Pressure treated wood is created by forcing chemical preservatives into the wood cells. These preservatives- creosote, pentachlorophenol and chromated copper arsenate salts – help make the wood resistant to decay by curbing fungus and insect attacks.
Although this is very appealing for raised bed construction, these chemicals leach into the soil and eventually into the plants.
There is an ongoing argument that a certain level of chromium must be in the soil before a plant can absorb these toxins. This would work the same way calcium and magnesium need each other in proper balance for a plant to absorb certain nutrients. But keep in mind, although these chemicals remain in the top few inches of soil, that is the area where our plants absorb most of their nutrients.
Pressure treated wood often takes on a greenish tint due to the copper residue. Lumber that is treated should be marked, but ask your dealer to be sure of what you are buying.
Also, stay away from old railroad ties and utility poles that are often covered in creosote. If you want to dispose of these items, take them to a hazardous waste disposal site. This practice also applies to any pressure-treated lumber. Burning it releases toxins into the air, which, could make you sick or worse if inhaled.
Building structures such as walkways or piers around ponds with pressure treated lumber can be a hazard for fish. Copper residue will kill fish. It is wiser to use a naturally resistant wood like cedar to make your raised beds and window boxes. You can also use stone, brick or cement block, but these can add lime, so check your ph often. So enjoy the benefits of raised beds and save the pressure treated wood for home construction.
Cindy Kerschner teachesRead more on backyardgardener.com