Chapter 9
Soil acid-base and redox reaction
Chapter 9 Soil acid-base and redox reaction
Section 1 The formation of soil acid-base
The acid-base reaction of soil
The acid-base reaction of soil is change in soil pH. Soil
pH is probably the single most important chemical
charecteristic of a soil. The knowledge of pH is needed to
understand important chemical processes such as ion
mobility, precipitation and dissolution equilibria, and
oxidation-reduction equilibria. A knowledge of pH is also
needed to understand nutrient availability to plants and the
negative response of many plant species to soil acidity.
The soil pH value in China is about 4.5-8.5.
一、Soil acidity
（一）The cause of soil acidity
1． Climatic factor
2．Biological factor
3．Fertilizer application and irrigation
1.The source of hydrogen ions
（1）Water dissociation
（2）Carbon dioxide
（3）Organic acid
（4）Inorganic acid (inorganic fertilizer)
（5） Acid rain
Acid rain
Acid rain is caused by the
burning of fossil fuels.
Burning oil, gas and coal in
power stations releases
Sulfuric Dioxide (SO2) into
the atmosphere.
Burning oil and gasoline in
motor vehicles puts nitrogen
oxides (NOX) into the
atmosphere.
These gases mix with water
droplets in the atmosphere
creating weak solutions of
nitric and sulfuric acids.
When precipitation occurs
these solutions fall as acid
rain.
2. Aluminum activation in soil
Exchangeable Hydrogen is the main
source of H+ at pH 6 and above.
Below pH 6 Aluminum is the main
source of H+ due to dissociation of Al
from clay minerals. Aluminum
becomes more soluble at lower pH’s
Al3+ + H20 ----> Al(OH)++ + H+
Al(OH)++ + H2O ---> Al(OH)2+ + H+
Al(OH)2+ + H20 ---> Al(OH)3 + H+
（二）The types of soil acidity
1．Active acidity
Active Acidity –The activity of hydrogen ion in
the aqueous phase of a soil expressed as a pH value.
2． Soil potential acidity
(1) Strong acidity in soil
Colloid-Al3+ = Colloid + Al3+
Al3+ + 3H2O Al（OH)3 + 3H+
(2) Acidity and weak acidity in soil
Al（OH）2+ + HOH Al（OH）2+ + H+
Al（OH）2+ + HOH Al（OH）3+ + H+
Or: Colloid-H+ H+
3. Soil exchangeable acidity
• Exchangeable acidity- The aluminum and
hydrogen that can be replaced from an acid
soil by an unbuffered salt solution such as
KCl or NaCl. (cmol（＋）/kg）
4. Soil hydrolytic acidity
CH3COONa + H2O CH3COOH + NaOH
H-Colloid Al + 4CH3COONa Na-Colloid 3Na
+ Al（OH）3 +4CH3COOH
Oxide -OH + CH3COONa Oxide-O-Na++ CH3COOH
二、Soil alkalinity
（一） The cause of soil alkalinity
1、Calcium carbonate
hydrolysis
CaCO3 + H2O Ca2+ + HCO3- + OH-
CO2 + H2O HCO3- + H+
2、 Sodium carbonate hydrolysis
(1)2NaHCO3 Na2CO3+H2O + CO2
(2)Na2SiO3 + H2CO3 Na2CO3 +SiO2+H2O
(3)CaCO3 +NaCl CaCl2 + Na2CO3
CaCO3 + Na2SO4 CaSO4+ Na2CO3
3、 Exchangeable Sodium hydrolysis
Ca2+ 2Na+
Colloid Colloid
Mg2+ + 4Na+ 2Na+ + Ca2+ + Mg2+
Colloid xNa + yH O +CO
2 2 （x-y）Na+ + yNaOH
Colloid
yH+
（二） Affect the factors of soil alklization：
1．Climatic factor
2．Biological factor
3．Parent factor
Section 2 Index of soil acidity
一、 Intensity index of soil acidity
（一）pH value
pH - the negative log of the hydrogen ion(H+)
concentration in the soil water solution.
pH = - log [ H+]
（二）Lime potential
Lime potential will be introduced later
course (Agricultural Chemistry)
Soil acidity is brief name of soil acid and base!
二、 Index of soil alkalinty
（一）Total alkalinty
Total alkalinty including carbonate and bicarbonate in
soil solution or irrigation water.
