NRES 201 Lecture 27 Outline of Last Lecture I. Cation exchange capacitiesII. Cation exchange and plant nutrient uptakeIII. Anion exchangeIV. Anion - exchange capacitiesV. Soil weathering and ion exchangeVI. Sorption of organic compounds VII. Clays and soil physical propertiesOutline of Current Lecture VIII. General ConceptsIX. Significance of soil acidity/alkalinityX. Active acidityXI. Potential acidityXII. Aluminum as a source of soil acidity Current LectureAcid - base behavior- Acid = H+ donor- Base = H+ acceptor Dissociation of acids- Weak acids: incomplete dissociation - Strong acids: complete dissociationpH - Fundamental aspects - - Defined as the negative logarithm of H+ activity - Expressed as -log[H+] or -log[H3O+]- For a dilute solution, concentration = activityFor pure water - - 10^-7- pH = -log(10^-7) = 7Scale = 1 - 14Soil pH range = 4 - 8Buffering 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 bufferedEquivalent weight = The weight of a substance that will contribute or react with 1 gram of H+Nutrient Availability - - Phosphoruso Highest availability at pH 6- 7- Exchangeable baseso Ca, Mg, or K deficiency in acid soils- Macronutrientso Acid soils better for Fe, Mn, B, Cu, and Zno Neutral soils better for MoMicrobial Availability - - Availability of N, P, and So Mineralization of organic formso Transformation of inorganic forms- Symbiotic N2 fixationo Reduced in acid soilsAlkalinity increases CECAlkalinity dissolves organic matterAcidity dissolves soil mineralsPotential 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 acidityo 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 cationso Soil aluminum - The key source of soil acidity at pH < 4.8Properties of aluminum- Hydrated as a soluble ion- Undergoes hydrolysis- Amphoteric (acid and base)- Insoluble at pH 4.8-8- Forms insoluble compounds- Toxic to
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