Exchange ReactionsCation exchangeSalt/Sodium Affected SoilsAcid Soils

Lecture 5

General Classes (layer build-up) of Phyllosilicate Minerals: Layer Type Charge† Trioctahedral Dioctahedral

1 octahedra 0 brucite, Mg(OH)2 gibbsite, Al(OH)3

1 tet. : 1 oct. 0 serpentine, Mg3Si2O5(OH)4 kaolinite, Al2Si2O5(OH)4

2 tet. : 1 oct. 0 talc, Mg3Si4O10(OH)2 pyrophyllite, Al2Si4O10(OH)2

2 tet: 1 oct. 1 phlogopite muscovite KMg3(AlSi3O10)(OH)2 KAl2(AlSi3O10)(OH)2 1 biotite KFe3(AlSi3O10)(OH)2 0.6-0.8 illite (hydrous mica) K(Na,Ca) Al1.3Fe0.4Mn0.2Si3.4Al0.6O10(OH)2

0.6-0.9 vermiculite 0.25-0.6 smectite † The layer charge per formula unit

• Humus 200 cmolc/kg

• Smectite/Vermiculite 100 cmolc /kg

• Illite 25 cmolc /kg

• Kaolinite 10 cmolc /kg

• Fe and Al oxides 5 cmolc /kg

Charge of Soil Components

ColloidNegativecharge

Positivecharge % constant % variable

Humus 200 0 10 90Vermiculite 120 0 95 5Smectite 100 0 95 5Illite 40 0 80 20Kaolinite 12 4 5 95Fe & Al Oxides 5 5 0 100

Origin of Charge

cmol / Kg

-

-

-

-

-

-

-

-

-

-

-

+

+

+

+

+

+

+

+

+

Ion Adsorption

Surface charge neutralized by ions from the soil solution

Adsorbed Cations

(a) arid region soils = "basic" cations Ca2+, Mg2+, K+, Na+

(b) humid region soils

= “acidic” cations as well Ca2+, Mg2+, H+ and Al3+

(c) strength of adsorption Al3+> Ca2+ = Mg2+ > K+ = NH4

+ > Na+

Cation Exchange

Exchange process

Ca2+-colloid + 2 Na+ 2 Na+-colloid + Ca2+

= Na+ replaces Ca+2 adsorbed to soil colloids

Ca-x + 2 Na+ 2 Na-X + Ca2+

X = the soil solid phase

Dispersion

before rainfallbefore rainfall

Saline-Sodic Soils

after rainfallafter rainfall

Saline Soils Saline Soils

EC > 4 ds/m = osmotic stress

* salt sensitive plants (EC = 2 ds/m)* salt sensitive plants (EC = 2 ds/m)

bean, onion, potato, raspberry, carrot, bean, onion, potato, raspberry, carrot, dogwood, larch, linden, peach, rose, tomatodogwood, larch, linden, peach, rose, tomato

* salt tolerant plants (EC = 10 ds/m)* salt tolerant plants (EC = 10 ds/m)

sugarbeets, barley, cotton, rosemary,sugarbeets, barley, cotton, rosemary,

wheat grass, wild ryewheat grass, wild rye

(see table 10.2 - 13(see table 10.2 - 13thth ed. or 10.3 – 12 ed. or 10.3 – 12thth ed.) ed.)

Sodic Soils (ESP > 15)Sodic Soils (ESP > 15)

flocculationflocculation

poorpoorwaterwaterinfiltrationinfiltration

dispersiondispersion

Sodium Ion EffectSodium Ion Effect

flocculationflocculation dispersiondispersion

attraction

CaCa2+2+ && Mg Mg2+2+

repulsion

NaNa++

SAR ParameterSAR Parameter

Predict sodium effect from saturated soil Predict sodium effect from saturated soil extract or irrigation waterextract or irrigation water

SAR is measured ESP/ESR is estimated in water or extract for soil solids

ESR = 0.015(SAR) - 0.01

Good quality irrigation water: for salt hazard = EC < 2 ds/m

for Na+ hazard = SAR < 15

Acid SoilsAcid Soils

Sources of Acidity Water: H2O H+ + OH-

CO2 from soil respiration CO2 + H2O H2CO3 H+ + HCO3

-

carbonic acid

Organic acids from O.M. decomposition RH R- + H+

Oxidation of S and N S H2SO4 2 H+ + SO4

2-

NH3 HNO3 H+ + NO3-

Human-Induced Acidity

* Chemical fertilizers

ammonium-based N materials

NH4+ (O2) HNO3

Ferrous-Fe materials

Fe2+ Fe3+ (+ 3 H2O)Fe(OH)3 + 3 H+

Elemental Sulfur

2 So + 3 O2 + 2 H2O 4 H+ + 2 SO42-

Acid Rain: N and S gases emitted from combustion processes

SO2 (O2, H2O) H2SO4

NOx (O2, H2O) HNO3

mining wastes, wetland drainage - oxidation of sulfide (S2-) minerals

S2- (O2, H2O) H2SO4

Human-Induced Acidity

Phases of Soil Acidity

bound acidityexchangeable

aciditysoluble acidity

As acidity is removed from or added to soil solution As acidity is removed from or added to soil solution maintain equilibrium within systemmaintain equilibrium within system must change all forms to change pHmust change all forms to change pH

