ESS Exam 3
ESS Exam 3 ESS 210 001
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This 6 page Study Guide was uploaded by Michaela Humby on Sunday February 14, 2016. The Study Guide belongs to ESS 210 001 at University of Tennessee - Knoxville taught by Andrew Conrad Sherfy (P) in Fall 2015. Since its upload, it has received 79 views. For similar materials see Intro to Soil Science in Environmental Science at University of Tennessee - Knoxville.
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Date Created: 02/14/16
Exam 3 Questions for ESS 210 The highlighted portions are the correct answers. For layer aluminosilicate clays, the aluminum sheets are comprised of OCTAHEDRAL Units. On soil colloids, variable charge is influenced by Exchange of Mg2+ for Al3+ in the aluminum sheets. Montmorillonite is an example of a 2:1 expanding clay Kaolinite is an example of a 1:1 clay Which of these closest represent the CEC of soil humus? 200 cmolc/kg The shrinking and swelling characteristics of some silicate clays are due primarily to the movement of water in and out of the interlayers of the crystals. In soils, base saturation refers to the number of Ca, K, Na, and Mg, ions present on the exchange sites. In soils, cation exchange capacity is reported in units of centimols of charge per kilogram of soil. As soil pH goes down, metal availability generally goes up and plant nutrients such as Ca and Mg go down. pH is a measure of H concentration As soil pH goes up, % base saturation goes up In order to manage soil acidity, we typically add which soil amendment? Lime The amount of exchangeable Al3+ and H+ on the soil colloids is referred to as reserve acidity. Exam 3 Questions for ESS 210 Which of the following elements would be considered “acidic cations” in the soil? H+ and Al3+ Soil phosphorus is primarily lost to erosion. Under acidic conditions, additional K+ will leach from soils. True Majority of soil potassium is in the form of soil minerals Majority of sulfur is in soil organic matter. When soil organic matter is decomposed aerobically, majority goes to the production of carbon dioxide Which of these is not a humic substance? Polysaccharide Which of these is a form of nitrogen which can be uptaken by a plant? NO3- Which C:N ration will most likely result in immobilization of soil nitrogen? 30:1 Mineralization of soil nitrogen involves which process? Microbial conversion of organic to inorganic N Nitrification of soil nitrogen involves which process? Bacterial reduction of NO3- to N2O and N2. 1. What are colloids and what are soil colloids? What are the properties of soilcolloids? Very small, usually < 1 mm diameter Chemically active particles a. Electrically charged surface Very high surface area per unit mass Common examples of colloidal suspensions b. Jello c. Milk Types of Soil Colloids Silicate clays ◦ aluminosilicate minerals ◦ crystalline and amorphous types Humus Al and Fe oxides and hydroxides ◦ sesquioxides ◦ amorphous, gel-like 2. Be able to describe the composition and structural units of layer silicate clays. e.g. what is micelle? Aluminosilicate minerals Crystalline structure Structural units 4+ a. Silicon (Si ) tetrahedral units b. Aluminum (Al ) octahedral units c. Individual units linked into SHEETS d. Sheets are combined in LAYERS e. Many layers = crystal or CLAY MICELLE 3. What is cation exchange capacity and how do clay particles develop charge? Definition: amount of exchangeable cations adsorbed per unit mass of soil. Units: centimoles of positive charge from cations per kilogram of soil. a. cmol ckg or cmol cg -1 Counts the nu2ber of excha3ge sites in s+il 1/2 cmol Ca , 1/3 cmol Al , or 1 cmol K will bind to 1 cmol /cg Soil scientists use CEC information to: 1) Determine the amount of limestone required to raise soil pH 2) Determine the amount of fertilizer nutrients required to provide adequate crop growth 3) Provide an index of soil fertility potentials 4. What factors affect cation exchange capacity? Soil solution composition