Acids & Bases in Soil Chemistry

1️. Bronsted-Lowry Theory

• Acid: A substance that donates H⁺ ions.
• Base: A substance that accepts H⁺ ions.
• Example: HCl→H++Cl−HCl —-à H^+ + Cl^-HCl→H++Cl−

2️. Arrhenius Theory

• Acid: Increases H⁺ ions in solution.
• Base: Increases OH⁻ ions in solution.
• Example: NaOH→Na++OH−NaOH —–à Na^+ + OH^-NaOH→Na++OH−

Acid-Base Neutralization

• When H⁺ and OH⁻ ions react, they form water and salt: HA+BOH→AB+H2OHA + BOH -à rightarrow AB + H_2OHA+BOH→AB+H2​O

Soil Reaction (pH & Ion Exchange)

Soil reaction refers to whether the soil is acidic, neutral, or alkaline, based on H⁺ ion concentration.

Cation Exchange & Soil pH

• Soil particles are negatively charged (~90%), attracting positively charged cations.
• Major cations in soil solution:
  • Acidic cations: H⁺, Al³⁺
  • Basic cations: Na⁺, K⁺, Mg²⁺, Ca²⁺

Soil pH Conditions

Soil Acidity

Acidic soils are common in areas with high rainfall, where basic cations (Na⁺, K⁺, Mg²⁺, Ca²⁺) are leached from the soil.

• Base saturation < 80% leads to soil acidity.
• Predominant acidic cations: H⁺ and Al³⁺.

Regions with Acidic Soils

• Eastern Terai (Nepal)
• Tropical & humid regions