this page is a draft in response to the recent events. This page is meant to evolve and will be update as time allows, including with reference from scientific publication.
In the early summer (July 14th) we tested our compost (at the Wortman Community Garden), and the soil form the site on 50th st and 39th ave; the compost was not contaminated, and the native soil was below the level that is considered concerning.
We are building raised beds for several reasons, but the most important in this context, is that it takes the roots away from the native soil, even though the tests didn't raise concerns. Raised bed and compost can be very helpful as compost retains a lots of moisture and is rich in nutrients the plants need to grow, avoiding the use of other fertilizers. It is also more convenient to work with raised beds as one does not have to crouch as low to work with the plants. When necessary, the beds will be topped off with more compost, as compaction and other mechanism might affect the level in the beds.
Why do we want the roots away from the native soil?
In urban areas, there are often diverse contaminants in the soil, for instance lead - and again, event though the July tests showed no concerns, we preferred to be prudent by building raised beds. As most vegetables have relatively shallow, the raised beds solution we adopted allows for the roots to develop without reaching the actual level of the soil. That means that the plant would not be able to absorb potential contaminants from the actual soil of the park. More over, compost, and other organic materials like wood chips, also have other beneficial effects on potential contaminants, by immobilizing contaminants like the heavy metal Lead, reducing its availability, and therefore the potential intake by plants (which again here, would not be an issue anyway, as the plants are in raised beds).
Additionally, we had already in some places - and will continue to do so - added a layer of compost as a mulch (the surplus that didn't fit in the beds); and plan on covering the whole area between raised beds, with a combination of compost and mulch depending on availability. This further isolate it and protect the volunteers taking care of the garden beds, and immobilize potential contaminants.
For those interested in the science:
Both Compost and wood chips/dust (as well as other organic materials) have the ability to immobilize lead and other contaminants. Compost and wood chips are made like all living things from what we call organic matter. Organic matter is able to bind strongly to a number of compounds, making it so that it cannot be transported by water, absorbed by living organism and so-on. This mechanism is call "organic matter adsorption". In particular It has been shown that lead and other heavy metals can form chemical complexes with humic acids of the compost, effectively trapping the lead.
In a study with only 5% of compost added to soil, then spiked with up to 2000 mg kg-1 of lead, compost reduced by 6-10 fold the amount of extractable lead, it also reduced the amount of lead in lettuce edible part by 2-5 fold (4).
Compost is also rich in nutrients overall, including plant nutrients that also have positive effect on the immobilization of contaminant. For example, phosphorus is commonly used to reduce the availability of a number of metals, metaloids and heavy metals.
Cd, Cr, Mn and Pb Bioaccumulation was significantly reduced by the addition of compost in contaminated soils. In particular Vermicompost (compost produced using composting worms) was the most successful in reducing the bioaccumulation in radish. It also translated in a reduction in the Health Index risk below 1 in most cases, a health risk below 1 practically means no risk. To be noted, the composts where mixed with soils coming from mine tailing, their level of contamination is many folds higher than in urban soils (5).
(Another research showed that As, Pb, Zn, Cd, Cu, and Ni uptake by plants and leaching though the water was reduced by the application of compost (3) )
1. Environ. Sci. Technol.2015, 49, 8948−8958
2. .S. EPA. The Use of Soil Amendments for Remediation, Revitalization, and Reuse,2007. http://www.clu-in.org/download/remed/epa-542-r-07-013.pdf (accessed 10/02/2020)
(3. Environ. Sci. Technol. 2012, 46, 2, 1019–1027)
4. Water, Air, Soil Pollut.2002,140(1-4), 261−277.