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Super Soil: Designing Soil for the Main Fountain Garden

By Patricia Evans, on June 28, 2016
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“Great soil is the foundation for a great landscape,” explains Claire Agre. A landscape architect, Agre is also a principal at West 8 urban design and landscape architecture, a design firm responsible for the execution of parks, gardens, waterfronts, and public spaces around the world. West 8’s latest project is to lead the landscape design of the Main Fountain Garden, including the planting of more than 2,600 boxwood in an exuberant hedge, 168 linden trees in Allee formation, and the development of additional pathways, furnishings, and fixtures for guests to enjoy.

Great care was taken in all aspects of planning the Garden, from plant selection to bench design, but perhaps the most intensive aspect of the project was designing the soil upon which the Garden would thrive. But how does one design soil? For many, this is an unfamiliar concept. Most homeowners are familiar with adding nutrients to soil or adjusting the pH level, but designed soil encompasses much more.

“There are a lot of things in the landscape that people don’t think of as designed,” Agre said, “and soil is one of them.” Designed soils can specify a mix of sand, silt, and clay, which determines the chemical and physical properties of a new soil so that objectives can be achieved. A soil designer can optimize soil for an area’s specific need, whether it's to promote lush plant growth, manage storm water, or more.

For the Main Fountain Garden, Longwood specified 13 attributes for the soil that included supporting the different plant material going into the Garden, withstanding increased guest foot traffic and vehicular access to the Garden, and providing good drainage, among others. “Part of the 21st century design for the Main Fountain Garden is to invite people into the Garden, so it needs to withstand a lot of use,” Agre explained.

To assist in the soil design, West 8 worked with Dr. Barrett Kays, a leading soil expert. Kays is well-known for his work designing the soils at the Great Lawn in New York’s Central Park. The work that Kays did bridges civil engineering, soil science, and horticulture, explains Agre, noting that Kays had to consider water issues, storm drainage, and runoff issues in addition to understanding the horticulture side of it, including the moisture and nutrient needs for each type of plant going into the Garden. “The breadth of knowledge required is immense,” said Agre.

Critical to getting the soil mix correct for the Main Fountain Garden was developing a formulation that would withstand compaction, which is the number one reason soil systems fail. Soil Compaction occurs when a stress (such as machinery or heavy foot traffic) is applied to a soil, displacing the air between the soil grains, causing the soil to become denser.

Designed soil can include many layers of materials to get the desired effect. Diagram courtesy of West 8.Designed soil can include many layers of materials to get the desired effect. Diagram courtesy of West 8.

Dr. Kays’ formulation for the Main Fountain Garden includes a fine gravel layer on the bottom with 2-3 feet of soil mixture on top. The soil mixture layer is 80% medium and coarse sand, and 20% topsoil. The sand has a large particle size, therefore the soil can be compacted while still maintaining large pore spaces for water infiltration and air for roots. In addition, the gravel and soil layers are designed to accomplish the seemingly incompatible tasks of promoting good drainage while also allowing enough water retention to support the plantings. This new soil will ensure that rain will promptly drain from the ground, allowing maximum use of the Garden. Formerly, after a rain event, Garden access would be limited to allow the ground to dry. The new soil is designed to absorb a major rainstorm—12 inches of rain in one day.

Seven thousand cubic yards of topsoil were removed from the Main Fountain Garden, sterilized, and stored offsite. The 4,000 truckloads are being returned to the Garden in time for planting to begin. Photo courtesy of Bancroft Construction.Seven thousand cubic yards of topsoil were removed from the Main Fountain Garden, sterilized, and stored offsite. The 4,000 truckloads are being returned to the Garden in time for planting to begin. Photo courtesy of Bancroft Construction.

Approximatel 7,000 cubic yards of topsoil from the Main Fountain Garden were removed during excavation and saved to become part of the new design mix. “Good topsoil is worth saving and Longwood has good topsoil,” explained Agre. The topsoil was sent to a nearby location where it was sterilized to remove unwanted noxious weeds—Bermuda grass and nut sedge. The topsoil is now returning, as part of the new soil mixture, in time for the planting of the boxwood and linden trees.

The first of 168 Tilia cordata ‘Greenspire’ trees are being planted in the designed soil of the Main Fountain Garden. Photo courtesy of West 8.The first of 168 Tilia cordata ‘Greenspire’ trees are being planted in the designed soil of the Main Fountain Garden. Photo courtesy of West 8.

Although homeowners may not go to such extremes in their own backyards, Agre believes there is a lesson to learn. “Homeowners may want to remember that some of the persistent problems they encounter in their own backyards might not be a plant problem at all. It might be the soil.”

Want to learn more about our designed soil? Join us on Twitter as we host Ground Chat on July 29, 2016 at 2:00 pm. Follow along using #GroundChat.

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