NRES 201 Lectures 29 31 Fall 2014 Soil Acidity Alkalinity Salinity NRES 201 Soil Acidity Alkalinity Salinity 1 Soil Acidity Alkalinity Salinity Today s lecture topics General concepts Significance of soil acidity alkalinity Active acidity Potential acidity Aluminum as a source of soil acidity 2 1 NRES 201 Lectures 29 31 Fall 2014 Soil Acidity Alkalinity Salinity Acid base behavior Definitions Acid H donor Base H acceptor Dissociation of acids Weak acid incomplete dissociation CH3COOH H2O CH3COO H3O Strong acid complete dissociation HCl H2O Cl H3O Ionization of water 2H2O H3O OH H3O OH 0 00000000000001 10 14 Kw 3 pH Fundamental aspects Defined as the negative logarithm of H activity Expressed as log H or log H3O For a dilute solution concentration activity For pure water H3O OH 10 14 10 7 pH log 10 7 7 0 4 2 NRES 201 Lectures 29 31 Fall 2014 Soil Acidity Alkalinity Salinity Soil pH Typical range is from 4 to 8 Alkaline 0 1 10 1 0 01 10 2 0 0000001 10 7 10 14 pH 1 2 7 14 Acid The pH scale H3O concentration M Source Brady and Weil 1996 5 Buffering Fundamental aspects Defined as resistance to a change in pH Buffer system weak acid or base its salt Simple illustration Titration of CH3COOH with NaOH Buffering is due to CH3COOH H CH3COOionizes to produce more H reacts with OH to make H2O Application to soil pH Soil pH is strongly buffered Source Tisdale et al 1993 6 3 NRES 201 Lectures 29 31 Fall 2014 Soil Acidity Alkalinity Salinity Equivalent weight Defined as The weight of a substance that will contribute or react with 1 gram of H Application to liming materials CaCO3 2 H H2CO3 Ca2 7 Equivalent weight Defined as The weight of a substance that will contribute or react with 1 gram of H Application to liming materials 8 4 NRES 201 Lectures 29 31 Fall 2014 Soil Acidity Alkalinity Salinity Significance of soil acidity alkalinity Range of crop requirements Source Brady and Weil 2002 9 Nutrient 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 Mo Microbial activity Availability of N P and S Mineralization of organic forms Transformations of inorganic forms Symbiotic N2 fixation Reduced in acid soils 10 5 NRES 201 Lectures 29 31 Fall 2014 Soil Acidity Alkalinity Salinity Cation exchange Alkalinity increases CEC Source SSSA Slide Set 1974 11 Solubilities Alkalinity dissolves organic matter Acidity dissolves soil minerals Al and Mn toxicities in acid soils Root distribution Acidity often inhibits root growth Ca is essential in cell walls and membranes Soil structure and tilth Source http www rib okayama u ac jp Improved by liming Ca increases aggregate stability 12 6 NRES 201 Lectures 29 31 Fall 2014 Soil Acidity Alkalinity Salinity Disease incidence Fungal diseases favored by acidic conditions Control by liming Bacterial and actinomycetous diseases favored by neutral or alkaline conditions Control by acidification Mobility of pesticides and pollutants Some are cations under acidic conditions Others are anions under alkaline conditions Clubroot on cabbage fungal disease Source http www gov mb ca Potato scab actinomycetous disease Source http www unece org 13 Active acidity H ions in the soil solution Measured as soil pH Present in minute quantities pH 5 10 5 M H Assuming 20 soil moisture 0 20 2 106 lb acre 6 400 000 lb H2O 181 440 L H2O 5 10 mole 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 14 7 NRES 201 Lectures 29 31 Fall 2014 Soil Acidity Alkalinity Salinity Potential acidity Defined as Soil constituents capable of contributing H ions to the soil solution through Ionization Dissociation Hydrolysis The major form of soil acidity And the reason why lime is applied with trucks instead of teaspoons Source http wps prenhall com 15 Sources of potential acidity Ionizable H Clay minerals Si Si O H OH Al Al Hydrous oxides Al OH Fe Al Fe O H CO2 CO2 H2O H2CO3 16 8 NRES 201 Lectures 29 31 Fall 2014 Soil Acidity Alkalinity Salinity Organic acids From soil organic matter From residues Exchangeable H H or H3O held on exchange sites Held more tightly than basic cations Soil aluminum The key source of soil acidity at pH 4 8 17 Properties of aluminum Hydrated as a soluble ion Source Black 1968 18 9 NRES 201 Lectures 29 31 Fall 2014 Soil Acidity Alkalinity Salinity Undergoes hydrolysis Dominant species pH Al H2O 63 4 0 Al OH 2 H2O 4 4 0 4 8 Al OH 3 H2O 3 4 8 8 0 Al OH 4 H2O 2 8 0 19 Amphoteric Al as an acid H 20 10 NRES 201 Lectures 29 31 Fall 2014 Soil Acidity Alkalinity Salinity Amphoteric Al as a base H 21 Solubility Insoluble at pH 4 8 8 0 Forms insoluble compounds Such as AlPO4 2H2O Toxic to plants Toxicity is due to soluble Al Source McLean 1976 22 11 NRES 201 Lectures 29 31 Fall 2014 Soil Acidity Alkalinity Salinity Sensitivity of various crops to aluminum Very low pineapple coffee tea rubber cassava tropical grasses rhododendron azaleas blueberries Low corn turnips redtop swede cauliflower kale potato Moderate radishes sorghum cabbage oats rye High Very high lettuce celery beets carrots timothy barley legumes tomato 23 Forms 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 bearing minerals A major form of Al in tropical soils Al phosphates From weathering or fertilizer reactions Important in tropical soils 24 12 NRES 201 Lectures 29 31 Fall 2014 Soil Acidity Alkalinity Salinity Exchangeable Al Forms Al H2O 63 and Al OH 2 H2O 4 Held more tightly than exchangeable bases Soluble Al Toxic to most plants The wheat variety on the right was sensitive to Al toxicity Source http www flickr com 25 Soil Acidity Alkalinity Salinity Today s lecture topics General concepts Significance of soil acidity alkalinity Active acidity Potential acidity Aluminum as a source of soil acidity 26 13 NRES 201 Lectures 29 31 Fall 2014 Soil Acidity Alkalinity Salinity Today s key concepts Soil acidity has important effects on plant growth microbial activity and soil properties Soils have very little active acidity but lots of potential acidity At pH 5 soil acidity is mainly due to soluble Al 27 Soil Acidity Alkalinity Salinity Today s lecture topics Factors contributing to soil acidity Soil as a buffered system Determination of soil acidity Estimating lime requirement 28 14 NRES 201 Lectures 29 31 Fall 2014 Soil Acidity Alkalinity