Extension connection: Be cautious when planting

Published 12:00 am Wednesday, April 30, 2008

By TONY GLOVER / Guest Columnist

Question: I have removed an old deck in an area where I want to plant a vegetable garden.

I have been told there are unsafe materials that could have leached into the soil from the treated wood.

What suggestions do you have to remedy this situation?


On Feb. 12, 2002, the Environmental Protection Agency (EPA) announced a voluntary decision by the wood preserving industry to phase out the use of wood preservatives that contain arsenic for any wood products destined for consumer use.

This affects virtually all residential uses of wood treated with chromated copper arsenate, also known as CCA, including wood used in play structures, decks, picnic tables, landscaping timbers, residential fencing, patios and walkways.

The EPA has not concluded that there is unreasonable risk to the public from CCA lumber, but believes that any reduction in exposure to arsenic is desirable.

There is still some concern among vegetable gardeners and those who have small children. It can be very expensive to have soil tested by a private lab, but Auburn University offers this service for a modest fee of $15 per sample.

They can test for chromium, copper and arsenic. A soil test form is available at any extension office in the state. The normal soil test for plant nutrients cost only $7, so make certain you request this special test.

Exposure to these elements can only occur if they move from the treated wood into soil or compost that is in contact with the wood, then are taken up by plants, and finally are ingested by humans. This is known as an “exposure pathway.”

Low concentrations of arsenic, chromium and copper occur naturally in water, soil, plants and the human body. Copper and possibly chromium is essential for plant nutrition, and all three of these metals are essential for human and animal nutrition.

Intake of excessive amounts, however, can have adverse effects on plants and humans.

On the positive side, both copper and chromium are held very tightly to soil particles of clay and organic matter when the soil pH is maintained around 6.5 on the pH scale. This is also the best pH for vegetable growth.

Arsenic is much more mobile within the soil and can be taken up into the plant roots and leaves. Most metals remain in the roots, with limited movement to edible portions above the ground.

There are exceptions, of course: leafy green vegetables such as lettuce, spinach and mustard greens tend to move arsenic from roots to leaves.

In general, however, the greatest human consumption of metals results from eating root crops such as beets, turnips, carrots and potatoes. Most of the metals remain in the surface skin and can be removed by peeling.

The ability of some plants to help take bad things out of the soil is called phytoremediation and a sun tolerant fern called brake fern has a remarkable ability to remove arsenic.

The arsenic accumulates in the foliage and would require the gardener to remove the foliage and dispose of it off site or they are just recycling the arsenic. To read the low down on this remarkable fern, visit http://npic.orst.edu