Introduction

For decades, the assumption has been simple: screened soil is better soil. It’s cleaner, more uniform, easier to manage — and is often seen as the gold standard. The golf industry, for example, has wholeheartedly embraced the properties of EPA-approved screen soils.

But here’s the problem: in the pursuit of consistency, we’re sifting away the very soil structure trees need to thrive. What’s good for golf is not good for trees. Soil peds, the naturally occurring clumps of aggregated soil particles, help create the structure that trees need to flourish, especially under pavement in Silva Cells where soils are not open to the elements (like they are on golf courses). The soil mixing process often removes peds from the equation. In pursuit of uniformity and predictability, we screen our soils — removing important structure that supports water movement, microbial life, and root expansion.

But it doesn’t have to be this way; in fact, it shouldn’t. Let’s explore why unscreened soil is actually ideal for city tree health — and how Silva Cells are a great choice to embrace this ped-friendly strategy.

Understands Soil Screening Guidance (SSG) and Peds

The EPA’s soil screen guidance (SSG) helped standardize and accelerate the evaluation of contaminated soils — a technology adopted by the golf industry where superintendents already vigorously test their course materials. The added sand to fairway and green mixes allows water to pass through quickly, crucial for preventing waterlogging, especially in areas with heavy rainfall.

But trees grow differently than grass. Sand has a lower organic matter (OM) content; also, the environment under pavement is completely different than fertilized and aerated golf courses. Tree roots grow better with more structured soil peds.

Peds are natural aggregates of soil particles — clusters that form over time through biological and environmental processes. Unlike clods, which are mechanically compacted and lack structure, peds are the result of healthy soil activity and decomposition. Their presence in soil is essential for:

• Maintaining texture and porosity
• Supporting air and water movement
• Facilitating infiltration and root expansion
• Hosting microbial life

In short, peds help soil function the way nature intended. They form the physical architecture that supports trees, soil organisms, and the entire below-ground ecosystem.

Screened vs. Unscreened Soil: What’s the Difference?

In urban construction, screened soil is commonly used for consistency and ease of placement. But this comes at a cost. What are the primary differences between screened and unscreened soil?

Screened Soil:

• Passes through a mesh to remove large aggregates, obliterating its structure
• Often supplemented with sand, reducing organic content and plant response
• Looser and prone to compaction, especially after installation
• Lacks natural micro and macropores essential for root growth and water movement
• Offers limited organic content, microbial life and biological function

Unscreened Soil:

• Retains its original structure, including peds and organic material
• Preserves natural network of pore spaces, promoting strong hydraulic conductivity
• Supports microbial communities and nutrient cycles
• More resilient to compaction and urban stressors
• Maintains natural ratio of micro and macropores and microbial habitats

Why Trees Prefer Unscreened Soil

Trees don’t grow in a vacuum — they grow in a complex, living system. Unscreened soil better replicates the conditions of natural forest soils, giving urban trees a stronger start and a longer future. What are the benefits to city trees of being planted in unscreened soil?

• Root growth: Peds create channels and spaces for deeper and wider root expansion
• Improved Water Properties: More effective infiltration and holding capacity
• Soil biology: Richer microbial life and nutrient availability
• Resilience: Better performance during droughts, heavy rainfall, and compaction
• Canopy growth: Healthier soil equals healthier trees, which deliver larger and more robust canopies

The difference in long-term tree performance is clear, particularly when cities are investing in urban greening as a climate and livability strategy. A prime example of this is the Martin Luther King Jr. Memorial in Washington DC — a plaza/streetscape project for which Silva Cell were utilized to support the new tree plantings. Unscreened soil was placed in the Silva Cells where the streetscape elm trees were planted, which, now more than a decade later, are exceeding all growth expectations and providing both the sidewalk and roadway with much-needed shade.

Silva Cells and On-Site Soil

Silva Cells provide a soil support and stormwater management system that is uniquely suited to take advantage of unscreened — or even on-site — soil. Oftentimes this on-location soil (with minor enhancement) can be placed back in the Silva Cell void space for ongoing tree support, a process for which we have soil specs and technical support as helpful resources.

Our “strongback” installation lids are our secret weapon: by not needing to sweep the soil into the system, and instead having large openings for soil placement, a wider variety of soils can be specified for urban forestry projects — including ped-friendly unscreened soil. (These strongback lids also have the advantage of maximizing the soil within the void space, guaranteeing appropriate soil volume and reducing subsidence issues.)

There are likewise environmental benefits to reusing on-site soil. Not only are the trees more likely to respond to this hospitable native soil, which preserves local biota in place before development, but the project itself can reduce energy expenditure by:

• Eliminating energy-intensive screening process
• Not relying on trucked-in outside soil

In short, Silva Cells don’t just make room for soil — they make room for better soil and ultimately better environmental sustainability. DeepRoot can also assist with this soil specification. Our soil specs (section 2.09) are derived from the blue-ribbon soil specification promoted by the nonprofit Urban Tree Foundation, which recommends 40-60% unscreened top soils in its blend. Our general principle is that trees will grow better in soil mixes with greater amounts of unscreened soil (with less sand). Drainage rates of 2-3” per hour at installation — typical of our soil specs — is great for both water treatment and trees.

Conclusion: Let Peds do the Work

Urban tree health doesn’t just come from the species you select or the amount of water they receive. It begins underground, with the structure and biology of the soil. By embracing unscreened soils — and the peds within them — we give trees the foundation they need to thrive.

And when paired with a system like Silva Cell, that foundation becomes even more powerful. It’s time to stop removing the very thing trees are asking for. Peds are your friends — let’s treat them that way.

Listen to This!

For even more insight on unscreened soils, check out our audio shorts!

 

 

Supporting Research

“Distinguishing urban soils with physical, chemical, and biological properties.” Study by Bryant C. Scharenbroch: A soil scientist at University of Wisconsin: Stevens Point, Scharenbroch completed a study that demonstrated that unscreened soil performed equal to or better than the screened soil mix while using less sand and requiring less processing. The experiment used three soil approaches: silty clay loam topsoil with limited screening.

“Soil Profile Rebuilding: An Alternative to Soil Replacement.” Study by Susan Day: A researcher at Virginia Tech, Day published a long-term study on different methods to improve compacted soil. She pioneered soil profile rebuilding which involved placing 4” of compost over the top of compacted soil and fractured it with a backhoe to a depth of 2’. This method outperformed screened soils.

“Amending Soils for Enhanced Infiltration of Stormwater.” Study by Barrett Kayes: In this large study, Kayes looked at infiltration rates of unscreened tilled soils in North Carolina. These soils perked as well as sandy soils even though they had clay content.

“Retaining soil structure to improve soil.” James Urban article in The Field: “Water holding capacity and plant respiration improved in the less processed soils, particularly when watering frequency was reduced.”