1. Exchange Reactions Cation exchange Acid Soils
Lecture 5
Exchange Reactions
Cation exchange
Acid Soils
Salt/Sodium Affected Soils

2. Charge of Soil Components
5.2
Charge of Soil Components
ò humus » 200 cmolc/kg
ò smectites » 100 cmolc /kg
ò illite » 25 cmolc /kg
ò kaolinite » 10 cmolc /kg
ò Fe and Al oxides » 5 cmolc /kg

3. 5.3
Origin of Charge

4. Adsorbed cations (a) arid region soils = "basic" cations
5.4
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. Cation Exchange Exchange process
5.5
Cation Exchange
Exchange process
Ca+2-colloid + 2 H+ ¬¾® 2 H+-colloid + Ca+2
= H+ replaces Ca+2 adsorbed to soil colloids
Ca-x + 2 H+ ¬¾® 2 H-x + Ca+2
x = the soil solid phase
Ca(ad) + 2 H+ ¬¾® 2 H(ad) + Ca+2
X(ad) = "adsorbed" cation X

6. 5.6
Saline-Sodic Soils
before rainfall

7. 5.7
Trout Farm?
Our fields!
Oh No...
after rainfall

8. Saline Soils EC > 4 ds/m = osmotic stress
5.8
Saline Soils
EC > 4 ds/m = osmotic stress
* salt sensitive plants (EC = 2 ds/m)
3 bean, onion, potato, raspberry, carrot, dogwood, larch, linden, peach, rose, tomato
* salt tolerant plants (EC = 10 ds/m)
3 sugarbeets, barley, cotton, rosemary,
wheat grass, wild rye
(see table 10.2)

9. 5.9
Sodic Soils
poor
water
infiltration
dispersion
flocculation

10. Sodium Ion Effect 5.10 flocculation dispersion è attraction ç
ç repulsion è
Na+
Ca+2 & Mg+2

11. 5.11
SAR Parameter
Predict sodium effect from saturated soil extract or irrigation water
SAR approximately equals ESP
SAR is measured ë ë ë ESP is estimated
in water or extract for soil solids
Good quality irrigation water:
4 for salt hazard = EC < 2 ds/m
4 for Na+ hazard = SAR < 15

12. Acid Soils

13. Y Sources of Acidity á Water: H2O ¬® H+ + OH-
5.13
Y Sources of Acidity
á Water: H2O ¬® H+ + OH-
á CO2 from soil respiration
CO2 + H2O ¨ H2CO3 ¨ H+ + HCO3-
carbonic acid
á Organic acids from O.M. decomposition
RH ¨ R- + H+
á Oxidation of S and N
S ¾® H2SO4 ¾® 2 H+ + SO4-2
NH3 ¾® HNO3 ¾® H+ + NO3-

14. Human-Induced Acidity
5.14
Human-Induced Acidity
* Chemical fertilizers
ü ammonium-based N materials
NH4+ ¾(O2)® HNO3
ü Ferrous-Fe materials
Fe+2 ® Fe+3 ¬(+ 3 H2O)® Fe(OH)3 + 3 H+
ü Elemental Sulfur
2 So + 3 O2 + 2 H2O ¾® 4 H+ + 2 SO4-2

15. Human-Induced Acidity
5.15
Human-Induced Acidity
ü Acid Rain: N and S gases emitted from
combustion processes
SO2 ¾(O2, H2O)® H2SO4
NOx ¾(O2, H2O)® HNO3
ü mining wastes, wetland drainage
- oxidation of sulfide (S-2) minerals
S-2 ¾(O2, H2O)® H2SO4

16. Acid Soils: Role of Aluminum
5.16
Acid Soils: Role of Aluminum
Al+3 ¨ Al(OH)+2 ¨ Al(OH)2+ ¨ Al(OH)3
|¬ strongly ¾®|¬ moderately ¾¾¾®|¬ alkaline
acid soils acid soils soils

17. Acid Soil Properties
5.17

18. 5.18
Phases of Soil Acidity
bound acidity
soluble acidity
exchangeable acidity
As acidity is removed from or added to soil solution
Ø maintain equilibrium within system
Ø must change all forms to change pH

19. Liming Materials Carbonate forms (a) "limestone" deposits and
5.19
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

20. Liming Materials (cont’d)
5.20
Liming Materials (cont’d)
Oxide and Hydroxide forms
(a) oxides formed by heating limestones
CaCO3 ¾(heat)® CaO + CO2
calcite gas
burned lime or quicklime
(b) add water to oxides to form hydroxides
CaO + H2O ¾® Ca(OH)2
hydrated lime

21. Lime Reactions in Soil 1. Neutralize acidity
5.21
Lime Reactions in Soil
1. Neutralize acidity
2 H-X + CaCO3 ¾® Ca-X + H2CO3 + H2O
2. Base Saturation increases
3. Soil pH increases
4. Al solubility decreases
Al OH- ¾® Al(OH)3
soluble insoluble
(toxic) (not toxic)