Chapter 5 Soil Acidity and Alkalinity第五章 土壤酸碱度The degree of acidity or alkalinity is a master（显 著的，主要的） variable （易变的） that affects nearly all soil properties—chemical,biological, and physical. Commonly expressed as soil pH, this variable largely controls plant nutrient availability and microbial reaction in soils.有1000克土样，分别含H+， Mg2+，Ca2+， Na+，K+和 Al3+离子8，5，16，1，1和4 cmol，土样中有机质含量 为2%（有机质平均CEC约为350 cmol/kg），粘粒含量 是25%，求:(1) 土壤的CEC(2) 土壤盐基饱和度(3) 若土样仅为一种粘粒矿物组成,试推断是何种矿物?•C.E.C.=8+5+16+1+1+4=35 cmol(+)/kg•BSP=(5+16+1+1) ×100/35=65.71%•Hu=2%, CEC=2×350/100=7 cmol(+)/kg——有机质•粘粒矿物CEC=35-7=28 cmol(+)/kg•若为单一矿物，则：CEC=28·100/25=112 cmol(+)/kg——蒙脱石一 土壤酸度的存在形式和相互关系(Forms and their relationship of soil acidities) 1 活性酸度：土壤溶液中游离的H+活度 Active acidity—the activity of hydrogen ion in the aqueous phase of a soil. 2 潜性酸度：土壤胶体上吸附的交换性H+，Al3+所显示的酸度 Potential acidity—the activity displayed by exchangeable H+ and Al3+ that were absorbed on soil colloid.第一节 土壤酸度 Section 1 Soil acidity3 总酸度(Total acidity)：活性酸与潜性酸的总和 活性酸与潜性酸的关系： (1) 处于相互转化的动态平衡 (2) 潜性酸远大于活性酸 (3) 土壤酸度的根本来源(起源)—活性酸H+ Al3+≒ H+,Al3+二 土壤酸度产生的原因(一)活性酸 1 土壤生物活动产生H+ (1) 生物呼吸放出CO2 + H2O — H2CO3 — H+ + HCO3-(2) 有机质分解产生有机酸 (3) 土壤中的的铵盐和硫化物在特殊微生物(如:硝化细菌,硫化细菌) 的作用下转化成酸(HNO3, H2SO4) 2 大气，土壤空气中的CO2 3 吸附态H+ ， Al3+通过解离和水解变成活性酸 4 土壤中H2O的解离(二)潜性酸 1 吸附态，结合态以及官能团上H+的解离和交换 2 吸附态Al3+(1) 吸附态Al3+的产生—氢铝转化过程(土壤胶体上吸附态H+ 减少而Al3+增加的现象)只有Al3+粘土，没有H+粘土Al + Ca2+Ca + Al3+ Once the aluminum enter the soil solution it reacts with water to form hydroxy aluminum compounds and free hydrogen ions. Al3+ + H2O ≒ Al(OH)2+ + H+ Al(OH)2+ + H2O ≒ Al(OH)2+ + H+ Al(OH)2+ + H2O ≒ Al(OH)3 + H+(2) 吸附态Al3+产生酸的机制思考:1 红壤用酸不断淋洗,最后得到什么胶体?2 南方强酸性土与弱酸性土通过Al3+产生酸度的机理是否相同?为什么?The main sources of net acid inputs are as follows:•The dissolution of CO2 in the soil water to form carbonic acid which dissociates according to CO2 + H2O H2CO3 H+ + HCO3- H+ + CO32-•The accumulation and humification of soil organic matter, producing humic residues with a high density of carboxyl and phenolic groups (羧基和酚基) that dissociate H+ ions.•Inputs of H2SO4, HNO3 and (NH4)2SO4 from the atmosphere or ‘acid rain’.•In soils formed on marine muds (海相沉积), or coal- bearing sedimentary rocks （碳基沉积岩）, the oxidation of iron pyrites （黄铁矿）FeS2 gives rise to acid sulphate （硫酸盐）soils.