Soil Reaction and Buffering Capacity

1. Introduction

Soils differ in their chemical nature — some are acidic, some neutral, and some alkaline.
This difference is due to the concentration of hydrogen ions (H⁺) in the soil solution and on the exchange sites of soil colloids.
The degree of soil acidity or alkalinity is expressed by pH.

2. Soil Reaction

2.1 Definition

Soil reaction is the degree of acidity or alkalinity of the soil, expressed in terms of pH (potential of hydrogen).

pH = −log₁₀[H+] 

• A low pH (<7) indicates acidity (more H⁺ ions).
• A high pH (>7) indicates alkalinity (more OH⁻ ions).
• Neutral soil: pH = 7.0

2.2 Soil pH Scale

2.3 Types of Soil Acidity / Alkalinity

2.4 Causes of Soil Acidity

• Leaching of bases (Ca²⁺, Mg²⁺, K⁺, Na⁺) by heavy rainfall.
• Accumulation of H⁺ and Al³⁺ due to hydrolysis.
• Parent material — granite, quartz-rich rocks produce acidic soils.
• Organic matter decomposition — releases organic acids.
• Use of acid-forming fertilizers (e.g., ammonium sulfate, urea).
• High rainfall and poor drainage.

2.5 Causes of Soil Alkalinity

• Accumulation of soluble salts and sodium carbonate.
• Parent material rich in bases (limestone, basalt).
• Low rainfall → poor leaching of basic cations.
• Irrigation with high sodium water.
• Poor drainage in arid and semi-arid regions.

2.6 Effects of Soil pH on Soil Properties

2.7 Measurement of Soil pH

2.8 Optimum Soil pH for Crops

2.9 Management of Acid and Alkaline Soils

3. Buffering Capacity of Soil

3.1 Definition

Buffering capacity is the ability of soil to resist changes in pH when acids or bases are added.

In simple terms, it’s the pH stability power of the soil.

3.2 Mechanism of Buffering

• Soils contain colloidal particles (clay and humus) with exchangeable H⁺, Al³⁺, and basic cations.
• When acid or base is added:
  • H⁺ or OH⁻ ions are neutralized by ion exchange or reactions with soil colloids.
  • Thus, pH does not change drastically.

3.3 Factors Affecting Buffering Capacity

3.4 Importance of Buffering Capacity

3.5 Relative Buffering Capacity of Different Soils

3.6 Example of Soil Buffering Reaction

• When acid (H⁺) is added: Soil – Ca + 2H⁺ → Soil – H₂+ Ca²⁺
• The H⁺ ions replace Ca²⁺, but the soil resists drastic pH drop →
• When lime (CaCO₃) is added: CaCO₃ + 2H⁺ → Ca²⁺ + H2O + CO₂↑
• H⁺ ions neutralized → pH stabilized.

3.7 Relationship Between Buffering and CEC

Buffering capacity ∝ CEC (Cation Exchange Capacity)

Soils with high CEC (clayey, humus-rich) are more resistant to pH change than sandy soils with low CEC.

4. Summary Table

6. Key Points to Remember

• Soil pH is the master variable controlling nutrient availability.
• Optimum soil pH for most crops:0–7.0
• Acid soils: Common in high rainfall areas (Northeast India, Western Ghats).
• Alkaline soils: Common in arid and semi-arid regions (Northwest India).
• High buffering capacity → slow but stable pH changes.
• Low buffering capacity → rapid pH change after fertilizer or acid rain.