Total alkalinty = CO32- + HCO3- (cmol(+)/L)
Lime reaction and lime soil:
（二）Exchangeable sodium percentage (ESP）
ESP is the percentage of exchangeable sodium ions to the total
soil exchangeable cations of all types in the soil sample.
ESP = [(exchangeable sodium ions )/(soil cations exchangeable capacity)]×100
Alkalinization-If a high proportion of exchange sites is occupied
by sodium ions, soils can become very basic with pH values of 8.5
to 10.5. These soils can become impermeable to water because
small soil particles that are dispersed by the sodium are entrapped
in the pores and seal them. These impermeable soils keep a water
layer on the surface, which cannot infiltrate, and these areas appear
“wet” longer than other adjacent soils.
Classification of solonetz in China: Light alkali-affected soil, ESP :
5－10％ ; Middle alkali-affected soil, ESP : 10－15％; Strong
alkali-affected soil, ESP : 15－20％ Alkali soil, ESP>30%.
三、 Affect the factors of soil acidity
1,Base saturation
2, CO2 differential pressure in the soil air
3, Soil moisture content
4, Condition of soil redox
Acid soil (Yunnan
Province, People's
Republic of China).
Section 3
Soil redox reaction
一、 Soil redox system
Characteristics of Soil redox system:
① Soil redox system can be divided two system:
inorganic and organic systems.
② The organisms participates in soil redox
reaction.
③ The soil is a heterogenesis system. Soil redox
potential is different in the field.
④ Equilibrium of soil redox is usually change.
二、 Index of soil redox
1. Soil redox potential (Eh)
Eh- The potential that is generated between or
oxidation or reduction half-reaction and the standard
hydrogen electrode.
Oxidation ＋ ne ==== Reduction
RT [氧化态 ] 0.059 [氧化态 ]
Eh E
0
log E
0
log
nF [还原态 ] n [还原态 ]
2、The negative logarithm of the apparent electron
activity（pe）
pe can be calculated by including the apparent activity of
electrons in equilibrium calculations of redox half-cell
reactions. In practice it is used as an alternative to Eh and at
25°C can be calculated from Eh values expressed in volts by
dividing by 0.059.
3、Relation between Eh and pH
Where m is number of protons in reaction.
三、Affect the factors of redox
1．Activity of microorganism
2．Content of labile organic matters
3 ． Content of easy oxidation-reduction
inorganic matters in soil
4． Metabolism of plant roots
5．Soil pH value
Section 4 Soil buffering
一、Concept of soil buffering
Soil buffering is the ability of soil to resist
change in pH.
二、Acid or alkali buffering of soil
（一） The principle of acid or alkali buffering of soil
HAc H+ + Ac-
NaAc + HCl HAc + NaCl
Colloid-M + HCl -H + MCl
Colloid
[盐]
pH pK a log
[酸]
（二） Acid or alkali buffering system of soil
1、Carbonate system
CaCO3 + H2O +CO2 Ca2+ + 2HCO3-
2
pH 6.03 log PCO2
3
2、Silicate system
Mg2SiO4 + 4H+ Mg2+ + Si(OH)4
3、Exchangeable ions system
Ultimate pH-当土壤胶体上吸附的阳离子全部是致酸离子
（H+和Al3+）时，称为“盐基完全不饱和状态”，此时土
壤的pH值称为“极限pH值”.
4、Aluminum system
When pH5.0 , Al ions become Al(OH)3
precipitation，it will lose it’s
buffering capacity.
5、Organic acid system
（三）The acid and alkali buffer capacity and
titration curve of soil (Studied by yourself!!)
(四) Affect the factors of acid or alkali buffering of soil
1．Soil inorganic colloid
2．Soil texture
3. Soil organic matters
三、The redox buffering of soil
(Studied by yourself!!)
Section 5 Soil acid-alkali and
redox with bioenvironment
一、The adaptability of organisms to
soil acid-alkali and redox
（一）The plants are suited to acid-alkali
Soil Acidity
in Australia
（二）Soil Eh Scope and plants growth
（三）Soil pH and Eh with soil microbial activity
二、 Soil acid-alkali and redox with nutrient
organisms effectively
（一) Influence of soil acid-alkali to nutrient effectively
（二） Influence of soil redox to nutrient effectively
三、 Soil acid-alkali and redox with
accumulation of toxicity matters
（一）Aluminum and manganese force and poison in
strong acid soil
pH6.0;水稻土排水解除锰的毒害。
（二） Soil redox with accumulation of
toxicity matters
EhAeration———>Oxidation
Irrigation———>Anaeration———>Reduction