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Cation Exchange Sites
Cations attached to clay particles/OM or free
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Charge Development
A. pH dependent charge
Deprotonation of surface functional groups (organic
matter)
COOH + OH-
= COO-
+ H2O 'Broken-edge charges': These occur at the broken
edges of clay mineral crystal sheets----- (Edges of layer
silicates; oxides of Fe and Al) ------the hydrogen of
hydroxyl groups dissociates slightly and the broken edge
is left with a negative charge carried by the oxygen.
pH dependent charge as pH
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Charge Development
B. Isomorphic substitution
replacement of one atom by another of
similar size in a crystal lattice withoutdisrupting or changing the crystal structure
of the mineral
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Isomorphic substitution of Mg2+ for Al3+ in the
octahedral layer of a 2:1 clay mineral.
Al2O2OH2 (no charge)
Al has a +3 charge, O has a -2 charge
AlMgO2OH2 (minus 1 charge)Mg has a +2 charge
http://www.soils1.cses.vt.edu/MJE/shockwave/cec_demo/version1.1/cec.shtml
http://www.soils1.cses.vt.edu/MJE/shockwave/cec_demo/version1.1/cec.shtmlhttp://www.soils1.cses.vt.edu/MJE/shockwave/cec_demo/version1.1/cec.shtmlhttp://www.soils1.cses.vt.edu/MJE/shockwave/cec_demo/version1.1/cec.shtmlhttp://www.soils1.cses.vt.edu/MJE/shockwave/cec_demo/version1.1/cec.shtml7/29/2019 Wastelands and weeds
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CATION EXCHANGE
Plant roots Vs Surface of Clay and Humus
Between
Two Clay Particle
Two Humus particlesClay and Humus particle
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CATION EXCHANGE
The exchange reaction is rapid
The exchange reaction is diffusion
controlled
The exchange reaction is reversible
The exchange reaction is
stoichiometric
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stoichiometric CATION EXCHANGE
1. Soil solution Vs Surface of Clay or Humus
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Selectivity of one ion over another
Some cations have a preference over others for the
negative surface charge of a soil colloid.
In general this is related to the charge of the cation andits hydrated radii.
Higher the charge, more strong is the adsorption
(exchangeable cation of aluminum, calcium or magnesium
stays close to the clay particle and initiate soil
aggregation)
When the valence of the cations are equal, the cation
with the smallest hydrated radius is more strongly
adsorbed.
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Radius Unit Na+ K+ Mg2+ Ca2+ Al3+
Hydrated nm 0.360 0.330 0.430 0.410 0.480
Order of preference for cations with a different valence (mixed series):
Al3+ > Ca2+ > Mg2+ > K+ = NH4+ > Na+ > H +
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Cation Concentration
Concentration of cations in the soil
solution: If the concentration of a cation in
soil solution is high, there is an increased
chance or tendency for that cation to beadsorbed.
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Cation Exchange Capacity
-vly charged sites: cation exchangesites.
Total number of sites = CEC
No. of cation adsorption sites/unit
weight of soilor
Sum total of exchangeable cations thata soil can adsorb
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Cation Exchange Capacity
-* Unit: milliequivalents (meq) per 100 g of oven dry soil
K+, AW: 39; eq wt=39
Ca++, AW: 40; eq wt =20
PO4-3 , AW: 95; eq wt =31.6
1 meq = 10-3 eq
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Predicting CEC
1) sum of cations : remove all cations andtotal the amount
2) NH4+ saturation: soil saturated withNH4+ NH4+ replaced by Ca++ Measure NH4+ removed3) CEC varies with texture:Sandy = 0-3 meq/100 g
Loamy = 10
15, Clay Loam = 10-30Clayey = > 30 (depends on kind of clay; 60-100)
Humus=400 meq/100 g
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High CEC value (>25) soil rich in clayand/OM
Low CEC value (
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% Base Saturation
(meq Ca+2 + meq Mg+2 + meq K+ + meq Na+)/CEC x 100
Base Cations
Calcium
Magnesium
PotassiumSodium
Acid Cations
Aluminum
Hydrogen
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