I would like to convert my son’s old built-in sandbox into a vegetable garden. It is located in a part of our backyard where it gets a lot of sun each day. It was built with plywood at the bottom.
Q: Do I need to remove all of the sand and does the wood bottom need to be removed?
It’s great that you want to re-purpose your old sandbox space and grow vegetables, especially if you’ve got a location that has ideal sun exposure.
The main consideration is to determine what to do with the existing wood and sand. You mention the sandbox is “old” and the bottom is made of plywood. Since you say it is “built-in”, then I’m assuming the sides are constructed of wood as well. Unless you know exactly when and how the wood was manufactured and/or treated, this is where you need to focus.
Lumber (including plywood) that is manufactured to withstand contact with soil and organic matter, over a long period of time, is treated with chemicals to preserve it. Until 2003, lumber was most commonly treated with chromated copper arsenate (CCA), which is composed of the metals chromium, copper and arsenic. Due to concerns about the safety of CCA or, more specifically, the arsenic in it and potential health effects from exposure, the lumber industry phased out the use of CCA for residential building products.
There is inconsistent data on exactly how much CCA leaches into the soil from CCA-treated lumber, how long it persists, and how far it spreads once in the soil. Soil acidity, duration of wood-to-soil contact, and degree of wood-to-soil contact are all contributing factors. The majority of leached metals are bound to organic soil components, with only a small amount available to be taken up by plants. In general, plants store most of these metals in their root structures, followed by leaves/stems and, lastly, fruit.
While it is generally accepted that the potential health risks from growing edible plants in soil that has come in contact with CCA-treated lumber is very low, there are measures that can be taken to ensure the lowest possible risk. A full list of these measures can be found in the final section of the resource Environmental Soil Issues: Garden Use of Treated Lumber, created by PennState Extension.
In the case of growing vegetables, one of the most important measures is to grow them at least 12 inches away from CCA-treated wood. Unless you are certain that the wood used to construct your old sandbox was NOT treated with CCA, the best approach would be to assume that it was. This means you should remove all the sand and existing wood from the sandbox, and assume that the soil beneath and surrounding the sandbox could potentially be contaminated. If you are able, remove underlying/adjacent soil to a depth/perimeter of one foot (12 inches). This may sound extreme but, since you plan on growing vegetables for consumption, this is the safest advice we can provide. You may wish to demolish and then rebuild the bed using new lumber.
If you are able to confirm that the sandbox lumber was NOT treated with CCA, it is still a good idea to remove the plywood bottom. Many vegetables require at least 12-18 inches of soil depth for proper root growth so, by removing the plywood, you will ensure the plants’ roots can extend deep into the soil. You can also see what type of native soil you are dealing with beneath the plywood. If it is heavily compacted, you can loosen it with a pitchfork and work in some of the new soil that will be used to fill the remaining depth.
Removal of all the sand is advised too. Sand is free-draining, does not retain moisture or provide adequate nutrients for vegetables to grow properly. A new soil medium could include a mix of topsoil, organic matter (ex. compost, well-rotted manure, mushroom compost) and peat, perlite or another soilless component. For exact proportions and more information on creating a new vegetable bed, the Raised Bed Checklist from the University of Illinois Extension provides helpful information for raised or in-ground vegetable gardens.
For general information on growing a vegetable garden, please check out these additional resources:
Vegetable Gardening Basics (University of Illinois Extension)
Organic Vegetable Gardening (Toronto Master Gardeners)
Planting the Vegetable Garden (University of Minnesota Extension)