Wastelands and weeds

7/29/2019 Wastelands and weeds

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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.shtml


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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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