NRES 201 Lecture 27 Outline of Last Lecture I Cation exchange capacities II Cation exchange and plant nutrient uptake III Anion exchange IV Anion exchange capacities V Soil weathering and ion exchange VI Sorption of organic compounds VII Clays and soil physical properties Outline of Current Lecture VIII General Concepts IX Significance of soil acidity alkalinity X Active acidity XI Potential acidity XII Aluminum as a source of soil acidity Current Lecture Acid base behavior Acid H donor Base H acceptor Dissociation of acids Weak acids incomplete dissociation Strong acids complete dissociation 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 10 7 pH log 10 7 7 Scale 1 14 Soil pH range 4 8 Buffering These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Fundamental aspects Defined as resistance to a change in pH Buffer system weak acid or base its salt Soil pH is strongly buffered Equivalent weight The weight of a substance that will contribute or react with 1 gram of H Nutrient Availability Phosphorus o Highest availability at pH 6 7 Exchangeable bases o Ca Mg or K deficiency in acid soils Macronutrients o Acid soils better for Fe Mn B Cu and Zn o Neutral soils better for Mo Microbial Availability Availability of N P and S o Mineralization of organic forms o Transformation of inorganic forms Symbiotic N2 fixation o Reduced in acid soils Alkalinity increases CEC Alkalinity dissolves organic matter Acidity dissolves soil minerals Potential acidity Soil constituents capable of contributing H ions to the soil solution through Ionization Dissociation and Hydrolysis The major form of soil acidity and the reason why lime is applied with trucks instead of teaspoons Sources of potential acidity o Ionizable H Clay minerals Hydrous oxides CO2 Organic acids from soil organic matter and from residues o Exchangeable H H held on exchange sites held more tightly than basic cations o Soil aluminum The key source of soil acidity at pH 4 8 Properties of aluminum Hydrated as a soluble ion Undergoes hydrolysis Amphoteric acid and base Insoluble at pH 4 8 8 Forms insoluble compounds Toxic to plants
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