Exchange ReactionsCation exchangeAcid SoilsSalt/Sodium Affected Soils

Lecture 5

humus 200 cmolc/kg

smectites 100 cmolc /kg

illite 25 cmolc /kg

kaolinite 10 cmolc /kg

Fe and Al oxides 5 cmolc /kg

Charge of Soil Components

5.2

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

5.3

Adsorbed cations(a) arid region soils = "basic" cations

Ca+2, Mg+2, K+, Na+

(b) humid region soils

= “acidic” cations as well Ca+2, Mg+2, H+ and Al+3

(c) strength of adsorption Al+3> Ca+2 = Mg+2 > K+ = NH4

+ > Na+

5.4

Cation Exchange

Exchange process

Ca+2-colloid + 2 H+ 2 H+-colloid + Ca+2

= H+ replaces Ca+2 adsorbed to soil colloids

Ca-x + 2 H+ 2 H-x + Ca+2

x = the soil solid phase

Ca(ad) + 2 H+ 2 H(ad) + Ca+2

X(ad) = "adsorbed" cation X

5.5

before rainfallbefore rainfall

Saline-Sodic Soils 5.6

after rainfallafter rainfall

Our fields! Oh No...Trout Farm?

5.7

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)(see table 10.2)

5.8

Sodic SoilsSodic Soils

flocculationflocculation

poorpoorwaterwaterinfiltrationinfiltration

dispersiondispersion

5.9

Sodium Ion EffectSodium Ion Effect

flocculationflocculation dispersiondispersion

attraction

CaCa+2+2 && Mg Mg+2+2

repulsion

NaNa++

5.10

SAR ParameterSAR Parameter

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

SAR approximately equals ESP SAR is measured ESP is estimated

in water or extract for soil solids

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

for Na+ hazard = SAR < 15

5.11

Acid 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-

5.13

Human-Induced Acidity

* Chemical fertilizers

ammonium-based N materials

NH4+ (O2) HNO3

Ferrous-Fe materials

Fe+2 Fe+3 (+ 3 H2O)Fe(OH)3 + 3 H+

Elemental Sulfur

2 So + 3 O2 + 2 H2O 4 H+ + 2 SO4-2

5.14

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 (S-2) minerals

S-2 (O2, H2O) H2SO4

Human-Induced Acidity

5.15

Acid Soils: Role of Aluminum

AlAl+3+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

5.16

Acid Soil Properties5.17

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

5.18

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

5.19

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

5.20

Lime Reactions in Soil

1. Neutralize acidity 2 H-X + CaCO3 Ca-X + H2CO3 + H2O

2. Base Saturation increases

3. Soil pH increases

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

soluble insoluble (toxic) (not toxic)

5.21