Green Team Part 17: For the Love of Dirt

Remember when we encouraged you to “look down” to examine the subsurface conditions that lie beneath our parks, buildings, and streetscapes? Now we’d like to call your attention to another material beneath our feet, one highly valued by landscape architects, soil. Our last post about stainless steel highlighted the importance of choosing the right material for a site. Similarly, the design of engineered soils for planting purposes has become increasingly important to landscape design.

But it’s just dirt, isn’t it?

Sure, but healthy soil is the backbone of a healthy site, especially in cities where the earth is disturbed and covered in rubble, or the existing soils are of poor quality and lack structure. The history of soils, their formation, properties and structure warrants a blog post of its own; there are also a number of texts by soil experts that cover this topic— James Urban’s Up by Roots or Phillip J. Craul’s Urban Soils in Landscape Design are two good examples.

A soil boring taken on a project site reveals telling information about soil texture.

Courtesy Lisa DuRussel  

Soils act as the infrastructure—a conveyance system—to feed trees and shrubs just as electricity and water lines feed buildings. Landscape architects love soil for its ability to sustain plant material and because it allows for the infiltration of stormwater runoff, encourages microbial processes that break down nutrients and chemicals, and provides a platform for decomposition that is ultimately linked to the fertility of the land. The complex balance of components (sand, silt, clay) mixed with organic matter, air, and water gives soil the necessary means for plants to absorb carbon dioxide, sequester carbon, and generate oxygen—a benefit to both humans and the landscape.

A view of the end of Pier 25 at Hudson River Park, where soils were engineered and installed to create healthy planting areas.

Courtesy Elizabeth Felicella

Soils as infrastructure

In an urban environment, landscape architects are often faced with designing a site from scratch. Our work relies on soil science and the design of engineered soils that have allowed us to effectively plan and design for landscape longevity on contaminated, abandoned, or poorly managed sites, developing strategies that mimic soil horizons found in nature and to create customized blends that respond to specific plant types at the volumes required to grow a mature plant.

Building healthy soils

At the Tribeca section of Hudson River Park, we were charged with establishing planting spaces on an existing pier over the Hudson River. The site contained areas of highly disturbed soil; some areas had no soil at all. Using the expertise of Pine & Swallow Environmental Services, our design team devised a specific recipe of engineered soil to respond to the existing conditions—and engineering—of the site.

Soils found in nature are made up of horizons. Each layer is distinctly different in physical characteristics from the surrounding layers. The upper layers are the most productive by way of decomposition, nutrient exchange and leaching, whereas the lower layers are for anchorage and drainage.

Courtesy Factmonster 

Soil blends were created specific to each planting type at the park. Our design called for a minimum soil depth of three feet in each planting area to provide adequate volume to support healthy root growth. A blend with a higher sand content was prescribed for lawns, encouraging drainage and minimizing compaction. In areas of tree planting, simulated soil horizons were created with layers of higher organic content intended to enhance nutrient availability and exchange at the soil surface and to maximize water-holding capacity.

A sand layer was located under each planting area to facilitate drainage. The right soil blend encourages plants to grow well-formed rooting structures, which help prevent the toppling of trees from wind and storms, common on the waterfront. By linking the soils and planting strategies, our design team was able to establish a viable planting system via a comprehensive soils infrastructure from project inception on, thereby ensuring the long-term success of the vegetation and reducing site maintenance.

Shown here is a typical section of the subgrade soil profile at Hudson River Park: drainage mat, foam (used to create a lightweight void to fulfill the loading constraints on structure), sand drainage layer, and subsoil.

Courtesy Mathews Nielsen

We also oversaw and managed the storage, handling, and installation of soils, which we will cover in a subsequent post.

In James Urban’s book Up By Roots, he advocates for 1000 CF of soil volume to grow a “big tree.”

Courtesy James Urban/Deeproot

Research in Action

Research related to soils and their influence on tree growth and survival calls for an average of 1,000 CF of soil (envision this as a cube, 18'x18'x3' in volume) in order for a tree to reach a healthy, mature size, averaged at 16" caliper. But maximizing soil volume is not always possible due to existing site conditions, engineering, or budget constraints. At Lincoln Center for the Performing Arts, 30 trees were proposed for a raised bosque on site. Because the available planting area was limited to a predetermined size with less than ideal amounts for available soil volume, our design team, in collaboration with Urban Trees + Soils, opted to use Silva Cells, a product that maximizes the subsurface area available for soils and provides an infiltration zone for stormwater runoff. These cells work by creating a continuous planting area below pavement for tree roots to thrive (and survive!) while providing the structural support required for adjacent construction and minimizing compaction.

Silva Cells are stacked between plywood voids created to hold tree rootballs. Planting soil is infilled into each cell to maximize rooting volume.

Courtesy Mathews Nielsen

The rule of thumb for urban sites: Begin with great soils from the start. You can rarely go back and change soil depth, but you can always add more plants! Soils are just one material on the designer’s checklist when it comes to making a healthy and sustainable landscape. Climate, exposure, aesthetics, and context also influence plant selection and longevity, which Zeina Zahalan will describe in our next Green Team post.

Trees are craned into the Lincoln Center site after Silva Cells are filled with soil.

Courtesy Mathews Nielsen

Soils and trees work in tandem at the Lincoln Center Bosque.

Courtesy Mathews Nielsen

The completed Lincoln Center bosque.

Courtesy Elizabeth Felicella

Lisa DuRussel, RLA, ASLA, LEED AP is a Midwestern transplant, avid coffee drinker, soils enthusiast, and practicing landscape architect in New York City. Since receiving her BS and MLA from the University of Michigan, she has worked on numerous urban revitalization and cultural landscape projects in the New York and Chicago areas, including the Governors Island Park and Public Space project.

This is one in a series of Metropolis blogs written by members of Mathews Nielsen Landscape Architects’ Green Team, which focuses on research as the groundswell of effective landscape design and implementation. Addressing the design challenges the Green Team encounters and how it resolves them, the series shares the team’s research in response to project constraints and questions that emerge, revealing their solutions. Along the way, the team also shares its knowledge about plants, geography, stormwater, sustainability, materials, and more.