwe
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.

31. Plate 21 Effect of poor drainage on soil color
Plate 21  Effect of poor drainage on soil color. Gray colors and red redox concentrations in the B horizons of a Plinthaquic Paleudalf.

32. Manganese concretions

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

34. Some conclusions about aeration:
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. Some conclusions about aeration:
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 season to develop anaerobic conditions in the upper part.
Oxygen is removed from groundwater by respiration of microbes, roots, soil fauna
Biological zero = 5°C
Why “growing season” is important: if period of saturation is too cold for microbial growth or plant root activity, may not have anaerobic conditions; it is anaerobic conditions that cause a place to be a wetland, not just saturation.

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. Hydric soils support growth and regeneration of hydrophytic plants.

41. Hydric Soils and Taxonomy
Histosols
(all Histosols except Folists)
(all Histels except Folistels)
Aquic suborders and subgroups
Definition of aquic soil moisture regime:
“reducing regime in soil virtually free of dissolved oxygen because it is saturated. Some soils are saturated at times while dissolved oxygen is present, either because the water is moving or the environment is unfavorable for microorganisms; such a regime is NOT considered aquic”.
Organic soils made up mostly of forest litter’ not saturated

42. Aquic Conditions: Periodic or continuous saturation
Redoximorphic features
Verify by measuring saturation or reduction

44. Exception to Aquic conditions:
Artificial drainage
Removal of free water from soils with aquic conditions
Artificially drained soils are included with aquic soils
Because soil Taxonomy is based on soil GENESIS and minimizes human disturbance
Pertains to Hydric soils also

45. Artificially wet soils are considered hydric
Artificially “dry” (drained) soils are considered hydric

46. Types of saturation endosaturation: all soil layers sat’d to 2 m depth
Episaturation: sat’d layers in upper 2 m (perched)
Anthric saturation: controlled flooding (rice, cranberries)

48. 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

50. Plate 30  Dark (black) humic accumulation and gray humus depletion spots in the A horizon are indicators of a hydric soil. Water table is 30 cm below the soil surface.

53. Figure 7.11  The relationship between the occurrence of some soil features and the annual duration of water-saturated conditions. The absence of iron concentrations (mottles) with colors of chroma >4, and the presence of strong expressions of the other features are indications that a soil may be hydric. [Adapted from Veneman et al. (1999)]

54. List of hydric soils
Click on hydric soils

55. Oxidized rhizosphere In some poorly aerated soils:
Red, oxidized iron in root channels
Oxygen diffused out of plant roots
Some plants transport oxygen through aerenchyma tissue in stems and leaves to roots (hydrophytic plants)

56. Plate 29  Oxidized (red) root zones in the A and E horizons indicate a hydric soil. They result from oxygen diffusion out from roots of wetland plants having aerenchyma tissues (air passages).

57. Black spruce
Shallow wide spreading roots in upper 8 “ organic soil; frequent fire intervals

58. Pitcher plant