Chapter 9

Soil acid-base and redox reaction

Chapter 9 Soil acid-base and redox reaction

 Section 1 The formation of soil acid-baseThe 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 irrigation1.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.

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+

2. Aluminum activation in soil

（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 soilColloid-Al3+ = Colloid + Al3+ Al3+ + 3H2O Al（ OH)3 + 3H+

(2) Acidity and weak acidity in soilAl（ 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 acidityCH3COONa + 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 hydrolysisCaCO3 + 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+

Mg2+ + 4Na+ 2Na+ + Ca2+ + Mg2+Colloid Colloid

xNa + yH2O +CO2 （ x-y） Na+ + yNaOH yH+

ColloidColloid

（二） Affect the factors of soil alklization： 1． Climatic factor 2． Biological factor 3． Parent factor

Section 2 Index of soil acidity

Soil acidity is brief name of soil acid and base!

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)

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.

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%.

ESP = [(exchangeable sodium ions )/(soil cations exchangeable capacity)]×100

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.

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

二、 Index of soil redox

][

][log

059.0

][

][log 00

还原态氧化态

还原态氧化态

nE

nF

RTEEh

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.

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

三、 Affect the factors of redox

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

-M + HCl -H + MCl ColloidColloid

][

][log酸盐

apKpH

（二） Acid or alkali buffering system of soil

1、 Carbonate system

pH PCO 6 032

3 2. log

CaCO3 + H2O +CO2 Ca2+ + 2HCO3-

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 pH<4.0, Al ions exist in Al(H2O)63+

form2 2 42 3

32 2 2 8

42Al OH OH Al OH H O H O( ) ( ) ( ) OHOHOHAlOHOHAl 2

612264

362 12])()([6)(4

When pH>5.0 , 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 colloid2． Soil texture3. 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

pH<5.5，游离的铝离子达 0.2C mol/kg土时 ,就可使农 作物受 害。

幼苗期对铝极为敏感。

铝害表现：根系变粗短，影响养分吸收。

措施：施用石灰。

当交换锰（Mn2+）达到 2-9Cmol/kg土 ,或植株干物质

含锰量超过 1000mg/kg 时产生锰害。

豆类植物易产生锰害，禾本科植物抗性较强

措施：施用石灰， Ph>6.0;水稻土排水解除锰的毒害。

（二） Soil redox with accumulation of toxicity matters

Eh<200mv时 ,土壤中的铁锰化合物就从氧化态转化为还原态 ,

当 Eh<-100mv时 ,则低价铁 (Fe2+)浓度已超过高价铁 (Fe3+),会使植物产生铁的毒害。

Eh<-200mv,就可能产生 H2S和丁酸等的过量积累 ,对水稻的含铁氧化还原酶的活动有抑制作用 ,影响其呼吸、减弱根系吸收养分的能力。在 H2S浓度高时 ,抑制植物根对磷、钾的

吸收，甚至出现磷、钾从根内渗出。

水田土壤大量施用绿肥等有机肥时常常发生 FeS的过量积累 ,使稻根发黑 ,土壤发臭变黑 ,影响其地上

部分的生长发育。

四、 Adjustment of Soil acid-alkali and redox 

（一） Adjustment of soil acidityThe soil acidity can be adjusted by lime. The lime can be divided by three main categories, which are Calcium oxide (CaO), Calcium hydroxide [Ca(OH)2]

and Calcium carbonate(CaCO3).

Lime requirment = Soil volume ×bulk density ×CEC (1- base saturation)

Lime requirment

Affect the factors of lime applied：

（ 1） Soil potential acidity and pH value, content of organic matter, base saturation and soil texture etc.

（ 2 ） Adaptability of the plant to acid-alkali

（ 3） Lime types and applying methods

假设某红壤的 pH为 5.0,耕层土壤为 2250000公斤 /公顷，土壤含水量为20％，阳离子交换量为 10Cmol/kg土 ,盐基饱和度为 60%,试计算达到 pH=7时 ,中和活性酸和潜性酸的石灰需要量 (理论值 )。

中和活性酸 pH=5时 ,土壤溶液中 [H+]=10-5,mol/kg土 ,则每公顷耕层土壤含 H+离子为 :

2250000×20%×10-5=4.5molH+/公顷

同理 :pH=7时 ,每公顷土壤中含 H+离子为

2250000×20%×10-7=0.045mol H+/公顷

所以需要中和活性酸量为 4.5-0.045=4.455mol H+/公顷

若以 CaO中和 : 其需要量 4.455×(56/2) = 124.74 克

中和潜性酸 :

2250000×(10 /100)×(1- 60/100)=90000 mol H+/公顷

90000×56/2=2520000克 =2520公斤 /公顷

案例1

（二） Adjustment of soil redox

Drainage ———>Aeration———>Oxidation

Irrigation———>Anaeration———>Reduction