Soil pH Properties and Effects

Soil pH

• Soil pH is the most important chemical property of soil.
• It affects numerous chemical processes, influencing nutrient availability and reactions.
• Soil pH determines plant growth suitability.
• pH measures the concentration of hydrogen ions ([H+] ).
• The maximum pH value is 14.
• At a neutral pH (7), the [H+] concentration equals the [OH-] concentration.
• pH = -log[H+]
• pH = log 1/[H+]

Chemical Nature of Soil Constituents

• If the [H+] concentration is 1.0 x 10⁻⁶ M, the pH is 6.0.
• At pH 6.0, there are 10 times more [H⁺] ions than at pH 7.0.
• At pH 5.0, there are 100 times more [H⁺] ions than at pH 7.0.
• If [H+] > [OH-], the soil is acidic (pH < 7.0).
• If [H+] < [OH-], the soil is basic or alkaline (pH > 7.0).
• If [H+] = [OH-], the soil is neutral (pH = 7.0).

Effects of Soil pH

• Microbial Activity: Fungi are unaffected, but bacteria and actinomycetes are inhibited at pH 5.
• Nutrient Availability:
  • Nitrogen (N) availability decreases at pH < 5.5.
  • Calcium (Ca) and Magnesium (Mg) availability decrease at pH < 6.0.
  • Phosphorus (P) availability decreases at pH < 6.0 and >7.0
  • Potassium (K), Sulfur (S), Molybdenum (Mo), Boron (B), Iron (Fe), Manganese (Mn), Zinc (Zn), Copper (Cu), and Cobalt (Co) availability are influenced by soil pH.

Problem Soils: Acid Soils

• Sources of Soil Acidity:
  • Hydrolysis of Aluminum Ions (Al³⁺): Al³⁺ indirectly contributes to acidity through hydrolysis
  • Carbonic Acid Dissociation (H₂CO₃): CO₂ dissolving in water forms carbonic acid then dissociates releasing H⁺ ions.
  • Organic Matter Decomposition: Organic matter decomposition releases organic acids.
  • Mineral Weathering: Some minerals also contribute H⁺ ions.
  • Heavy Cropping: Excessive cropping removes basic cations and replaces them with acidic ones.
  • Acidifying Fertilizers: Fertilizers like ammonium nitrate, ammonium sulfate, urea, elemental sulfur, iron sulfate and aluminum sulfate can acidify the soil.
  • Natural Acidic Fertilizers: Peat moss, compost, and manure release acids when decomposing.
  • Nitrification: The conversion of ammonium to nitrate releases hydrogen ions into the soil. This is the most common in acid soils
  • Acid Rain: Atmospheric pollutants forming acids during rainfall.

Problem Soils: Acid Soils (Management)

• Management Strategies:

  • Use acid-tolerant crops.
  • Apply agricultural lime.
  • Importance of considering the economics of using lime.

• Calcium Carbonate Equivalent (CCE) or Relative Neutralizing Value (RNV): Measures a liming material's capacity to neutralize acidity relative to pure calcium carbonate (CaCO₃).

• Factors Affecting Lime Effectiveness:

  • Type of lime (order of reactivity: CaO > Ca(OH)₂ > CaMg(CO₃)₂ > CaCO₃)
  • Particle size (smaller particles are more reactive).
  • Lime requirement (the amount needed to reach desired pH)

• Effects of Lime on the Soil:

  • Physical: Increased soil granulation due to flocculation by calcium (Ca).
  • Chemical: Neutralization of H⁺ ions, increased availability of phosphorus (P), molybdenum (Mo), calcium (Ca), and potassium (K), reduced solubility/toxicity of iron (Fe), aluminum (Al), and manganese (Mn).
  • Biological: Promotes microbial activity (decomposers, etc.), enhances mineralization of nutrients, and nitrogen fixation.

Problem Soils: Alkaline and Salt-Affected Soils

• These soils are prevalent in arid and semi-arid regions where evapotranspiration exceeds precipitation.
• Insufficient water prevents leaching of basic cations from minerals.
• Saline Soils: High concentration of soluble salts like chlorides and sulfates of sodium (Na⁺), Calcium (Ca²⁺) and Magnesium (Mg²⁺), which impacts plant growth. (pH < 8.5, white alkali)
• Sodic Soils: High sodium (Na⁺) concentration on the soil's exchange sites (pH > 8.5, black alkali).

Reclamation of Salt-Affected Soils

• Saline Soils:
  • Leaching excess salts out of the root zone.
  • Retardation of evaporation (e.g., using sulfate mulch).
  • Use of salt-tolerant crops.
• Sodic Soils:
  • Replace Na⁺ with Ca²⁺ on the exchange sites.
  • Leaching of exchanged sodium (Na).
  • Gypsum (CaSO₄·2H₂O) is the most common liming material used.

This quiz focuses on the vital role of soil pH in determining soil health and plant growth. It examines how pH influences chemical processes, nutrient availability, and microbial activity in the soil. Understanding soil pH is essential for effective soil management and agriculture.