NRES 201 Lectures 29-31 (Fall 2014):Soil Acidity, Alkalinity, & Salinity1NRES 201Soil Acidity, Alkalinity, & Salinity12Today’s lecture topics- General concepts- Significance of soil acidity/alkalinity- Active acidity- Potential acidity- Aluminum as a source of soil aciditySoil Acidity, Alkalinity, & SalinityNRES 201 Lectures 29-31 (Fall 2014):Soil Acidity, Alkalinity, & Salinity2Acid-base behaviorDefinitions- Acid = H+donor- Base = H+acceptorDissociation of acids- Weak acid: incomplete dissociationCH3COOH + H2O CH3COO-+ H3O+- Strong acid: complete dissociationHCl + H2O Cl-+ H3O+Ionization of water2H2O H3O++ OH-[H3O+][OH-] = 0.00000000000001 = 10-14= Kw3pHFundamental aspects- Defined as the negative logarithm of H+activity- Expressed as -log [H+] or -log [H3O+]- For a dilute solution, concentration  activityFor pure water- [H3O+] = [OH-] = 10-14= 10-7- pH = -log(10-7) = 7.04NRES 201 Lectures 29-31 (Fall 2014):Soil Acidity, Alkalinity, & Salinity3The pH scaleH3O+concentration(M) pH_________________ __0.1 = 10-110.01 = 10-22••••••0.0000001 = 10-77••••••10-1414Soil pH- Typical range is from 4 to 85Source: Brady and Weil (1996)AcidAlkalineBufferingFundamental aspects- Defined as resistance to a change in pH- Buffer system = weak acid or base + its saltSimple illustration- Titration of CH3COOH with NaOH- Buffering is due to:CH3COOH  H++ CH3COO-Application to soil pH- Soil pH is strongly bufferedSource: Tisdale et al. (1993)6reacts with OH-to make H2Oionizes to produce more H+NRES 201 Lectures 29-31 (Fall 2014):Soil Acidity, Alkalinity, & Salinity4Defined as- The weight of a substance that will contribute orreact with 1 gram of H+Application to liming materialsEquivalent weightCaCO3+ 2H+ H2CO3+ Ca2+7Defined as- The weight of a substance that will contribute orreact with 1 gram of H+Application to liming materialsEquivalent weight8NRES 201 Lectures 29-31 (Fall 2014):Soil Acidity, Alkalinity, & Salinity5Significance of soil acidity/alkalinityRange of crop requirementsSource: Brady and Weil (2002)9Nutrient availability- Phosphorus- Highest availability at pH 6.0-7.0- Exchangeable bases- Ca, Mg, or K deficiency in acid soils- Micronutrients- Acid soils better for Fe, Mn, B, Cu, and Zn- Neutral soils better for MoMicrobial activity- Availability of N, P, and S- Mineralization of organic forms- Transformations of inorganic forms- Symbiotic N2fixation- Reduced in acid soils10NRES 201 Lectures 29-31 (Fall 2014):Soil Acidity, Alkalinity, & Salinity6Cation exchange- Alkalinity increases CECSource: SSSA Slide Set (1974)11Solubilities- Alkalinity dissolves organicmatter- Acidity dissolves soil minerals- Al and Mn toxicities in acidsoilsRoot distribution- Acidity often inhibits root growth- Ca is essential in cell walls andmembranesSoil structure and tilth- Improved by liming- Ca increases aggregate stability12Source: http://www.rib.okayama-u.ac.jpNRES 201 Lectures 29-31 (Fall 2014):Soil Acidity, Alkalinity, & Salinity7Disease incidence- Fungal diseases favoredby acidic conditions- Control by liming- Bacterial and actinomycetousdiseases favored by neutralor alkaline conditions- Control by acidificationMobility of pesticides and pollutants- Some are cations under acidicconditions- Others are anions under alkalineconditions13Source: http://www.gov.mb.caPotato scab (actino-mycetous disease)Source: http://www.unece.orgClubroot on cabbage(fungal disease)Active acidity- H+ions in the soil solution- Measured as soil pH- Present in minute quantitiespH 5 = 10-5M H+Assuming 20% soil moisture:(0.20)(2  106lb/acre-6”) = 400,000 lb H2O= 181,440 L H2O(10-5mole H+/L)(1 g H+/mole)(181,440 L) (1 lb/453.6 g)(16 oz/lb) =0.064 oz H+per acre-6”- Can be neutralized by “liming with a teaspoon”14NRES 201 Lectures 29-31 (Fall 2014):Soil Acidity, Alkalinity, & Salinity8Potential acidityDefined as- Soil constituents capable of contributing H+ionsto the soil solution through- Ionization- Dissociation-HydrolysisThe major form of soil acidity- And the reason why limeis applied with trucksinstead of teaspoons15Source: http://wps.prenhall.comSources of potential acidityIonizable H- Clay minerals- Hydrous oxides- CO2-CO2+ H2O  H2CO3SiAlOHSiAlO-+ H+AlFeOHAlFeO-+ H+16NRES 201 Lectures 29-31 (Fall 2014):Soil Acidity, Alkalinity, & Salinity9- Organic acids- From soil organic matter- From residuesExchangeable H- H+(or H3O+) held on exchange sites- Held more tightly than basic cationsSoil aluminum- The key source of soil acidity at pH  4.817Properties of aluminum- Hydrated as a soluble ionSource: Black (1968)18NRES 201 Lectures 29-31 (Fall 2014):Soil Acidity, Alkalinity, & Salinity10- Undergoes hydrolysisDominantspecies pHAl(H2O)63+< 4.0Al(OH)2(H2O)4+ 4.0-4.8Al(OH)3(H2O)3 4.8-8.0Al(OH)4(H2O)2-> 8.019- Amphoteric- Al as an acidH+20NRES 201 Lectures 29-31 (Fall 2014):Soil Acidity, Alkalinity, & Salinity11- Amphoteric- Al as a baseH+21- Solubility- Insoluble at pH 4.8-8.0- Forms insolublecompounds- Such as AlPO4·2H2O- Toxic to plants- Toxicity is due tosoluble AlSource: McLean (1976)22NRES 201 Lectures 29-31 (Fall 2014):Soil Acidity, Alkalinity, & Salinity12Sensitivity of various crops to aluminumVery lowpineapplecoffeetearubbercassavatropicalgrassesrhododendronazaleasblueberriesLowcornturnipsredtopswedecauliflowerkalepotatoModerateradishessorghumcabbageoatsryeHighlettucebeetstimothybarleylegumestomatoVery highcelerycarrots23Forms of soil aluminum- Al in minerals- The major form of soil Al- Occurs mainly in the octahedral sheet- Al hydroxides- Produced by weathering of Al-bearingminerals- A major form of Al in tropical soils- Al phosphates- From weathering or fertilizer reactions- Important in tropical soils24NRES 201 Lectures 29-31 (Fall 2014):Soil Acidity, Alkalinity, & Salinity13- Exchangeable Al- Forms: Al(H2O)63+and Al(OH)2(H2O)4+- Held more tightly thanexchangeable bases- Soluble Al- Toxic to most plants25Source: http://www.flickr.comThe wheat variety on the right was sensitive to Al toxicity.26Today’s lecture topics- General concepts- Significance of soil acidity/alkalinity- Active acidity- Potential acidity- Aluminum as a source of soil aciditySoil Acidity, Alkalinity, & SalinityNRES 201 Lectures 29-31 (Fall 2014):Soil Acidity, Alkalinity, & Salinity1427Today’s key concepts- Soil acidity has important effects on plant growth,microbial activity, and soil properties.- Soils have very little active acidity, but