Certified Arborist Test Soil Science and Root Management 1 — Questions and Answers
Question 1: Which soil texture has the smallest individual particle size and tends to retain the most water but drains poorly?
- Sand
- Silt
- Clay (Correct answer)
- Loam
Correct answer: Clay
Clay particles are the smallest of the three primary soil separates (sand, silt, clay), giving clay soils a very high surface area that holds water tightly. This poor drainage can limit oxygen availability in the root zone and impede root development.
Question 2: What is the primary purpose of a soil percolation test in arboriculture?
- To measure the pH level of the soil
- To determine the rate at which water drains through the soil profile (Correct answer)
- To identify the presence of mycorrhizal fungi
- To assess the nitrogen content available to tree roots
Correct answer: To determine the rate at which water drains through the soil profile
A percolation test measures how quickly water moves downward through the soil profile. This tells arborists whether a site has adequate drainage to support healthy root respiration, since waterlogged soils deprive roots of oxygen.
Question 3: What is the critical root zone (CRZ) of a tree generally defined as?
- The area directly beneath the tree's canopy drip line only
- A circular area with a radius of 1 foot per inch of trunk diameter (Correct answer)
- The area within 3 feet of the base of the trunk in all directions
- The zone extending to twice the height of the tree
Correct answer: A circular area with a radius of 1 foot per inch of trunk diameter
The critical root zone is typically calculated as a circular area with a radius of 1 foot for every inch of trunk diameter measured at breast height (DBH). This defines the minimum area around a tree that should be protected from disturbance during construction or other activities.
Question 4: Which of the following best describes a 'soil horizon'?
- The depth at which roots stop growing due to lack of oxygen
- A distinct layer within a soil profile that differs in physical or chemical properties from adjacent layers (Correct answer)
- The boundary between clay and sandy soils at a construction site
- The zone of highest microbial activity around tree feeder roots
Correct answer: A distinct layer within a soil profile that differs in physical or chemical properties from adjacent layers
Soil horizons are distinct layers visible in a soil profile, each with its own texture, color, structure, and chemical composition. Understanding horizons helps arborists assess rooting depth potential, drainage characteristics, and nutrient availability at a site.
Question 5: What does soil pH primarily affect in the context of tree nutrition?
- The physical structure and aggregate stability of the soil
- The availability of nutrients to tree roots (Correct answer)
- The rate at which organic matter decomposes in the root zone
- The depth to which feeder roots can penetrate
Correct answer: The availability of nutrients to tree roots
Soil pH controls the chemical form and solubility of nutrients, directly affecting how available they are for root uptake. For example, iron and manganese become less available at high pH (alkaline soils), while aluminum and manganese can reach toxic levels at very low pH (acidic soils).
Question 6: What is 'girdling root' and what is its primary threat to tree health?
- A root that grows straight down, competing with the taproot for water
- A root that encircles the trunk or another root, restricting vascular flow (Correct answer)
- A surface root that is repeatedly severed by lawn maintenance equipment
- A root that grows into a drain pipe and causes soil subsidence
Correct answer: A root that encircles the trunk or another root, restricting vascular flow
A girdling root wraps around the base of the trunk or around another structural root, constricting the phloem and xylem as both the root and the trunk expand over time. This restriction progressively limits the movement of water, nutrients, and carbohydrates, and can eventually kill the tree.
