Structural Soil for Urban Trees

Currently most urban trees are planted directly into existing compacted urban soil or tree pits with limited root space. Trees that are planted in areas surrounded by paving tend to struggle for air space and usually decline in ten years. Where soil volume is limited by pavement, tree roots suffer and tend to take the path of least resistance searching for air, usually in and around pipes, foundations, or to the surface. Healthy trees need a large volume of non-compacted soil with adequate drainage and aeration and reasonable fertility. The need and desire for large trees in the urban landscape still is the desired intent, unfortunately,
the trees do not live long enough to fill the need. Not planning for root growth is ignoring the biological requirements of trees and is not economical. The failure to provide adequate soil drainage  is the most frequent cause of newly transplanted trees dying. Insuring a good supply of air to the tree roots is essential for satisfactory tree growth, however in urban situations, the movement of air into the soil is often restricted. By providing additional root space below the pavement in what otherwise was compacted urban soil, Stalite Structural Soil can allow most newly planted trees to have a chance for healthy growth. This mix consists of 80% Stalite rotary kiln fired expanded slate aggregates of a gradation of 3/4 to a # 4 screen size and 20% approved sandy clay loam. The aggregates bear on one another, providing the structural stability for the pavement above. As they bear, the additional voids remaining for uncompacted soil in between the aggregates providing space for air and future root expansion. This design provides a resource for root growth beyond the tree pit, not as a replacement for tree pit soil.

The Stalite expanded slate provides sufficient compressive strength to support pavement while the added porosity of the aggregate itself provided additional surface area for air, water and nutrients for the fine feeder root hairs. The cation-exchage-capacity (CEC) of soil is tremendously important to prevent nutrient loss from leaching. The CEC of the expanded slate is around 21.5 me/100g reducing the need for additional clay and allowing the use of sandy-clay-loam as the soil portion of the mix.

Since 1995, Stalite Structural soil has been used on several tree planting projects in the eastern U.S. The ItSaul Natural Company first used Stalite structural soil for tree plantings in Atlanta for the 1996 Olympics. The trees planted in the mix in Atlanta in 1996 on Marietta Street, North Avenue and Tenth Street are thriving years later. The National Park Service used this specification for the retrofit to the Korean War Veterans Memorial in Washington D.C. in 1998. The lindens have been in place for over a decade and are very large and healthy without any fatalities. Dozens more projects have been completed with the trees doing very well with the roots growing deep under the pavement. The trees in downtown Greenville, NC planted in Stalite structural soil survived hurricane Floyd in 1999. This history of success is another reason to specify Stalite Structural Soil.

Benefits of Stalite Structural Soils

  • Cation-exchage-capacity of 20 to 25 me/100g
  • Water and nutrient retention
  • High water release curves
  • Less clay needed in the mix
  • High compressive strength
  • Lightweight and easy to mix and work with
  • Contains voids for additional surface area for fine feeder roots
  • Additional air space available if soil proportion shifts

 Watch the installation videos taken in 2000 at the construction of Phase I of the downtown district at Council Street in Salisbury NC.  The structural soil is installed first to support the pavement and provide additional aeration, drainage and area for root growth under the pavement.  Tree planting is the last phase of construction which assures correct planting depth and reduces injury risks to the trees.



Council St. Sweetgum "Rotundiloba' Trees October 2009


Structural Soil Summary 

Structural Soil Specification