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: http://soils.cals.uidaho.edu/SoilORDERS Textbook
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
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.

10. Aerator sandals!

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

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

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

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

15. Oxidized/Reduced forms of…
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

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

17. ethylene
ethanol
Hydrogen sulfide

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

19. 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

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

21. Reducing agent
Substance donates electrons easily

22. 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

23. 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:

24. 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

25. 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
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

26. 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

27. 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

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

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

30. 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

33. Manganese concretions

34. 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)

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

39. 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.

40. 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

41. 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

42. 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

43. Hydric Soils

44. Wetland criteria :
Hydrology
Hydric soils
Hydrophytic plants

45. 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.

46. 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!!!

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

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

49. 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

51. 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.

54. 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

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

57. 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

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

59. 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)

61. 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)

62. 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).

63. Crops requiring saturated conditions:
Rice: wild rice, paddy rice
Cranberries

64. "In waters with high sulfate, we've struggled to find any wild rice,“
John Pastor damage to wild rice probably occurs when sulfate is converted to hydrogen sulfide. In an oxygen-starved environment such as the sediment under wild-rice beds, bacteria "breathe in" sulfate and "exhale" hydrogen sulfide, which can be toxic to plants.
Pastor knows from previous research that the availability of adequate nitrogen is the biggest limiting factor for the growth of wild rice.
Now he is seeing wild-rice plants exposed to high sulfate that "didn't look poisoned. They looked starved." Pastor's hypothesis is that sulfate transformed to sulfides is affecting root growth and blocking nutrients from getting into plants. Now he will see if the research supports his hypothesis.

65. Paddy rice Important food staple for over half of world’s population
Important for food security, produced overwhelmingly in developing nations
Asia: 90 % of world production
China 30%
India 21 %

66. Process

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

73. Pitcher plant