Dirt vs. Soil: Unlocking the Root Problem for Arborists
Soil stores more carbon than all the world’s plants and atmosphere combined, making it a crucial ally in fighting climate change.
As arborists, we’re caretakers of trees and stewards of the landscapes they inhabit. But the key to thriving trees lies beneath the surface, in the living and breathing world of soil. Soil is more than just “dirt.” It’s a complex ecosystem that nurtures life, sustains trees, and supports the environment in ways we’re only beginning to fully understand.
In this article, we’ll explore how soil is created, what makes it different from lifeless dirt, and how trees, fungi, and microbes team up to form one of nature’s most fascinating partnerships. We’ll also address the challenges of urban soils and how arborists can improve them using organic and regenerative methods.
How Soil is Born: From Rock to Life
Soil begins as parent material—rocks and minerals broken down by a combination of physical, chemical, and biological processes. Over time, this material transforms into soil through these steps:
Weathering: Rocks are broken down into smaller particles by water, wind, freezing, and thawing.
Organic Addition: Plants grow, die, and decompose, adding organic matter that enriches the developing soil.
Biological Activity: Microbes, fungi, and insects populate the soil, recycling nutrients and creating structure.
Horizon Formation: Over thousands of years, soil develops distinct layers, or horizons:
O Horizon: Rich in organic matter like decomposed leaves.
A Horizon (Topsoil): A blend of minerals and organic material, teeming with life.
B Horizon (Subsoil): Less organic matter, more miner
The Problem with Urban Soils
In natural environments, these layers work in harmony, but urban soils often lack this structure due to human activity. Construction, grading, and compaction strip away topsoil and disrupt the natural soil horizons, leaving behind a mix of subsoil and fill material.
How Urban Soil Differs
Compaction: Urban soils are heavily compacted, reducing pore space and water infiltration.
Low Organic Matter: With no natural addition of leaf litter or plant material, organic content is minimal.
Fragmented Microbial Communities: Without healthy horizons, the soil food web is often incomplete.
Fun fact: In many urban areas, the “soil” under lawns is closer to lifeless dirt—a compacted, nutrient-poor medium that struggles to support deep-rooted plants like trees.
Soil Under Grass vs. Soil Under Forests
Soil beneath a lawn differs significantly from that under a forest:
Lawn Soil: Dense, compact, and low in organic matter, offering minimal microbial diversity.
Forest Soil: Looser and richer, with thriving fungal networks and a high concentration of organic material.
This difference is critical for arborists managing trees in urban environments. Trees evolved to grow in forest soil, and replicating those conditions can improve their health and longevity
The Structure and Texture of Soil
Healthy soil is a balance of:
Mineral Particles: Sand (large), silt (medium), and clay (tiny) determine soil texture.
Organic Matter: Decomposed plant material adds nutrients and supports soil life.
Pore Space: Spaces between particles make up 50% of healthy soil, split evenly between air and water.
Why Pore Space Matters:
Water Movement: Pores allow water to infiltrate, preventing runoff and erosion.
Oxygen for Roots: Roots need oxygen for respiration, and microbes rely on it to cycle nutrients.
Root Growth: Looser soil with adequate pore space enables deeper, stronger root systems.
How Trees, Microbes, and Fungi Shape Soil
Soil is alive because of the organisms that inhabit it, many of which work directly with plants and trees to improve structure and fertility.
Plant Roots as Soil Engineers
Roots don’t just absorb nutrients; they create channels that aerate the soil and encourage water infiltration. They also release exudates—sugars and chemicals that attract microbes and stimulate soil life.
Microbial Builders
Bacteria break down organic material and release nutrients in plant-available forms.
Fungi, particularly mycorrhizal fungi, form symbiotic relationships with trees, connecting roots to a vast underground network.
Fun fact: Mycorrhizal fungi can extend a tree’s root system by up to 700 times, allowing it to access nutrients and water far beyond its reach.
Fungal and Organic Contributions
Fungi produce sticky proteins like glomalin that bind soil particles together, improving structure.
Decomposing organic matter, like wood chips and leaves, creates humus, which retains moisture and nutrients.
How Arborists Can Revive Urban Soils
1. Vertical Mulching
Drill narrow holes into compacted soil and fill them with compost or wood chips. This improves aeration, restores pore space, and adds organic matter without disturbing tree roots.
2. Mulching with Arborist Waste
Use wood chips and leaves from tree care to create mulch layers under trees and other perennial beds or natural areas. It retains moisture, suppresses weeds, and decomposes into nutrient-rich humus over time. I recommend 3-5” thick.
3. Compost Tea
Formulate and spray compost teas or extracts over soil / mulch layers to boost microbial activity and nutrient availability. This is especially helpful if you have already vertically mulched or aerated the soil. Avoid “tilling” which turns over soils and breaks established fungal connections.
4. Encourage Native Plants
Planting native ground covers and shrubs improves biodiversity and mimics the natural forest floor, fostering healthier soil.
Cool Soil Facts to Share with Clients
Soil acts as a sponge, holding up to 20,000 gallons of water per acre, reducing runoff and preventing erosion.
Fungal networks can stretch for miles within a single gram of soil, acting as an underground internet for trees.
One acre of healthy soil contains enough microorganisms to weigh as much as two cows!
Soil stores more carbon than all the world’s plants and atmosphere combined, making it a crucial ally in fighting climate change.
A Soil-First Approach for Arborists
Healthy soil is the foundation of healthy trees. By understanding how soil is created, why urban soils are often deficient, and how fungi, microbes, and plants interact, arborists can improve the landscapes they manage while creating a greener future.
By offering soil-focused services—like vertical mulching, compost applications, and soil education—you can address soil challenges and turn sustainable practices into a revenue stream.
So, let’s stop thinking of soil as “just dirt.” Instead, let’s embrace it as a living system critical to trees, ecosystems, and the planet. Get your hands dirty (with soil, not lifeless dirt!), and start transforming the underground magic today! 🌱
Kurt Stenberg, Arborist
ISA PR-5709A
Founder, Atmos Tree
Creator, Arborist Blueprint Podcast
