I have known James for over six years. We met at an ASLA Annual Meeting when I heard him speak. Subsequently, I invited him to speak at all four of the Organic Landcare Symposiums that Atlanta BeltLine put on. His breadth of knowledge is inspiring and every time I hear him, I learn something new. I hope you will find this post enlightening and that it might even encourage you to explore more about creating environments for healthy soil microbiology.
-Kevin Burke, ASLA, Sustainable Design and Development PPN Officer
Located in Midtown at the intersection of Fifth Avenue and 59th Street, Grand Army Plaza stands as a gateway to New York City’s Central Park. Its grand gesture design and historical significance have made it a notable place since its original construction in 1916.
In September 2015, the Central Park Conservancy completed a major restoration of the northern section of the plaza, including the General Sherman statue. Site work included reconstruction of paving, stonework, benches, and lighting, all designed to be in keeping with the original historic design. Electric, drainage, and irrigation infrastructure were fully replaced. The trees at the plaza perimeter, previously lost in an October 2011 snowstorm, were replaced with a double row of London Plane trees, to be consistent with the original design. The placement of CU-Structural Soil™ was incorporated beneath all pavements to provide adequate soil volume for mature tree root systems.
The Leading Tone
Several months prior to the anticipated project completion, concerns developed for the health of the newly installed London Plane trees.
Due to unanticipated project delays, the nursery-dug trees were held above ground in the Conservancy’s plant compound much longer than planned. Despite receiving adequate care, the trees displayed signs of stress prior to planting. Combined with the challenges of being planted in a highly urban, restricted construction site with ever-changing environmental conditions, the vitality of the trees was tested.
The development of a program to increase tree and surrounding soil health was necessary, but required careful planning so as to not impact the persistent flowing of city traffic and set back the plaza’s reopening—a seemingly impossible challenge often found in urban design and management.
Establishing a baseline health report was the first step in our approach. The return of data would give us a deeper understanding of current conditions while also serving as a comparative resource. The tree’s soil microbial biomass measured an average of 282 ug/g, with an estimated 590 pounds of microbial carbon per 1000 square feet and 1.28 pounds of microbial nitrogen per 1000 square feet. All numbers indicated a low productive range. Root mycorrhizal colonization rates also returned weak readings, with a 6 percent average. While expected from newly planted trees, the need for amendment was critical for long term sustainability and continued establishment.
In determining a solution(s), the focus remained in designing prescriptions that would correct both microbial biomass and mycorrhizal colonization while supporting the overarching system and preserving the aesthetic.
It was recognized that building both pieces would require adjustments to the surrounding soil profile, as the material had become compacted through project progression. Amending the soil would allow the root systems to grow without restriction and the microbial community to operate, all of which would aid in the founding of a successful environment below and above ground. But how can this be achieved?
Cue the cover crop concept.
Rather than employing mechanical methods, often leading to effects such as pricey resources and damage to the soil’s microscopic mycelium, an alternative and natural program was implemented—a program where nature could amend nature.
Cover crops allow the break up of soil compaction by developing their own strong root systems while also releasing sugars and carbon for soil bacteria and fungi to break down into organic matter. A Sorghum cover crop seed was chosen after scientific literature indicated its role in commercial mycorrhizal spore production and its germination rates. The seed would grow harmoniously with the London Plane trees while enhancing the soil with its own ecological processes.
The program began with subsurface injections of specific Liquid Biological Amendments (LBAs) administered at a 2-4 inch depth around each tree pit. LBAs were created by employing the gains of Ecopiles™ and applicable technology where the extraction of both specific and indigenous organisms can be transferred into solutions for application.
Following applications, the Sorghum seed was planted and then drenched with a vesicular-arbuscular mycorrhizal (VAM) spore agent to aid in the establishment of the interdependent relationship between root and microbe.
Within 10 days of installation, observed germination effects were taking shape. At two weeks, roots of the Sorghum had grown to a depth of 4 – 6 inches, creating an un-compacted root zone and making room for the London Plane trees to flourish. A second set of Sorghum leaves even pushed above ground in areas impacted by steam pipes.
Within 6 months, microbial biomass measurements increased to 473 ug/g, an estimated 1090 pounds of microbial carbon per 1000 square feet, and 2.15 pounds of microbial carbon per 1000 square feet. The root mycorrhizal colonization percentage shot to 14 percent.
It is also important to note that these tree pits are not irrigated, only hand watered by the Central Park Conservancy staff. A focus on soil moisture management rather than irrigation management has helped to maintain the ideal conditions for health and growth.
The trees displayed a beautiful leaf out and maintained dense leaf coverage during the 2016 seasons. This season will bring another year of maturity to the plaza’s landscape and ecological health. As we approach the third growing season, the trees are ready with a healthy bud set. The space, bridging past to present, continues to inspire the ever beating melodies of Midtown through successful design and the natural rhythms of nature.
by James Sottilo, Lead Consultant and Founder, Ecological Landscape Management