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Week 7 Clay And Ion Exchange (1)

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  • no protein enzymes in existence to form the first nucleotides or catalyze the first formation of RNA strands because these proteins are only formed by RNA … all that was required was an assemblage of RNAs capable of both catalysis and replication with change. The prebiotic synthesis of RNA may have been carried out on the surfaces of clay minerals (and other layered minerals) using montmorillonite clay-catalyzed reactions of activated monomers (the building blocks of RNA)
  • ALL clay minerals have edge charges.
  • (a) 13 negative charges and 5 positive charges; (b) 3 negative charges and 6 positive charges
  • Upper case takes place readily as Ca2+ binds more strongly than does K+ (lyotropic series) Second case: need more than 3 K+ for the reaction to take place even though the reaction is a charge-balanced one (I.e., only 3 of the K+ are involved). This is because the Al3+ is higher on the lyotropic series. Note also that these are REVERSIBLE (unless something precipitates, volatilizes, or is strongly adsorbed).
  • E- remember to put this on test
  • Teton Dam failure story: used silt instead of clay as dam’s core, and it failed, killing 11 people and wiping out thousands of homes.
  • Teton Dam failure story: used silt instead of clay as dam’s core, and it failed, killing 11 people and wiping out thousands of homes.

Transcript

  • 1. Announcements
    • Midterm Exam 2 on Monday, Feb. 22nd –
    • IN THIS ROOM, at 11 AM
      • Physical properties
      • Soil water
      • Clay mineralogy
      • Ion exchange
  • 2. Visual comparison of common silicate clays Smectite Kaolinite1:1 Vermiculite Fine-grained mica Chlorite 2:1 clays 1:1 clays
  • 3. Fine-grained Mica (2:1)
    • Al 3+ substitution for Si 4+ on the tetrahedral sheet
    • Strong surface charge
    • “ fairly nutrient poor”
    • Non-swelling , only moderately plastic
    • Stable under moderate to low pH , common in midwestern US
    • Potassium bound in interlayer
  • 4. Structure of soil mica (Illite) ~ 20% of the silica sites are occupied by aluminium (tetrahedral substitution) yielding a VERY strong net negative charge
  • 5. Structure of soil mica (Illite) Explore Soil Mica (muscovite) HERE 1. Isomorphous substitution is in the tetrahedral sheets 2. K+ in the interlayer space to satisfy the charge and “locks up” the structure K+ K+
  • 6. Visual comparison of common silicate clays Smectite Kaolinite1:1 Vermiculite Fine-grained mica Chlorite 2:1 clays 1:1 clays
  • 7. Chlorites (2:1:1)
    • Hydroxy sheet in the interlayer space
    • Restricted swelling
    • “ Nutrient poor”
    • Common in sedimentary rocks and the soils derived from them
    • Isomorphic substitution in both tetrahedral and octahedral sheets
  • 8. Structure of Chlorite Mg-Al-Fe hydroxy sheet Mg-Al-Fe hydroxy sheet
    • Iron-rich
    • “ locked” structure
    • Low nutrient supply capacity
    = Al = Fe = Mg Explore Chlorite HERE
  • 9. Explore Nacrite HERE Nacrite Nacrite, Lodève Basin, France                                        Field of view approx. 200 microns wide
  • 10. Visual comparison of common silicate clays Smectite Kaolinite1:1 Fine-grained mica Chlorite 2:1 clays 1:1 clays H bonding Vermiculite
  • 11. Comparison of common silicate clays Edges only – NO isomorphic substitution Property Kaolinite Smectite Fine-grained mica General class 1:1 (TO) 2:1 (TOT) 2:1 (TOT) Swelling Low High Low or none Nutrient supply capacity Low High Moderate Charge location Octahedral sheets Tetrahedral sheets Bonding Hydrogen ( strong ) Van der Waal’s ( weak ) Click here Potassium ions ( strong )
  • 12. Smectite Kaolinite1:1 Fine-grained mica Chlorite 1:1 clays Location of isomorphic substitution and resulting internal charge imbalance No charge except at edges octahedral tetrahedral octahedral tetrahedral tetrahedral octahedral tetrahedral tetrahedral octahedral tetrahedral tetrahedral tetrahedral tetrahedral tetrahedral tetrahedral octahedral tetrahedral octahedral Vermiculite 2:1 clays H bonding Consider: hydration, cation adsorption, swelling, shrinking, plasticity
  • 13. Genesis of silicate clays
    • Dissolution and recrystallization
