Do chemicals leach from the sides of raised beds?

Question

Around a year ago I went to a local hardware store to buy some treated pine logs to make a raised garden bed. The guy at the store asked if I was using it for a vegetable bed and he explained that chemicals from the treated pine would leach into the soil and I should use a special type of wood specifically designed for use in garden beds.

I have since abandoned the idea of purchasing from the hardware store and currently I only have one raised bed which I am using for potatoes which I have built up using an old bookshelf I picked up for free and knocked the back out. This is made out of what I assume is melamine coated particle board.

Is there research around where leaching of wood materials has been properly researched and documented?

What healthy as well as sustainable / recycled / reusable solution should a person use when building a raised bed? Note: This doesn't necessarily need to be wood it could be something like recycled brick.

Answer 1

Mdf tends to fall apart in the presence of moisture. After a while exposed to wet soil or soil with a clay component it returns to being wood chips and paper. What happens to the formaldehyde resins that bind it together and the urea formaldehyde that is slowly released is a good question.

This detailed article from Fine Gardening explains that since 2003 in the United States pressure treated wood has used copper based compounds instead of arsenic. Details on the EPA's policy can be found here. Similar legislation is in effect in Europe, Australia and Canada. Theoretically food plants will die before they absorb toxic levels of copper from wood produced after 2003 and this is only likely to happen in soils low in phosphorus.

Even though I do not aspire to have my garden certified as organic which requires no pressure treated wood I do not use it as there are many alternatives.

• nothing- just bank the soil
• stone - highway construction sites are usually happy if you take away blast rock they would otherwise haul away. (ask first if you are not sure!)
• cedar contains natural preservatives which lengthen it's lifespan
• recyclers can provide breezeblocks, paving slabs, old bathtubs
• shipping pallets are readily available. They make good firewood or excellent retaining walls

Bricks can be problematic. Most house bricks are only fired so they are waterproof on one side. When exposed to constant moisture and winter/summer they flake and spall in a few years. Interlock bricks are cement and can be used.

Answer 2

Your hardware store guy is right — chemically treated wood will leach into the surrounding soil. Lumber is typically treated with Chromated copper arsenate (CCA) to preserve it. The oxides and salts of the three metals — chromium, copper and arsenic — together provide as an effective fungicide and insecticide and also as a barrier against termites. From the wiki article:

CCA is known by many trade names, including the worldwide brands "Tanalith" "SupaTimber" and "Celcure". The chromium acts as a chemical fixing agent and has little or no preserving properties; it helps the other chemicals to fix in the timber, binding them through chemical complexes to the wood's cellulose and lignin. The copper acts primarily to protect the wood against decay fungi and bacteria, while the arsenic is the main insecticidal component of CCA.

These chemicals can, over time, leach into the surrounding soil and potentially even contaminate the water table. Here is some information from the National Pesticide Information Center

Chromium, copper and arsenic can leach into soil or water when wood treated with chromated copper arsenate (CCA) is exposed to the environment. Many factors can affect the amount of leaching that occurs from treated wood. Such factors include how long the wood has been exposed to the environment, the size and type of wood that was treated, whether the wood is coated with a sealant, water movement, and the type of soil. The chemicals that leach from CCA-treated wood can accumulate in soils near the wood, but under certain conditions, the chemicals can travel farther. In general, CCA chemicals are least mobile in organic soils, slightly more mobile in clay soils, and most mobile in sandy soils or water.

You can use untreated wood to create your raised beds. One way to tell untreated wood apart from treated is the absence of a greenish tint (mostly due to the copper in CCA). Wood that is used to make palettes and box crates are generally untreated (so raw that they don't even bother finishing it). However, you should be careful when working with them because they are so rough that you could easily get splinters. Best to get a few planks and run them through the planer if you have one.