Soil Chemical properties

Soil Chemistry Topics

  • Key Areas to Study:

    • Soil pH

    • Charge Properties of Soils

    • Speciation of Nutrients and Availability to Plants

    • Fate of Pollutants in Soil

    • Activity and Survival of Soil Microorganisms

    • Choice of Plants to Grow

    • Soil Physical Properties

    • Influence of Chemical Properties on Environmental Factors

Soil pH

  • Definition:

    • Represents the concentration of hydrogen ions (H+) in soil solution (water in soil pores).

    • Formula:

      • pH = – log [H+]

  • Neutrality of pH:

    • Pure Water: H+ concentration = 10^-7 M; therefore, pH = 7 (neutral).

    • Circumneutral pH: Range between 6.5 to 7.5, recognized as healthy for soil.

Importance of Soil pH

  • Influences:

    • Nutrient Availability:

      • Nutrient availability is optimal around neutral pH (6.5 to 7.5).

    • Microbial Activity:

      • Bacterial activity affected when pH drops below 5.

      • Aluminum toxicity arises when pH is below 5, harming vegetation.

    • Plant Growth:

      • Soil acidity directly impacts plant health and growth rates.

  • Crop Selection Guide:

    • Most edible crops thrive in slightly acidic soils (pH 6 - 6.8).

    • Specific crops benefiting from acidic soils: Huckleberry, Potatoes, Crabapples (pH 5 - 6).

    • Nearly all evergreens (Pines, Firs, Spruces, Yews) flourish in acidic conditions.

Management of Soil pH

Increasing Soil pH (Reducing Acidity)

  • Lime Application:

    • Use of Calcium and Magnesium compounds such as

      • Calcium Carbonate (CaCO3)

      • Calcium Hydroxide (Ca(OH)2)

      • Calcium Oxides (CaO)

      • Magnesium Oxides (MgO)

  • Choosing a Liming Agent:

    • Chemical Composition:

      • More reactive lime (CaO, Ca(OH)2) might require careful management.

      • CaCO3 is less reactive and widely used due to cost-effectiveness and manageability.

    • Purity:

      • High purity enhances lime's ability to neutralize acidity.

    • Fineness:

      • Smaller particle sizes react quicker and more completely than larger particles.

Decreasing Soil pH (Increasing Acidity)

  • Acidifying Amendments:

    • Iron Sulfide (FeS):

      • Reacts to produce acidifying conditions.

    • Organic Matter:

      • Adds acidity naturally and improves soil structure.

    • Ammonium Sulfate:

      • Fertilizer that contributes to lowering soil pH.

Electrochemical Properties of Soil

  • Soil Composition:

    • Sand and Silt: Chemically inert, coarse size, low surface area.

    • Clays and Humus: Chemically active, large specific surface area, charged.

      • Focus on clay and humus to study soil chemical properties effectively.

    • Surface Charges of Clay Minerals:

      • Montmorillonite: -80 to -150 cmolc/kg

      • Vermiculite: -100 to -200 cmolc/kg

      • Illite and Chlorite: -10 to -40 cmolc/kg

      • Kaolinite: -1 to -15 cmolc/kg

      • Gibbsite: +10 to -5 cmolc/kg

      • Goethite: +20 to -5 cmolc/kg

Ion Storage in Soil

  • Adsorption Properties:

    • Clays can adsorb cations (positively charged) and anions (negatively charged).

      • Cations: Cadmium (Cd2+), Lead (Pb2+), Calcium (Ca2+), Magnesium (Mg2+)

      • Anions: Arsenate (AsO4^3-), Selenate (SeO4^2-), Phosphate (HPO4^2-), Chromate (CrO4^2-)

  • Cation Exchange Capacity (CEC):

    • Influences soil's ability to store and supply nutrients.

    • Factors Affecting CEC:

      • Amount and type of clay, organic matter content, and soil pH.

Summary Points

  • Soil and Nutrient Efficiency:

    • Fine clays and organic matter are effective in nutrient retention and contaminant remediation.

  • Sand and Silt Limitations:

    • Coarser particles are not effective in treating pollutants or storing nutrients.