equivalence when forming compounds H+ and Cl- are both monovalent H+ and NO3 - are both monovalent H+ is monovalent, SO4 -2 is divalent Na+ and Cl- are both monovalent Ca+2 is divalent, Cl- is monovalent Mg+2 is divalent, Cl- is monovalent HCl HNO3 H2 SO4 NaCl CaCl2 MgCl2 essential chemistry 1. Clay Mineralogy 2. Cation Exchange 3. Soil Acidity 6 Outline
in diameter Intermediate between dissolved and particulate fractions 7 1 nm = 10-9 m size surface area external surface area internal surface area electrostatic charge negative positive 8 clay, OM particles 10 to >800 m2/g
cations and anions ionic double layer adsorption of water 9 Soil Colloids Crystalline Silicate Clays Primary Silicate Clays Secondary Silicate Clays Noncrystalline Silicate Clays Iron and Aluminum Oxides Organic 10 definite order sheets no definite order e.g. volcanic ash highly weathered soils
O OH H bond Prevents expansion when clay is wetted 22 non-expanding Less shrink-swell Less plastic Less sticky little isomorphous substitution Low cation adsorption low specific surface area Good for engineering, construction, cultivation Less adsorptive capacity Dominated by external surface area Nutrient management needed
shrink-swell plastic sticky Al+3 to Mg+2 isomorphous substitution high cation adsorption high specific surface area Difficult for cultivation, construction high adsorptive capacity External and internal surface area 1 octahedral sheet sandwiched between 2 tetrahedral sheets 26 Si+4 to Al+3 isomorphous substitution K+ adsorbed (fixed)
to Mg+2 isomorphous substitution Some adsorption of cations Additional layer of hydroxide (Mg-dominated) Non-expanding Allophane No definite composition or shape Imogolite Similar to nanotubes 28 volcanic origin Amorphous High water holding capacity High amounts of + and - charge Image: Guimarães, Luciana, Andrey N. Enyashin, Johannes Frenzel, Thomas Heine, Hélio A. Duarte, and Gotthard Seifert. 2007. Imogolite nanotubes: Stability, electronic, and mechanical properties.ACS Nano 1 (4): 362-8.
should know: What are ions? What are cations and anions? How are cations and anions formed? Ion vs. atom 43 loose attraction between the charged surface of the soil colloid and the solute adsorption 44
a bridge between colloid surface and adsorbed ion 45 Outer sphere complexes Ions are weakly held to the colloid Ions are easily replaced Direct bonds between ions and colloid 46 inner sphere complexes Ions are strongly held to the colloid Ions are not easily replaced
equivalence Ratio law Anion effects Cation selectivity Complementary cations Principles governing ion exchange 49 50 Reversibility Reaction goes to the left if Na+ is added Na+ H+ H+ Na+
equivalence Ca+2 H+ 2H+ Ca+2 H+ Na+ H+ H+ Na+ equivalence of charges, not weights or number of ions 52 Ratio Law Ratio of Ca and Mg on the colloid = ratio of Ca and Mg in soil solution 20 Ca+2 5 Mg+2 16 Ca+2 4 Mg+2 1 Mg+2 4 Ca+2 Ratio on colloid 4:1 Ratio in soil solution 4:1
effects Reaction is pulled to the right H+ Ca+2 CaCO3 H2 O H+ CO2 ↑ H+ Ca+2 CaCl2 2H+ H+ 2Cl- Lost from the system Greater cation exchange with CaCO3 CaCO3 used to neutralize soil acidity Al+3 > Sr+2 > Ca+2 > Mg+2 > Cs+ > K+ = NH4+ > Na+ > Li+ Cation selectivity Held more tightly to the colloid Less likely to be displaced Higher charge Smaller hydrated radius 54 Not all cations are adsorbed equally Soil colloids are dominated by Al+3 in humid regions
eq = 1 mole valence quantity of an ion that possesses 1 mole of charge 1 eq Na+ = 1 mole Na+ = 6.02 x 1023 ions 1 eq Ca+2 = 1/2 mole Ca+2 = (6.02 x 1023)/2 ions equivalent weight = molecular weight valence Atomic weight of Ca+2 = ~40 g 1 mole of Ca+2 weighs 40 g 1 eq Ca+2 = atomic weight/valence = 40/2 = 20 g 1 meq of Ca+2 = 20 mg = 0.02 g What is the mass of a meq of calcium? 58
(1M) solution = 1 mole of solute in 1 L of solution 1 Normal (1N) solution = 1 g equivalent weight in 1 L of solution 59 Molarity and Normality HCl, molecular weight ~36.5 1 mole HCl = 36.5 g HCl 1 eq HCl = 36.5 g HCl 1M HCl = 36.5 g HCl in 1L solution 1N HCl = 36.5 g HCl in 1L solution H2 SO4 , molecular weight ~98 1 mole H2 SO4 = 98 g H2 SO4 1 eq H2 SO4 = 49.5 g H2 SO4 1M H2 SO4 = 98 g H2 SO4 in 1L solution 1N H2 SO4 = 49.5 g H2 SO4 in 1L solution essential chemistry Units meq/100 g or cmolc kg-1 number of centimoles of positive charge (cmolc ) that can be adsorbed per unit mass 60
decreases with decreasing pH AEC increases with decreasing pH Buffered methods CEC measured at pH 7.0 or 8.2 “potential”/ “maximum” CEC, if acidic soils Unbuffered methods CEC measured at actual pH of soil “effective CEC” 68 The pH of the method is important
of CEC satisfied by a particular cation Cation cmolc /kg Na+ 5 K+ 10 Ca+2 50 Mg+2 30 Others 5 Total 100 What is the calcium saturation percentage? 50% 70 High ion saturation = Ion is more available for plant uptake
/kg Na+ 5 K+ 10 Ca+2 50 Mg+2 30 Others 5 Total 100 What is the calcium saturation percentage? proportion of CEC satisfied by base cations 95% 71 Base cations 1. K+ 2. Ca+2 3. Mg+2 4. Na+ Na+ is non-essential for plants Loamy Sand Silt Loam 50% percentage of CEC occupied by base cations CEC = 5 meq/100g CEC = 20 meq/100g 50% 50% BS = 2.5 meq/100g + 2 meq/100g = 4.5 meq/100g new BS = 4.5/5 = 90% 72 90% before after 60% before after How does %BS change after adding 2 meq/100g of base cations to the soil cation exchange sites?