    • Chemical alteration of primary minerals
    Why different clays form…
    • Weathering conditions
    • Ions present & concentration in weathering solution at time of crystallization (PM)
  • 14. Crystal Growth
    • http://www.mpipks-dresden.mpg.de/mpi-doc/kantzgruppe/wiki/projects/Crystal_Growth.html
  • 15. Factors affecting mineral stability
    • Number and type of base cations in the structure ( base cations are soluble…more base cations = less stable )
      • Ca 2+ , Mg 2+ , Na + , K +
    • Number of silica tetrahedra that are linked ( more sharing of oxygens = more stable )
    • Al 3+ substitute for Si 4+ ( more subs = less stable )
  • 16. Weathering pattern of clay formation Oxisols Ultisols Entisols, Inceptisols Vertisols
  • 17. Where to find different clays
  • 18. Types of charge
    • Permanent
    • pH-dependent
    (due to isomorphous substitution) (variable, due to edge phenomena)
  • 19. Permanent charge Octahedral sheet neutral Net negative charge -
  • 20. + pH-dependent charge: on edges!!! Espec. Important in kaolinite, humus, where no internal charge imbalance (exchange on edge only) H + bound tightly at low pH, so the lower the pH , the less exchange there is (i.e., lower nutrient availability ) As pH increases, hydrogen is held loosely and can be exchanged for other cations Under moderately acidic conditions – little or no charge results pH dependent charges are associated with the edges of inorganic (clay crystals) and organic colloids (OM) Low pH (moderately acidic) Al +2 OH OH OH - - - NH 4 + K + Na + High pH (less acidic)
  • 21. Kaolinite Well crystallized kaolinite from the Keokuk geode, USA                                        Field of view approx. 18 microns wide
  • 22. Ideas about Origin of Life on Earth
    • Pre-formed life came crashing in on a meteorite?: extraterrestrial
    • Formed in place from abiotic materials?: abiogenesis
    • “ Primodial Soup”:
    • Miller-Uery experiment (1952): CH 4 , H 2 O, NH 3 , H 2 , heat, spark
      • 11 out of 20 amino acids formed
    Requires self-replication, metabolism
  • 23.
    • RNA is a chemical messenger between DNA (blueprints) and the ribosomes (protein factory)
    • Chicken and egg problem
      • no protein enzymes in existence to form the first nucleotides or catalyze the first formation of RNA strands because these proteins are only formed by RNA
    • New discovery (mid 1980’s): in some microbes, RNA can behave like a simple enzyme- creating new proteins on its own (Tom Cech, Nobel Prize)
    How does replication originate? RNA
  • 24. Clays and the origins of life on Earth
    • “ Naked replicators” – crystals first “life” forms
      • information stored and replicated in crystalline structure
      • Crystals appear likely to prove to be the most plausible self-replicating agents in a pre-biotic environment.
      • Individual nucleotides poured onto surface of smectite join together into RNA strands
      • Smectite also causes free abiotically formed fatty acids to join together into vesicles
  • 25. Clay is a replication platform/template
  • 26. “ Sweet crystal” hypothesis: clay minerals have grooves that catalyze synthesis of polysaccharide chains Polysaccharides – polymer chains, contain carbon, hydrogen and oxygen in a 1:2:1 ratio A.G. Cairns-Smith – 1987, Clay minerals and the origin of life (book) R.M. Hazen – 2001, Life’s rocky start - Scientific American
  • 27. END OF Clay structure & properties
  • 28. Chapter 8 How plants get nutrients from soils (ion exchange)
  • 29. Ch. 8 Learning Objectives
    • Describe the process of cation exchange and know what drives it
    • Calculate the cation exchange capacity (CEC) of a soil given the amount of clay and organic matter
    • Explain how CEC is related to nutrient supply capacity
  • 30. Ion exchange
    • The exchange of one ion for another on the surface of or in the interstitial spaces and edges of a clay crystal (or surfaces of humus molecules)
      • Cation exchange (e.g., Ca 2 + for K + )
      • Anion exchange (e.g., H 2 PO 4 - for NO 3 - )
  • 31. What’s so great about ion exchange?
    • Retards the release of pollutants to groundwater
    • Affects permeability, with implications for landfills, ponds, etc.
