Lecture 12: Soil Acidity, Alkalinity, Salinity, and Sodicity

Chapter 9: Soil Acidity, Alkalinity, Salinity, and Sodicity

Section 9.1: Introduction to Soil pH

  • Definition of pH: A soil’s degree of acidity or alkalinity.

    • Balance between H+ and OH- ions in water:

    • Reaction: H2O ⇌ H+ + OH-

    • Water dissociation constant: K_W = 10^{-14}, where KW=[H^{+}]\cdot[OH^{-}] .

    • pH is quantified using the scale: \left\lbrack-{log}\left(H^{+}\right)\right\rbrack .

Section 9.2: Importance of Soil pH

  • Soil pH is a master variable controlling a wide range of chemical and biological properties of soils, including:

    • Availability and mobility of plant nutrients, toxic elements, and pollutants.

    • Activity and composition of soil microbial communities.

Section 9.3: Global Soil pH Distribution

  • Global map of soil pH classes:

    • Strongly Acidic

    • Mildly Acidic

    • Neutral

    • Mildly Alkaline

Section 9.4: Effects of Soil pH on Microbes

  • Soil pH critically influences:

    • Diversity of soil bacteria.

    • Relative abundance of key nutrients and their availability at various pH levels, e.g.:

    • Fungi and bacteria diversity plotted against soil pH range (4-9).

      • Specific nutrients affected at different pH levels include:

      • Nitrogen (N), Potassium (K), Sulfur (S), Calcium (Ca), Magnesium (Mg), Phosphorus (P), Iron (Fe), Manganese (Mn), Molybdenum (Mo), Copper (Cu), Boron (B), Zinc (Zn).

Section 9.5: Soil pH Ranges for Plant Growth

  • Appropriate pH ranges for optimal growth of various plant species, highlighting:

    • Different plants thrive in different pH conditions, with ideal ranges specified for mineral soils.

Section 9.6: Effects of Low Soil pH

  • Consequences of low pH:

    • Loss of macronutrients: Ca, Mg, K.

    • Increased Aluminum (Al) toxicity and Phosphorus (P) deficiency.

    • Instability in soil structure and further deficiencies in Fe and P.

Section 9.7: Overview of Acid Soils

  • Classification of acidic soils: Separated into categories based on their acidity, such as:

    • Extreme acidity, Very Strong, Strong, Medium, Slight, Neutral.

Section 9.8: Pools of Soil Acidity

  • Components of soil acidity:

  • Acidic cations: H+ and Al3+ (Aluminum ions).

  • Hydrolysis reaction of Aluminum: 

  • Three pools of acidity characterized by:

  • pH electrode measurements and Cation Exchange Capacity (CEC);

  • Base saturation analysis for acid and nonacid cations.

Section 9.9: Pathways to Acidic Soils

  • Key Acidifying processes include:

    • Formation of carbonic acid from CO2.

    • Acid dissociation reactions.

    • Oxidation processes of nitrogen (N), sulfur (S), and iron (Fe) compounds.

    • Atmospheric deposition of sulfuric acid (H₂SO₄) and nitric acid (HNO₃).

    • Cation uptake by plants leading to soil acidification.

    • Accumulation of acidic organic matter in certain environments.

    • Cation precipitation reactions involving aluminum.

Section 9.10: Acidifying Processes

  • Organic matter accumulation effects:

    • Soil organic matter contains functional groups allowing H+ ions to dissociate, facilitating cation leaching.

    • Effects of applying organic materials:

    • Accumulation of O and A soil horizons.

    • Enhancements via sewage sludge, manure, compost, and mulch.

  • Oxidation of nitrogen (N) and sulfur (S) fertilizers leads to pH decrease via ammonium dissociation:

  • Reaction: NH4^+ + 2O2 → H2O + 2H^+ + NO3^-.

  • Allows for understanding the dissociation mechanisms of ammonium sulfate (e.g., ((NH4)2SO4 + 4O2 → 2HNO3 + 2H2SO4 + 2H2O).

  • Aluminum contribution to soil acidity:

    • Weathering and hydrolysis release Al3+ into the soil.

    • Al3+ is derived from aluminosilicates and silicate clays, showing high toxicity and negatively impacting plants and aquatic organisms.

  • Acidification via cation loss: Cation uptake by roots and leaching result in:

    • The exchange of H+ ions, with a net acidifying effect. ex: Ca^{2+} + 2H^+ → Ca^{2+}(ratio) .

Section 9.11: Ameliorating Acid Soils

  • Raising soil pH:

    • Use of alkaline materials to amend acid soils includes:

    • Conjugate bases such as carbonate (CO₃²⁻), hydroxide (OH⁻), and sulfite (SO₃²⁻).

    • Material examples:

      • Burnt lime: Ca(OH)_2.

      • Carbonates: CaMg(CO3)2

  • Common liming materials and their effectiveness:

    • Calcitic limestone
      ightarrow CaCO_3: Natural rock, low solubility.

    • Dolomitic limestone
      ightarrow CaMg(CO3)2: Supplies magnesium to plants.

    • Burned lime
      ightarrow CaO: Fast-acting but caustic.

    • Hydrated lime
      ightarrow Ca(OH)_2: Highly reactive, must be stored properly.

    • Basic slag: A by-product of the pig-iron industry, contains phosphorus and should be finely ground.

  • Improvement of aluminum toxicity: Organics can bind Al3+, thereby preventing toxicity without necessarily raising pH.

Section 9.12: Key Learning Objectives

  • Key points to be understood/learned:

    • Analyze how soil pH affects microbial activity and nutrient availability.

    • Define base saturation and the implications of Al3+ as an acidic cation.

    • Differentiate between active, salt-replaceable/exchangeable, residual, and total acidity.

    • Understand spatial distribution of acid soils relative to climate, and the pH ranges relevant to specific vegetation types.

    • Identify acidification processes accelerated by human activity and their effect on nutrient availability.

    • Learn management strategies for acid soils to improve agricultural uses and protect water resources.