a unitless soil characteristic strongly associated with soil CEC high CEC = greater buffer capacity 73 Acid buffering pH of the soil remains relatively constant 74 high CEC = greater buffer capacity = more limestone needed to increase pH low CEC = low buffer capacity = less limestone needed to increase pH Determining the amount of limestone needed to increase the pH to 6.5: effect of CEC limestone: calcium carbonate, CaCO3
limestone needed if initial pH is 5.5 Soil A Soil B CEC (meq/100g) 100 60 BS (%) 70 70 1. Which soil is more buffered? 2. How many meq/100g of bases would be required to increase the BS of soil A to 80% 3. How much would be required as mg of Ca? 76 A 10 meq/100g 5 mmoles, 200 mg
H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ active acidity exchange ions exchangeable acidity H H H Al Al Al Al H H H Non-exchangeable H and Al are released at high pH residual acidity Negative charges are freed up CEC increases The equilibrium between the three pools of acidity helps buffer soils of intermediate pH (5-7). pH values in salt solution are lower than in water 92 pH can be measured in water, or in salt solution
availability pH Co Cu Fe Mn Ni Zn Mo P: available pH 5-7 Available at high pH Most macronutrients Mo micronutrients base cations: greater availability at high pH acid cations: greater availability at low pH 96 Nutrient availability: Aluminum toxicity Aluminum becomes more available Root uptake of Al Damages root membranes Stunted root system Drought stress (because of roots)
availability: phosphorus 98 Soil biology Preferred pH of microbe groups Bacteria 6 to 8 Actinomycetes 7 to 7.5 Fungi 4 to 8 Predominant at low pH Different organisms have different preferred pH ranges
2 K, 3 M g, 10 Ca, 65 Al/H , 20 Ap Modified from Bear (1964) % 99 An “Ideal” Ap Horizon CEC Composition East Bear Exchangeable Cation Composition cmol c kg-1 0 5 10 15 20 25 30 C 25-C 5-25 5-cm O-Hor Na K Mg Ca Al H pH = 3.77 pH = 4.17 pH = 4.51 pH = 4.74 pH = 4.70 O 5 (Bh/s) 5-25 (Bs) 25-C (BC) C A “Representative” Undisturbed Maine Forest Soil Exchange Complex 100
increase pH Lime (calcium carbonate) Wood ash (rich in calcium) OM (rich in calcium) Some amendments decrease pH OM (forming organic acids) Sulfur 2S + 3O2 + 2H2 O → 2H2 SO4 101 102 Neutralizing soil acidity by liming Reaction is pulled to the right H+ Ca+2 CaCO3 H2 O H+ CO2 ↑ Lost from the system
The Nitrogen Cycle Organic N e.g. amino acids Microbial biomass N mineralization deposition manure biosolids ATMOSPHERE SOIL PLANTS ammonium nitrate inorganic forms of N N2 inert N-fixation ammonia immobilization leaching, loss to streams Nitrous oxide greenhouse gas nitrification denitrification decomposition uptake
15 20 25 30 35 1984 1989 1994 1999 2004 Year Sulfate deposition (kg/ha) Adirondack s (NY 08) Poconos/ Catskills (NY 65) Southern New England (MA 08) Central New England (NH 02) Maine (ME 09) Temporal trends in SO4 deposition (kg/ha) from 1984-2004 at five NADP sites representing each ELS-II region. After Rosfjord 2006 109 Bear Brook Watershed in Maine 110
Brook Treated East Bear Brook Reference 111 112 West Bear Treatments Initiated Nov, 1989 1800 eq ha-1 yr-1 (NH4 )2 SO4 = 25.2 and 28.8 kg ha-1 yr-1 N and S Added in 6 bi-mo. applications
IV III V VI VII Constant N and S Loading DBC+ DBC- Soil BS% Stream Conc. Conceptual Model of Soil and Stream Base Cations 115 Acidic inputs to soil Base cations leached from soil and exported to stream Base cations depleted from soil Acid deposition stops Soils adsorb base cations, so less input to streams Soil Chemistry 116
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