Concept of Soil pH

• Soil pH is a measure of the acidity or alkalinity of soil and is one of the most important factors affecting plant growth, microbial activity, and nutrient availability.

Definition of Soil pH

• Soil pH is the negative logarithm of hydrogen ion (H⁺) activity in grams per liter:

pH=−log⁡[H+]pH = -log [H^+]pH=−log[H+]

• Neutral soil: pH = 7 (Equal H⁺ & OH⁻ ions)
• Acidic soil: pH < 7 (More H⁺ than OH⁻ ions)
• Alkaline soil: pH > 7 (More OH⁻ than H⁺ ions)

Agricultural soils generally range from pH 4.0 to 9.0, with the ideal range for most crops being 6.0 to 7.5.

Importance of Soil pH

a. Nutrient availability:

• Iron (Fe) solubility increases in acidic soils.
• Manganese (Mn) solubility increases in alkaline soils.

b. Microbial activity:

• Beneficial microbes thrive in neutral pH (6.0–7.5).
• Acidic soils reduce nitrogen-fixing bacteria.

c. Soil structure:

• Acidic soils can lead to Al³⁺ toxicity, affecting root growth.
• Alkaline soils may accumulate salts, harming plants.