Acid Soils: Role of Aluminum

AlAl3+3+ Al(OH)Al(OH)2+2+ Al(OH) Al(OH)22++ Al(OH) Al(OH)33

|| strongly strongly || moderately moderately || alkaline alkaline acid soils acid soilsacid soils acid soils soils soils

Al3+ + H2O Al(OH)2+ + H+ K = 10-4.93

 

Al(OH)2+ + H2O Al(OH)2+ + H+ K = 10-4.97

 

Al(OH)2+ + H2O Al(OH)3o + H+ K = 10-5.7

 

Al(OH)3o + H2O Al(OH)4

- + H+ K = 10-7.4

Acid Soils: Role of Aluminum

AlAl+3+3 Al(OH)Al(OH)+2+2 Al(OH) Al(OH)22++ Al(OH) Al(OH)33

Changes in Al Speciation

- - - - - -

- - - - - -

Clay Interlayer Soil Solution

pH 4 pH 6

H+

H+

Why [Al3+] ~ [H+] in Acid Soils

Fe3+ + H2O <--> Fe(OH)2+ + H+ K = 10-2.19

 

Fe(OH)2+ + H2O <--> Fe(OH)2+ + H+ K = 10-3.5

 

Fe(OH)2+ + H2O <--> Fe(OH)3o + H+ K = 10-7.4

 

Fe(OH)3o + H2O <--> Fe(OH)4

- + H+ K = 10-8.5

Why Not Iron?

Liming Materials

Carbonate forms(a) "limestone" deposits and

industrial byproducts

(b) calcite = (CaCO3) = calcium carbonate and

dolomite = CaMg(CO3)2

(c) dolomitic limestone maintains Ca:Mg balance

Liming Materials (cont’d)

Oxide and Hydroxide forms(a) oxides formed by heating limestones

CaCO3 (heat) CaO + CO2

calcite gas

burned lime or quicklime

(b) add water to oxides to form hydroxides CaO + H2O Ca(OH)2

hydrated lime

Lime Reactions in Soil1. Neutralize acidity

2 H-X + CaCO3 Ca-X + H2CO3 + H2O

2. Base Saturation increases

BS = (CEC – [Al3+][H+]) / (CEC) * 100

BS = {[Na]+[K]+[Ca]+[Mg]}/CEC *100

3. Soil pH increases

4. Al solubility decreases Al+3 + 3 OH- Al(OH)3

soluble insoluble (toxic) (not toxic)

Acid Soil Properties

Wetland (Hydric) Soils

and

Redox Conditions

Anaerobic Organisms

Food Source

• Organic carbon*

• Ammonium Ion (NH4+)

• Ferrous Iron (Fe2+)

• Hydrogen Sulfide (H2S)

Electron Acceptor

• Nitrate (NO3-)

• Manganese (Mn4+)

•Ferric Iron (Fe3+)

• Sulfate (SO42-)

CH2O

CO2

O2

H2O

NO3-CH2O

CO2 N2

Fe(OH)3CH2O

CO2 Fe2+

CH2O

CO2

SO42-

H2S

Energy YieldsDonor Acceptor

700

400

100

Eh (mV)* Condition

oxic

suboxic

anoxic

*pH 7

MnO2CH2O

CO2 Mn2+

Redoximorphic FeaturesRedoximorphic Features

- Soil colors

- Color Distribution

Soil Colors

Yellow -> Orange -> Red Fe(III) minerals

Black (veneer) Mn(IV) minerals

Dark Brown (disseminated) Organic Matter

Aerobic Environments

Gray -> Green -> Black Fe(II) minerals

Dark Brown (disseminated) Organic Matter

Anaerobic Environments

Iron massesRedox depletions

Root linings

MottlingNodules

Gleyed colors

Redoximorphic Features

Histic HorizonsHistic Horizons

“Rotten Eggs”

Plant Effects on Redox Conditions

FeIII(OH)3

deposit

O2(g)

Plaque Formation on Plant Roots

Fe(OH)3

O2

H2O

Fe2+