Soil Management for the ISA Exam: Key Concepts and Common Questions

Domain Review: This topic is one of the [10 ISA exam domains](/blog/isa-arborist-exam-domains-explained) you must master.

Soil is where tree health begins. This domain (12%) is heavy on chemistry and physics.

Texture vs. Structure

• Texture: What the soil is made of (Sand, Silt, Clay). You cannot change this.
  • Sand: Large particles, large macropores, drains fast, low nutrient holding.
  • Clay: Tiny particles, small micropores, holds water/nutrients well, compacts easily.
  • Loam: The ideal mix.
• Structure: How the particles are arranged (Clumps, Plates, Blocks). You CAN change this (compaction destroys structure).

Pore Space

Ideal soil is 50% solid and 50% pore space.

• Macropores: Filled with Air. (Provide oxygen to roots).
• Micropores: Filled with Water.

Compaction

Compaction crushes macropores.

• Result: Roots suffocate (no oxygen).
• Symptom: Dieback, stunted growth.
• Fix: Air spade, vertical mulching (adding organic matter).

Chemistry: pH and CEC

• pH: Acidity/Alkalinity.
  • Low pH (Acidic): < 7.0.
  • High pH (Alkaline): > 7.0.
  • The Trap: High pH locks up micro-nutrients like Iron and Manganese. This causes Chlorosis (yellow leaves with green veins).
• Cation Exchange Capacity (CEC): A measure of fertility.
  • High CEC: Clay/Organic matter (Holds nutrients like a magnet).
  • Low CEC: Sand (Nutrients leach out).

Water Movement

• Infiltration: Entry into soil.
• Percolation: Movement through soil.
• Problem: Crusty surface = poor infiltration. Compacted subsoil = poor percolation.

Summary

If a tree looks sick, look down. Most "diseases" are actually soil problems.