Notes on Soil Acidity

Soil acidity significantly limits soil productivity worldwide.
Understanding soil's fundamental chemistry is crucial for management and remediation strategies that are relevant to local conditions.
pH Value: The pH value is a critical chemical characteristic of soil, likened to a ‘master variable’ in soil chemistry, affecting:
- Nutrient availability to plants
- Toxicity of trace metals
- The response of plants to soil acidity.

Soil pH measures the intensity of acidity in soil. Due to its solid-phase nature, determining pH in soils can be complex.
Measurement Methods:
- pH can be measured by equilibrating soil with distilled water or a salt solution (0.01M CaCl2 or 1M KCl).
- Laboratory Measurement: pH is measured using a specific H+-ion electrode.
- Field Measurement: pH-sensitive dyes can be used.

More precise measurements (to 0.01 units) can be made using an electromotive force (emf) cell, which includes:
- Two electrodes immersed in solutions, connected via a salt bridge.
- The potential difference (ΔE) between electrodes correlates to pH changes (ideal glass electrode response is 0.059V per unit change in pH).

Colloidal Suspensions:
- pH measurement in soils with colloidal suspensions (like clay) affects results due to concentration gradient changes created by cation attraction.
- An apparent decrease in pH can occur, known as the junction or suspension effect.

Measured pHH2O is influenced by:
- Natural seasonal variation in soil solution salt concentrations
- Recent fertilization effects.
- To counteract these, 0.01M CaCl2 or 1M KCl are added before pH measurement.

Total Acidity measures all forms of acidity and can be determined by:
- Titration of soil suspension in salt solution until reference pH is reached.
- Use of pH buffer-salt mixtures.

Includes all titratable acidity in the solid phase (
- A measure of a soil's buffering capacity against pH changes. It sums exchangeable and non-exchangeable acidity.

Exchangeable Acidity is comprised of:
- Al³⁺ and H⁺ ions that are easily extracted by neutral salts (e.g., 1M KCl).
- This form is prevalent in highly acidic soils (pH < 4).

Soil acidity arises from natural and human-induced factors, including:
- Acid rain from SO₂ and NOₓ emissions.
- Mineral weathering, organic matter decomposition, ion uptake by plants, and chemical amendments modifications affect acidity levels.

Carbon Dioxide: In waterlogged soils, CO₂ raises carbonic acid levels, contributing to acidity.
Organic Matter: Organic acids are more significant at pH < 5.5, increasing acidity through weak acid functionality.
Aluminum Hydrolysis: Al³⁺ present in acidic soils leads to further acidity through its reactions with water.
Mineral Weathering: Certain assemblages like pyrite oxidation lead to high acidity, resulting in acid sulfate soils.

Soils can resist changes in pH due to:
- Carbonate Buffering: Neutralizes acidity from rain via carbonate minerals like CaCO₃.
- Base Cation Exchange: Exchange of H⁺ ions for base cations on clay, affecting soil pH.
- Mineral Decomposition: Protonation of variables surfaces can help buffer against changes in acidity.

Crop Selection: Using acid-tolerant crops can sustain production despite soil acidity.
Nutrient Application: More fertilizers may be necessary to maintain production, as fewer nutrients are available.
Use of Lime: Lime serves as the foremost method to correct soil acidity, utilizing various forms like CaCO₃ and Ca(OH)₂.
Lime Requirement Determination: Calculated based on desired pH, CEC, and buffering capacity. Two methods include the pH-base saturation method and buffer solution method.