    • Affects nutrient availability to plants (constant supply, protection vs. leaching)
  • 32. “ Next to photosynthesis and respiration, probably no process in nature is as vital to plant and animal life as the exchange of ions between soil particles and growing plant roots.” Nyle C. Brady
  • 33. 2 Controls on ion exchange
    • Relative concentration of ion in soil solution
    • Strength of adsorption
      • Related to ionic radius and valence (charge)
        • The smaller the radius and greater the valence , the greater the force of attraction.
    r 2  charge F = Coulomb’s Law
  • 34. Controls on ion exchange
    • Relative concentration of ion in soil solution
    • Strength of adsorption
      • Related to ionic radius and valence (charge)
        • The smaller the radius and greater the valence , the more closely and strongly the ion is adsorbed.
    r 2  charge F = Coulomb’s Law
  • 35. Controls on ion exchange
    • Relative concentration of ion in soil solution
    • Strength of adsorption
      • Related to ionic radius and valence (charge)
      • The smaller the radius and greater the valence , the greater the force of attraction.
    r 2  charge F = Coulomb’s Law
  • 36. Strength of adsorption is related to the combined effects of Charge and Size
  • 37. Exchange affinity Held more strongly Held more weakly This is referred to as the “ Lyotropic series” Al 3+ > Ca 2+ > Mg 2+ > NH 4 + = K + > Na + Strength of adsorption related to valence (charge) ÷ hydrated radius
  • 38. Definitions
    • cation : An ion that carries a positive (+) charge
    • anion : An ion that carries a negative (-) charge
    • cation exchange : A process: cations in solution exchanged with cations on exchange sites of minerals and OM
    • (CEC) cation exchange capacity : The total amount of positive charge that a particular material or soil can potentially adsorb at a given pH
    • (AEC) anion exchange capacity : The total amount of negative charge that a particular material or soil can potentially adsorb at a given pH
  • 39. Announcements
    • Monday’s and today’s lectures are posted on Blackboard
    • My office hours today: after class or 2-4 pm
    • Web Soil Survey assignment
      • Due next week in lab
  • 40. What is clay?
    • A particle size class (<0.002 mm)
    • A textural class (soils with >45% clay)
    • A specific type of mineral, phyllosilicate, with either 1:1 or 2:1 crystal structure, which often has net negative charge
  • 41.  
  • 42. Smectite Kaolinite1:1 Fine-grained mica Chlorite 1:1 clays Location of isomorphic substitution and resulting internal charge imbalance No charge except at edges octahedral tetrahedral octahedral tetrahedral tetrahedral octahedral tetrahedral tetrahedral octahedral tetrahedral tetrahedral tetrahedral tetrahedral tetrahedral tetrahedral octahedral tetrahedral octahedral Vermiculite 2:1 clays H bonding Consider: hydration, cation adsorption, swelling, shrinking, plasticity
  • 43.  
  • 44. Ion exchange vs. CEC Sandy loam VERY acidic soil How many charges are there to fill??? H + H + NO 3 - NO 3 - NO 3 - H + HSO 4 - exchange surface CEC = 7; AEC = 2 NH 4 + Ca 2+ H + Mg 2+ K + NO 3 - Cl -
  • 45. Cation Exchange Capacity (CEC)
    • The sum total of all exchangeable cation charges that a soil can potentially adsorb
    • Determined experimentally in the lab
    • Expressed in terms of positive charge adsorbed per unit mass [cmols charge /kg soil ]
    • If: CEC = 10 cmol c /kg
    • Then: soil adsorbs 10 cmol of H + which can be exchanged with 10 cmol K + , or 5 cmol Ca 2+
    Number of charges, not the number of ions , is what matters 1 mol = 6.022 X 10 23 atoms (1 cmol = 1/100 mol)
  • 46. CEC depends upon
    • Amount of clay and organic matter (colloids)
    • Type of clay minerals present (i.e. kaolinites, smectites, vermiculites, micas, chlorites)
    Which of the 12 soil orders would have the highest CEC?