a. If K+ dominates solution, also dominates exchange sites Soil pH b. acid soils: high Al3+and H in solution and on sites c. alkaline: low Al3+and H , high Ca , Mg , K , Na + Strength of adsorption increases as: d. cation valence increases e. hydrated size decreases f. strength of negative charge on colloid increases g. Relative affinity: 5. What is buffering capacity? a. Ability to moderate large changes in pH (or nutrient concentrations) b. Increases with increasing CEC 6. How do we calculate CEC of a soil? Measure or estimate % clay and humus Give each component an average CEC per 1% of that clay or humus a. e.g., 1 kg montmorillonite = 100% = 100 cmol c b. so, 1% mont. clay = 1 cmol c Multiply CEC/1% by the actual % Add all components 7. How do we determine amounts of exchangeable bases in a soil? 8. Know the various classes of silicate clay minerals and their physical and chemical properties. Amorphous Silicate Clays : Mixtures of silica and alumina that have not formed crystals - imagine a gel Young materials ◦ insufficient time ◦ insufficient weathering ◦ common in volcanic materials pH dependent charge high charge (at neutral pH) ◦ ~50 to 100 cmol /kg c Sesquioxides - Al and Fe Oxides : As weathering continues: ◦ H 4iO 4s leached ◦ much Al is also lost Low solubility, stable minerals pH dependent in acid soils, may have positive charge low total charge - 0 to 3 cmolc/kg 9. Describe the characteristics of other soil colloids and their charges. 10. How does weathering affect cation exchange capacity? Weathering a. rain = pH 5.6 b. dissolution of minerals releases Ca, Mg, K, Na and Al c. Basic cations leached more readily, leaves Al behind d. Why is Al3+ an acidic cation? Al3+ + 3HOH <____-----3 Al(OH) 3 + 3 H + ◦ “Acid Rain” 11. What is pH and understand the idea of soil pH. The Concept: Definition: negative logarithm of the H concentration in solution. - log [H ], where [H ] = mol/liter = M + if [H ] = 0.000-4M, then pH = - log 0.0001 = 4 0.0001 M = 10 M pH ranges from 0 to 14 ◦ pH < 7 = acidic ◦ pH > 7 = basic or alkaline ◦ pH 7 is neutral [OH-] = [H+] Insensitive: pH 4 is 10 times more acid than pH 5 soils generally range from 4.5 to 9.5 12. What is active acidity vs. reserve acidity? Active acidity a. [H+] in soil solution, form measured by standard pH measurement b. Very small amount, may take only a few kg of CaCO to 3eutralize c. Acidity that the plant “sees” Reserve d. Exchangeable Al and H e. As pH increases, these H+ and Al3+ can be released - pH dependent f. 1,000 to 100,000 times greater than active acidity g. more acidity ill move out of the reserve if h2o is added to the soil solution 13. How does pH affect nutrient and metal availability in soils? Weathering a. rain = pH 5.6 b. dissolution of minerals releases Ca, Mg, K, Na and Al c. Basic cations leached more readily, leaves Al behind d. Why is Al3+ an acidic cation? Hydrolysis- the splitting of water e. “Acid Rain” 14. What is base cation saturation a measure of? As pH increases, amounts of Ca, Mg, K, Na increase - these are “basic” cations Relative measure of weathering % Base saturation: cmolc as Ca, Mg, K, Na/kg x CEC (cmolc/kg) 15. How can we manage soil acidity? 100 Reduction of acidity a. application of liming materials Increasing acidity or lowering alkalinity b. application of acid-forming materials Liming materials Carbonates, oxides or hydroxides of calcium and magnesium Calcite - Ca3O Dolomite - MgCO3 Calcium oxide or quicklime - CaO Calcium hydroxide or hydrated lime - C2(OH) All these neutralize 2 H per mole compound Determining pH and Lime Requirement pH: ◦ Measure in soil-water suspension with pH meter. ◦ Usually 1 or 2 parts water to 1 part soil (volume). ◦ Sometimes use CaCl solution. 2 Lime Requirement: ◦ Add pH 8 buffer to soil-water suspension (Adams-Evans buffer in Tennessee, SMP in other northern states). ◦ Measure pH. ◦ Decrease from pH 8 is from exchange acidity. Can calculate H released and lime required.
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