•Nitrification of NH4+ions, producing H+ ions, and NO3- which is susceptible (易受影响的) to leaching.Removal of Ca2+ and Mg2+Plant and animal residuesATMOSPHERESOILAtmospheric inputs Of acidityWet and dry depositionH+ in solutionBiomass activity-acid production Mineralization and nitrificationH+NO3 Respiration H2CO3 Production of organic acidsAcid attack of soil minerals-acid consumptionH+H+Solution Al3+,Ca2+,Mg2+,K+,H+e.g. Al(OH)3 + 3H+ Al3+ + 3H2O CaCO3 + 2H+ Ca2+ + H2O + CO2Root activity-acid productionCation and anion uptakeH+ or OH- releaseRespiration H2CO3Exudation e.g. Citric acidLeachingCation exchange- acid storageExchangeable Ca2+ Mg2+ K+ Al3+ H+H+,Al3+Ca2+,Mg2+ K+Solution+-+-ClayHumus-----Leaching Into groundwaterAl3+ Ca2+ Mg2+ K+ H+The process of soil acidification三土壤酸度的表示方法 (Indicative methods of soil acidity)(一)活性酸度 — pH值： 是土壤酸度的强度指标(Intensity index)Active acidity — due to the H+ and Al3+ ions in the soil solution.一般地 pHH2O > pHKCl在pH分级方面，各国的均不一致；我国土壤酸碱反应大 致呈“南酸北碱”，确切地说是“东南酸而西北碱”的分布趋 势；大体分五级：pH酸碱度级别 Grade8.5强碱性 Strongly alkaline soil(二)交换性酸度 Exchange acidity—the titratable (可滴定) hydrogen and aluminum that can replaced from the adsorption complex by a neutral salt solution.+ 4KCl ≒H Al4K+ Al3+ + H+ + 4Cl-Al3+ + H2O ≒ Al(OH)3 + 3H+性质：容量指标(Capacity index)；单位：cmol/kg特点:(1) 包括活性酸；(2) 不能将胶体上全部的H+ 、Al3+代换下来，因此，它只是 潜性酸的大部分，而非全部。
Salt-replaceable acidity, involving the aluminum and hydrogen that are easily exchangeable by other cations in a simple unbuffered salt solution such as KCl.Salt-replaceable acidityThe quantity of salt-replaceable acidity (exchangeable acidity) is much higher, commonly more than 100 times that needed to neutralize the soil solution (active) acidity.(三)水解性酸度(Hydrolysis acidity) 用弱酸强碱盐类(如NaAc)浸提土壤，将交换性H+ , Al3+置 换到土壤溶液中所显示的酸度性质：容量指标(Capacity index)；单位：cmol/kg特点：(1)包括活性酸2) 它可以代表土壤总酸度(改良酸性土壤计算石灰施用量的 依据)。
4NaAc + 4H2O ≒ 4NaOH + 4HAcH Al+4NaOH +4HAc ≒4Na + Al(OH)3 + H2O +4HAcResidual acidity (Hydrolysis acidity)Residual acidity, which is associated with the large quantity of Al3+,H+ and Al(OH)x ions that are bound in nonexchangeable forms by organic matter and silicate (硅酸盐) clays.The residual acidity is far greater than either the active or salt-replaceable acidity. It may be 1000 times greater than the soil solution or active acidity in sandy soil and 50,000 or even 100,000 times greater in a clayey soil high in organic matter.第二节 土壤碱度 Section 2 soil alkalinity一土壤碱度产生的原因 1 土壤中的碱金属和碱土金属盐类的水解(主要是碳酸盐和重 碳酸盐) 2 有些土壤中的中性盐(Na2SO4)的水解 一3 胶体上吸附的 Na, Ca, Mg 离子的水解MicelleCa2++ 2H2OMicelleH+ H++ Ca2++ 2OH-(soil solid)(soil solution)(soil 。