  • 47. Anion Exchange Capacity (AEC)
    • The sum total of all exchangeable anion charges that a soil can potentially adsorb
    • Determined experimentally in the lab
    • Expressed in terms of negative charge adsorbed per unit mass [cmols charge /kg soil ]
    Number of charges, not the number of ions , is what matters
  • 48. Positive Charge in soils: (creates Anion Exchange Capacity)
    • Fe and Al oxides (important in ultisols and oxisols)
    Al or Fe
  • 49. Charges on soil colloids* * Itty bitty soil components – silicate clays, oxides, humic substances So what will those negative charges adsorb? Colloid type Negative charge Positive charge Humus (O.M.) Silicate clays Oxides of Al and Fe 200 cmol c /kg 0 cmol c /kg 100 cmol c /kg 0 cmol c /kg 4 cmol c /kg 5 cmol c /kg
  • 50. Source of charge on 1:1 clays Broken edge of a kaolinite crystal showing oxygen atoms as the source of NEGATIVE charge Charge is pH dependent
  • 51. Source of charge for the smectites Isomorphous substitution here, in the octahedral sheet means a net NEGATIVE charge Permanent charge
  • 52. Source of charge for the micas 3. Charge imbalance mostly on edges Permanent charge K+ K+ 2. K+ satisfies charge and “locks up” the structure no internal exchange surfaces 1. Isomorphous substitution in tetrahedral sheets
  • 53. Negative charges on humus Central unit of a humus colloid (mostly C and H) ENORMOUS external surface area! (but no internal surface – all edges) Charge is pH dependent Explore Soil Organic Matter (SOM) HERE
  • 54. Surface charge comparison 13 out of 18 “sites” are negative (72%) 3 out of 9 “sites” are negative ( 33%) But the numbers will vary as the pH of the soil varies!
  • 55. Examples of cation exchange + Ca 2+  The interchange between a cation in solution and one on a colloid must be CHARGE balanced. K + K + K + K + K + K + K + Strength of adsorption of ions in solution Relative concentration of ions in solution + 2K + Ca 2+ K + K + Al 3+ + 3K +  K + K + K + + Al 3+
  • 56. Sources of acidity: Hydrolysis (weathering reaction)  H + (requires water) Biological decomposition  H + (requires water) Dissolution of minerals (weathering reaction)  Al 3+ (requires water)
  • 57. “ Acid” cations
    • H + , Al 3+
    • Al 3+ + H 2 O Al(OH) 2+ + H +
    • Al(OH) 2+ + H 2 O Al(OH) 2 + + H +
    • Al(OH) 2 + + H 2 O Al(OH) 3 + H +
  • 58. Adsorbed cations: climate effect Humid region soil Arid region soil Low pH (acidic) High pH (basic) H + H + H + Al 3+ K + K + Ca 2+ Mg 2+ H + Mg 2+ NH 4 +
  • 59. Typical Adsorbed cations (%) Soil order “ Acid cations ” (H + , Al 3+ ) “ Base cations ” (everything else, e.g., Ca 2+ , NH 4 + , K + , etc.) Ultisol Alfisol Mollisol 45 55 65 35 30 70
  • 60. A real-life application:
    • How lime raises pH (lowers acidity) --
      • CaCO 3 + 2 H +  H 2 O + CO 2 + Ca 2+
  • 61. CEC (cmol c /kg): soil order 1:1 clays 2:1 clays O.M. Low pH 128.0 3.5 Lower moisture Ultisols Alfisols 9.0 Mollisols 18.7 Vertisols 35.6 Histosols low high
  • 62. OM has highest CEC 2:1 clays 1:1 clays Non-clayey soils Highly weathered oxides
  • 63. Measuring CEC saturate w/ ammonium remove xs salt leach measure
  • 64. Calculating CEC
    • Charge is the fundamental reacting unit
    • 4 cmol c /kg = 2 Ca 2+ or 4 K + or 1 Al 3+ and 1 H +
    K + Al 3+ H + Ca 2+ Ca 2+ K + K + K +
  • 65. Ion Exchange Animation
    • http://www.wiley.com/college/strahler/0471480533/animations/ch21_animations/soilph.html
  • 66. Announcements
    • Midterm Exam 2 on Monday, Feb. 22nd –
    • IN THIS ROOM, at 11 AM
    • Bring a calculator!
    • Web Soil Survey: 6-8 pages (MOST of this is tables, maps, legends printed off the internet) Each of the questions only requires a few sentences to answer.
      • Find a good internet connection- this is a heavily trafficked website.
  • 67. Controls on Ion exchange?
    • ?
    • ?
  • 68. Remember , CEC depends upon
    • Amount of clay and/or organic matter (OM)
    • Type of clay minerals present
  • 69. Rough Rule of thumb CEC = ( % O.M. x 200 CEC ) + ( % clay x 50 CEC ) But …CEC of clay minerals ranges from 3 to 150 !
