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Chapter 9

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

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Chapter 9

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  1. Chapter 9 Soil acid-base and redox reaction

  2. 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.

  3. 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

  4. 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.

  5. 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+

  6. （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.

  7. 2． Soil potential acidity • 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+

  8. 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）

  9. 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

  10. 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

  11. 3、 Exchangeable Sodium hydrolysis Ca2+ 2Na+ Mg2+ + 4Na+ 2Na+ + Ca2+ + Mg2+ Colloid Colloid Colloid xNa + yH2O +CO2（x-y）Na+ + yNaOH yH+ Colloid （二） Affect the factors of soil alklization： 1．Climatic factor 2．Biological factor 3．Parent factor

  12. 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!

  13. 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:

  14. （二）Exchangeable sodium percentage (ESP） ESP is the percentage of exchangeable sodium ions tothe 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%.

  15. 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).

  16. Section 3 Soil redox reaction

  17. 一、 Soil redox system

  18. Characteristics of Soil redox system: ①Soil redox system can be divided two system: inorganic and organic systems. ②The 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.

  19. 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

  20. 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.

  21. 3、Relation between Eh and pH Where m is number of protons in reaction.

  22. 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

  23. 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 Colloid Colloid

  24. （二） Acid or alkali buffering system of soil 1、Carbonate system 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”.

  25. 4、Aluminum system When pH<4.0, Al ions exist in Al(H2O)63+ form When pH>5.0 , Al ions become Al(OH)3 precipitation，it will lose it’s buffering capacity. 5、Organic acid system

  26. （三）The acid and alkalibuffer capacity and titration curve of soil (Studied by yourself!!) (四) Affect the factors of acid or alkali bufferingof soil 1．Soil inorganic colloid 2．Soil texture 3. Soil organic matters 三、The redox buffering of soil (Studied by yourself!!)

  27. Section 5 Soilacid-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

  28. （二）Soil Eh Scope and plants growth （三）Soil pH and Eh with soil microbial activity

  29. 二、 Soilacid-alkali and redox with nutrientorganismseffectively （一) Influence of soilacid-alkali to nutrienteffectively  

  30. （二） Influence of soilredoxto nutrienteffectively 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。

  31. （二） Soilredox 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。

  32. 四、Adjustment of Soilacid-alkali and redox （一） Adjustment of soilacidity The 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).

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

  34. 案例1 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

  35. （二） Adjustment of soil redox Drainage ———>Aeration———>Oxidation Irrigation———>Anaeration———>Reduction

  36. The End

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