2. Visual comparison of common silicate clays Smectite Kaolinite1:1 Vermiculite Fine-grained mica Chlorite 2:1 clays 1:1 clays
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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
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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 )
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)
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
28. Chapter 8 How plants get nutrients from soils (ion exchange)
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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
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
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 -
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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)
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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
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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
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?
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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)
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106. Shape of silicon tetrahedron and aluminum octahedron O OH O O O OH OH OH OH Si Al
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
121. Exchange affinity This is referred to as the “ Lyotropic series” Strength of adsorption proportional to valence (charge) ÷ hydrated radius
Editor's Notes
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.
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