  • 70. Organic matter challenge
    • How many effects of organic matter in soil can you think of?
    • Increases CEC
    • Colors soil darker
    • Removes carbon from atmosphere, stores it in soil
    • Energy substrate (food) for soil fauna: (microbes, fungi, animals)
    • Key agent in formation and stabilization of soil aggregates- especially granular/crumb
      • Reduces formation of surface crusts
      • Reduces soil erosion
      • Reduces surface water runoff
      • Increases macropores, increases %PS, decreases D b increases infiltration
      • Increases water-holding capacity
  • 71. (cmol c /kg) low really high high
  • 72. Sample calculation Calculate the net negative charge of a soil sample containing 5% organic matter (OM) and 20% montmorillonite (smectite) type of clay. Net negative charge = (% OM * its’ CEC) + (% clay * its’ CEC) Net negative charge = [( 5 /100) kg * 200 cmol c /kg] + [( 20 /100) kg * 100 cmol c /kg] CEC OM ~ 200 cmol c /kg CEC montmorillonite ~ 100 cmolc/kg 10 cmolc/kg + 20 cmolc/kg = 30 cmolc/kg
  • 73. More . . .
    • Which would have a higher CEC,
    • a soil with 5% organic matter and 20% kaolinite
    • (b) a soil with 1% organic matter, 20% smectitic montmorillonite , and 1% vermiculite ?
    See your text, Box 8.2, p. 209, for more (0.05 * 200) + (0.20 * 3) = 10.6 cmol c /kg (0.01 * 200) + (0.20 * 100) + (0.01 * 150) = 23.5 cmolc/kg
  • 74. CEC and pH – for 1:1 clays and humus CEC low high 3 8 Soil pH Why?
  • 75. pH-dependent charge
  • 76. Influence of pH on the CEC of smectite and humus or Kaolinite Edge charge = Internal charge
  • 77. Charge characteristics Colloid type Total charge Constant (%) Variable (%) Positive charge Organic 200 Smectite 200 Kaolinite 8 10 90 0 5 95 2 95 5 0 Permanent charge pH dependent charge
  • 78. CEC and weathering intensity Alfisols, Vertisols, Argiudolls Ultisols Oxisols
  • 79. Base saturation
    • A measure of the proportion of basic cations occupying the exchange sites
    • Base cations are those that do not form acids
      • Ca 2+ , Mg 2+ , K + , Na + , NH 4 + . . .,
    (replace with non-acid cation)
  • 80. Cations – acid forming vs base forming Sources of acidity: Hydrolysis (weathering reaction)  H + (requires water) Biological decomposition  H + (requires water) Dissolution (weathering reaction)  Al 3 + (requires water) Al 3 + + 3H 2 O  Al(OH 3 ) + 3H + non-acid “ Acid cations ” (H + , Al 3+ ) “ Base cations ” (everything else, e.g., Ca 2+ , NH 4 + , K + , etc.)
  • 81. Equation for base saturation non-acid cation “ Acid cations ” (H + , Al 3+ ) “ Base cations ” (everything else, e.g., Ca 2+ , NH 4 + , K + , etc.)
  • 82. Back to why we might care… … plants (i.e. FOOD!)
  • 83. How a plant works Nutrients
  • 84. How a plant works
  • 85. Nutrient Availability: Mechanism
    • H + ions from root hairs and microorganisms replace nutrient cations on the exchange complex
    • Nutrients are thus forced into soil solution , where they can be assimilated by adsorptive surfaces of roots or lost to drainage
  • 86. Availability of nutrients
    • Cation saturation : if high, nutrient readily available
    • Influence of complementary adsorbed cations : if other ions held more strongly, nutrient is more readily available
    • Effect of colloid type : some clays give cations up easy, others don’t (e.g., illite (fine grain mica))
  • 87. Cation Saturation
    • Compare availability of
    • 6 cmol c Ca 2+ /kg in a soil (CEC of 8 cmol c /kg) 6 cmol c Ca 2+ /kg in a soil (CEC of 30 cmol c /kg)
    • Plant demands differ:
      • Alfalfa requires Ca 2+ saturation of ~ 80%.
    Ca2+ displacement easier and more rapid
    • 6 / 8 = 75% vs. 6 / 30 = 20%
  • 88. Cation Complementarity
    • Remember the Lyotropic series??
    • Would you fear ammonium (NH 4 + ) limitation in a moderately acidic soil or a sodic soil (basic soil with lots of Na + )?
