SOIL222- Lecture 2: Soil Acidity

Soil Acidity and Acidification

  • Definition of Acidity:

    • Refers to the concentration of hydrogen ions (H+) in soil solution.

    • pH calculation: pH = - \log_{10}[H^+].

    • More H+ results in lower pH and higher acidity.

Sources of H+ Ions in Soil

  • Natural Sources:

    • Dissolved CO2 in water forms carbonic acid (H2CO3), leading to increased H+.

    • Biological activities can increase soil CO2 levels significantly.

    • Rain contributes carbonic acid, while industrial activities cause acid rain (HNO3, H2SO4).

    • Decomposition of organic matter produces organic acids.

  • Human Activity Sources:

    • Agricultural practices lead to soil acidification through the addition of nitrogen fertilizers (e.g., ammonium).

    • Leaching and product removal during harvest can lead to localized acidification.

    • Recent data indicate varying rates of acidification based on fertilization practices.

Characteristics of Acid Soils

  • Definition: Acid soils lead to dissolution of Al-containing minerals, increasing Al3+ concentrations as pH decreases.

  • Susceptibility Factors:

    • Initial pH, buffering capacity, and amount of H+ input influence acidity.

    • Buffers resist pH changes; dissolution of minerals occurs at extreme pH levels.

Effects and Toxicity from Acidity

  • Challenges:

    • Acid soils cause toxicity from Al and Mn, and deficiencies in Ca, Mg, Mo, and P, leading to reduced microbial activity.

    • Al toxicity affects root growth negatively, reducing plant health.

Reclamation Strategies

  • Remediation Options:

    • Liming (application of lime materials) can increase soil pH and reduce Al saturation.

    • Manage carbon and nitrogen cycles better and use acid-tolerant plant varieties.

  • Liming Process:

    • CaCO3 neutralizes H+ ions effectively and can replace Al ions, balancing soil pH.

  • Challenges in Subsoils:

    • Harder to remediate acid subsoils due to lime mobility issues.

    • Applying soluble calcium sources (e.g., gypsum) can assist in growth improvement.

  • Acid Sulfate Soils:

    • Require significant lime additions to control acidity from drainage.

Importance of Soil pH

  • Influences nutrient availability and microbial activity.

  • Land management practices like fertilizing or drainage can impact soil pH, promoting acidification.