Soils

Soils are one of the most crucial considerations when proposing plants within the ROW. Soils that possess favorable horticultural characteristics (pH, nutrients, drainage, soil texture, etc.) will allow the greatest array of plant species to thrive and reduce maintenance costs. These characteristics must be carefully considered and balanced in context of proposed species, plant tolerances, and overall project goals. Soils that do not support selected plant species, or alternatively, selecting species that do not thrive in a particular soil will have significant impacts on plant survivability and the overall success of a proposed landscape.

It is particularly important that enough soil volume is provided for the proposed planting; this measure contemplates depth as well as the total area of soil. Limited soil volume will stunt plant growth and contribute to plant decline or death. As such, maximize soil volume and choose plants that will grow well in the available soil volume. As most roots grow within the top layer of soil, increasing the soil surface area is most beneficial. However, also provide appropriate soil depth: at least 18 inches for grass or ground cover, 18 to 24 inches for shrubs, and 18 to 30 inches for trees. The soil depth should also relate to the root ball depth at planting. For soil surface area, ensure plant beds are sufficiently wide to support healthy growth. A standard tree bed is typically 5 feet wide by 10 feet long. In general, planting areas should not be less than 3 feet wide at the narrowest point.

There are additional specific considerations when evaluating soils for use in green infrastructure installations. Features such as rock outcroppings, glacial erratic’s, or a highwater table may preclude the use of green infrastructure and must be understood prior to siting such installations. Refer to DEP’s geotechnical investigation procedures, standard details, and standard specifications, which can be found on DEP’s Green Infrastructure website. Additionally, soils used in green infrastructure installations are typically highly engineered to ensure appropriate performance. Consider the use of highly absorbent soils, such as biochar. Carefully select plants that grow well within these soils to ensure long-term success.

Where pavement is necessary next to trees or plantings, consider a suspended pavement system (i.e. structural soils or structural framing systems or cells) to provide greater rooting area. Suspended pavement systems support the engineering requirements of pavement, while still allowing plant roots to grow in the subbase below. In most cases, pavements must be able to withstand typical loading and compaction requirements (95% proctor density), with some locations requiring truck loading (AASHTO HS-20) standards be met. The added growing medium can also serve to increase the area for stormwater capture or detention. Additionally, by encouraging root growth within the subbase, these systems typically reduce sidewalk heaving caused by tree roots and the associated maintenance burden.

In order to determine if an existing or proposed soil has the appropriate characteristics it must be examined by a qualified laboratory. Soils should be tested for features including but not limited to texture, pH, organic content, permeability, salt content, nutrients, bulk density, and presence of contamination. Additional investigation must be done when specifying soils for use in Green Infrastructure. New soils should meet appropriate DOT and Parks specifications, which may vary depending on project specific conditions. Existing soils may be amended to bring soil characteristics within acceptable ranges. Methods for amending soils include applying lime or sulfur to adjust pH, adding sand or silt or clay to adjust soil texture, adding organic matter to improve permeability and correct nutrient deficiencies, loosening compacted soil via mechanical or hand tools, and several other techniques. Qualified laboratories, following an examination, will provide recommendations for amending soils, if possible, to achieve optimal plant growth. If amendments cannot be made, the soil will need to be replaced to the required depth and width accordingly or plant selection must be carefully tailored to the available soil. Should there be contaminated soils on site, consult with the appropriate city or state agencies on the process for disposal.

The following table describes generally desirable soil characteristics, however, soil must be examined and selected based on site-specific conditions and tailored to fit the needs of the project. Different values may apply for soils used in Green Infrastructure. It is important that proper sampling and testing methods are followed to ensure test results are accurate and represent the actual soil. A composite sample (taken from several locations within the area of interest, to a consistent depth, and at the same time) can provide an appropriate representative sample for testing purposes. 

Where possible, preserve existing soils that have supported or are capable of supporting plant growth. Stockpiling existing soils for re-use is also a good strategy for reducing embodied carbon. Care should be taken to manage existing weed seeds that are typically present in the top layer of urban soils; tarp stockpiled soil on site if it is to be reused later or if new soil is to sit idle for an extended time before being used. If time permits, techniques like solarization or steam sterilization can be employed to kill weeds. Do not work the soil if it is frozen or sodden. Use pneumatic excavation, or similar methods, under the supervision of a consulting arborist, to preserve tree roots. Where new soil must be imported, pursue techniques to reduce the carbon impacts of associated transportation such as sourcing locally or utilizing more efficient means of transportation. For projects within NYC, consider using the NYC Clean Soil Bank. New soil should be installed in lifts and adequately tamped so as to not unduly compact the soil or create air pockets that could desiccate plant roots. For new soil, it is important that appropriate sampling and testing is conducted prior to delivery as well as prior to installation to ensure quality soil is being provided; soil tests should not be older than 6 months from the date of proposed delivery to the project site. Ultimately, soils are the foundation for a successful landscape and must be given special attention to ensure success.