    The availability of the nutrient depends on the other cations in solution/adsorbed assume system is at equilibrium H + > Al 3+ > Ca 2+ > Mg 2+ > NH 4 + = K + > Na + loosely held cations, relative to the other cations present are more readily available pH = 5 - Al 3+ - H + - H + - H + - NH 4 + + NO 3 - pH = 8.5 - Na + - Na + - Na + - Ca 2+ - NH 4 + - Ca 2+ Held strongly Held weakly Al 3+ > Ca 2+ > Mg 2+ > NH 4 + = K + > Na +
  • 89. Cation exchange in action The fate of the K is limited by the other ions present on the exchange surface
  • 90. Root uptake of nutrients
    • Mass flow
    • Diffusion
    • Interception
  • 91. Test review
    • Calculate (from memory)
      • D b - Bulk density (understand the influence of compaction on D b )
      • % pore space
      • Gravimetric water content
      • Volumetric water content
    BRING A CALCULATOR
  • 92.
    • Woodburn silt loam in the OSU quad
    • You excavate a hole in the surface horizon and reserve all of the removed material, which you then weigh, dry, and re-weigh
    • Freshly collected soil weighs 470 g
    • The weight of the dried material is 390 g .
    • The volume of the hole is 300 cm 3 .
    • What is the bulk density of the surface horizon?
    • What is the % pore space of this horizon?
    Example test question… = 100(1-[1.3 g/cm 3 /2.65 g/cm3 ]) = 51% = 390g/300cm 3 = 1.3g/cm 3 D b =mass dry soil/bulk volume % PS = 100 * (1-[D b /D p ]) You will recall…
  • 93. Gravimetric Moisture Content: The amount (by wt.) of water contained in a soil sample at a given time.
    • Expressed as a ratio or percentage
    • Example: wet weight 150g, dry weight = 102g: [(150-102)/102]*100 = 47%
    • or [(150-102)/102] = 0.47
    • Calculated on an OVEN-DRY BASIS: = 100*((wet weight – dry weight ) / dry weight )
    Mass of water relative to the mass of the dry soil particles
  • 94. Particle Size Clay Silt Sand Pore size , infiltration rate, drainage rate, aeration
  • 95. Surface area, pore volume , nutrient supply capacity, plasticity and cohesion, swelling Particle Size Clay Silt Sand
  • 96. A soil is said to be at field capacity just after all the gravitational water has drained from the soil. Water potential (suction) is -10 cbar at field capacity. Many crop plants are most productive when soil moisture levels are kept at levels of at least 60 - 90% of field capacity. For soil a , what is the minimum range of soil volumetric water content you would aim for to keep your crops most productive? For soil a, field capacity (-10 cbar) is at at ~38%. 60% of 38% is 0.6*38% or ~ 23% 90% of 38% is 0.9*38% or ~34% You would want to keep your volumetric moisture content up at at least 23 to 34%.
  • 97. Which soil (a or b) has a higher bulk density? What is the bulk density of soil a ? Max volumetric water content for soil a is ~45%, which means the %PS for soil a is ~45%. %PS = [1 – (D b /D p )] * 100 45% = [1 – (X/2.65)] * 100 X= 1.46 g/cm 3 Soil a has higher bulk density. You can tell because it’s maximum water content is lower.
  • 98. If your field started completely dry, and your irrigation system delivers 1 cm water per hour, how long would you have to irrigate soil a for to bring the top 30 cm of your field to a moisture content that is 60% of field capacity?
  • 99.  
  • 100. If your field started completely dry, and your irrigation system delivers 1 cm water per hour, how long would you have to irrigate soil a for to bring the top 30 cm of your field to a moisture content that is 60% of field capacity?
        • θ v = ~38% at field capacity for soil a
        • 60% of 38% is 23%
        • 23% is the volumetric water content you are aiming for, and you are starting at 0% (dry soil)
        • 30 cm * 23% = 6.8 cm water needed
        • at 1 cm/ hour irrigation rate, you need to irrigate for 6.8 hours
  • 101. Which soil ( a or b ) holds more water at wilting point? (Wilting point is at a suction of -1500 cbar). Is this water available to plants?
    • Soil b holds more than twice as much water at wilting point, compared to soil a, but this water is unavailable to plants (held at too great suction or tension).
  • 102. Adhesion & cohesion
    • Adhesion : the attraction of water molecules for solid surfaces
    • Cohesion : the attraction of water molecules for each other
    • = contact angle
  • 103.  (cbar)  (%) Moisture Characteristic Curve Suction units: 1 bar 100cbar 1.01971 x 10 5 Pa 0.9869 atm 10 6 dynes/cm 2 14.5 psi 1019.753 cm H 2 O ~33 feet H 2 O volumetric water content Suction 0 -10 -100 -10 3 -10 4 -10 5 0 10 20 30 40 50 60
  • 104. Difference is a function of texture, structure, & OM content.  (cbar)  (%) Estimating moisture content using Moisture Characteristic Curve and Tension data -15 bar -490 bar  (bar) soil A soil B soil C soil D -1 -10 -100 -10 3 -10 4 -10 5 0 10 20 30 40 50 60 -0.01 -0.1 -1 -10 -100 -1000 -10 -100 -1000 -10 4 -10 5 -10 6  (cm)
  • 105. Darcy’s law (K sat )
    • The flow of water through saturated media is a function of the pressure head change (  H) across the media, the length (L) and cross-section area (A) of the media, and the texture of the media, i.e., coarse or fine.
    h 2 h 1 L inlet outlet A K = intrinsic permeability of media and the hydraulic properties of the liquid Air bubble Q = K A  H L K sat = sat. hydraulic conductivity  H = h 1 – h 2 (head change) A = cross sectional area L = media column length Q = V/t (infiltration rate) Q = discharge (cm 3 /t)
  • 106. Shape of silicon tetrahedron and aluminum octahedron O OH O O O OH OH OH OH Si Al
  • 107. Octahedral sheet Octahedral sheet Tetrahedral sheet Tetrahedral sheet Tetrahedral sheet Tetrahedral sheet
  • 108. Isomorphous substitution
    • The replacement of one ion for another of similar size within the crystalline structure of the clay
    • This changes the total charge and location of the charge on the mineral (greatly affecting the properties of the clay)
    ~ equal & shape/size (ionic radii)
  • 109. Permanent charge Octahedral sheet neutral Net negative charge
  • 110. pH-dependent charge: on edges!!! Espec. Important in kaolinite, humus, where no internal charge imbalance H + bound tightly, so the lower the pH , the less exchange there is (i.e., lower nutrient availability )
  • 111. CEC and pH – for 1:1 clays and humus CEC low high 3 8 Soil pH
  • 112. Smectite Kaolinite1:1 Fine-grained mica Chlorite 1:1 clays Location of internal charge imbalance No charge except at edges octahedral tetrahedral octahedral tetrahedral tetrahedral octahedral tetrahedral tetrahedral octahedral tetrahedral tetrahedral tetrahedral tetrahedral tetrahedral tetrahedral octahedral tetrahedral octahedral Vermiculite 2:1 clays H bonding Consider: hydration, cation adsorption, swelling, shrinking, plasticity “ Young” Clays “ Old” Clays
  • 113. 100% silt 100% sand 100% clay % clay % silt % sand
  • 114. Color
    • Hue (e.g., 5 R ) tells you general shade ( red ); indicator of mineralogical composition DOES NOT tell you how dark the soil is
    • Value (e.g., 10R 5/ ) tells you how dark the soil is: ( 0 is darkest) may indicate current moisture status (dark=wet) and/or amount of organic matter
    • Chroma (e.g., 10R 5 /8 ) tells you color intensity ( 0 = gray ). Indicator of hydrologic regime (well drained =  O 2 = high chroma)
    “ quantified” using the Munsell system 8/ 6/ 5/ 4/ 3/ 2/ Hue Value /8 /4 /3 /2 /1 /6 7/ Chroma 5Y 5R 5YR 5G 5RP
  • 115. Aggregate stability
  • 116. Why Are Aggregates Important?
    • Increase porosity
    • Increase water infiltration, drainage, decrease runoff
    • Increase water holding capacity
  • 117. When aggregates break down…
  • 118. Conditions that Promote Aggregate Stability
    • Low disturbance
    • High root abundance
    • High fungal biomass
    • High OM
    • High clay content
  • 119.  
  • 120. Controls on ion exchange
    • Relative concentration of ion in soil solution
    • Strength of adsorption
      • Related to ionic radius and valence (charge)
  • 121. Exchange affinity This is referred to as the “ Lyotropic series” Strength of adsorption proportional to valence (charge) ÷ hydrated radius