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Elisabeth C. Miller Library

Keyword: Soil contamination

Safety of gardening in arsenic-affected surfaces

This is about composting from raised beds constructed of railroad ties. I went to a workshop on growing edible plants, and was informed that one cannot eat anything grown in a railroad tie bed because of arsenic and other nasties, and if one has such beds, they should ONLY BE USED for ornamentals.

I try to compost everything in my garden, so I need to know if it is safe to use compost made from plants growing in railroad beds on the beds where I am growing edibles. If it is not safe, would time, weather, or decomposition EVER make it safe? I’m willing from now on to put all the soil-contaminated clippings in the city yard waste bin that goes to Cedar Grove, but I’d rather be able to make use of them in my own garden.

 

Your question about the safety of compost made from plant matter grown in a railroad-tie bed is complex. Railroad ties are treated with wood preservative that contains arsenic. Arsenic never goes away entirely, but the amount may be at lower levels than Washington State’s law on clean-up, based on parts-per-million. I would definitely recommend a soil test. Here is information from the Northwest Coalition for Alternatives to Pesticides, which has a page on chromated copper arsenate.

The Center for Disease Control has published a Public Health Statement on arsenic, excerpted below:
“About 90% of all arsenic produced is used as a preservative for wood to make it resistant to rotting and decay. The preservative is copper chromated arsenate (CCA) and the treated wood is referred to as ‘pressure-treated.’

Arsenic cannot be destroyed in the environment. It can only change its form, or become attached to or separated from particles. It may change its form by reacting with oxygen or other molecules present in air, water, or soil, or by the action of bacteria that live in soil or sediment. Arsenic released from power plants and other combustion processes is usually attached to very small particles. Arsenic contained in wind-borne soil is generally found in larger particles. These particles settle to the ground or are washed out of the air by rain. Arsenic that is attached to very small particles may stay in the air for many days and travel long distances. Many common arsenic compounds can dissolve in water. Thus, arsenic can get into lakes, rivers, or underground water by dissolving in rain or snow or through the discharge of industrial wastes. Some of the arsenic will stick to particles in the water or sediment on the bottom of lakes or rivers, and some will be carried along by the water. Ultimately, most arsenic ends up in the soil or sediment. Although some fish and shellfish take in arsenic, which may build up in tissues, most of this arsenic is in an organic form called arsenobetaine (commonly called ‘fish arsenic’) that is much less harmful.”

Washington State University has information on gardening on arsenic- or lead-affected soil which may be of interest to you.

To be cautious, you should keep the compost from these beds separate from your other compost, and only use it on your ornamental plants already being grown in those beds. I don’t recommend putting even slightly arsenic-contaminated yard waste into the city compost, since that means the problem is being spread farther afield. It would be worthwhile testing every so often for soil contaminants. Here is King County Public Health’s guide on arsenic, and testing information.

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Colored bark mulch

We are planning to put down a seasonal mulch this spring and fall, and wondered what your opinion is of colored bark. Would brown, black, red, or some other color be best in our ornamental beds?

 

May I answer none of the above? There are better options. I can’t think of any good reason to apply mulches which have added dyes and colorants to any landscape. Check the Material Safety Data Sheets for each product to verify the source of the dye to be sure that they are not going to cause harm (to the landscape and the people and animals in it, and to stormwater). Some packaged colored mulches also have herbicides added, and I would highly recommend not using them. The sources of the “bark” may be an even more important cause for concern. This information from University of Massachusetts Extension points out that dyed bark mulch made from recycled treated wood can introduce toxic substances you would not want in your garden.

Although this is an aesthetic judgment call, to my eye, colored mulch always looks unnatural in the landscape compared to materials such as compost, leaf mulch, and arborist wood chips. Washington State University Extension professor of horticulture Linda Chalker-Scott is a strong advocate for the use of wood chips as mulch. My own observation is that bark mulch (in general, not just the colored bark products) often introduces weeds into a landscape. Chalker-Scott supports this observation in her book, The Informed Gardener (University of Washington Press, 2008):
“I have seen a number of landscaped sites where applied bark mulch immediately gave birth to horsetail seedlings.”

She further states that bark mulch from trees which have been kept in salt water can increase salt levels in your soil. Tree bark has a waxy covering, so bark mulch is not the best choice for absorbing and releasing water.

To summarize, unless you have your heart set on the look of dyed bark mulch, your garden beds would benefit from the alternative mulch materials (compost, leaf mulch, free arborist wood chips) mentioned above, plus planting an appealing and naturally colorful selection of ground cover plants in areas where that is possible.

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Growing edible plants in galvanized containers

Is it safe to use galvanized containers (in this case a large
galvanized tub like the kind used for watering livestock) in which to
grow root vegetables, herbs, etc? I’ve seen some sites sell these for this
use, but there are also some postings referring to the potential for ill
effects of anti-rust coatings.

 

You may want to consider both what goes into the galvanizing process, and what could be leaching out of containers as the coating wears down over time.

Here is information on the process of galvanizing from the American
Galvanizers Association. Excerpt:

“What are the steps in the galvanizing process?”

  1. Pre-inspection – where the fabricated structural steel is viewed to
    ensure it has, if necessary, the proper venting and draining holes,
    bracing, and overall design characteristics necessary to yield a quality
    galvanized coating
  2. Cleaning – steel is immersed in a caustic solution to remove organic
    material such as grease and dirt, followed by dipping in an acid bath
    (hydrochloric or sulfuric) to remove mill scale and rust, and
    finally lowered into a bath of flux that promotes zinc & steel reaction
    and retards further oxidation of the steel… (steel will not react with
    zinc unless it is perfectly clean).
  3. Galvanizing – the clean steel is lowered into a kettle containing 850
    F molten zinc where the steel and zinc metallurgically react to form
    three zinc-iron intermetallic layers and one pure zinc layer

Based on the above, one concern would be whether the zinc would be
harmful. Zinc is one of many nutrients needed by plants, but I couldn’t
hazard a guess as to what effect the zinc from the coated steel would
have, if any, or whether the galvanizing process involves other
substances.

The book The Edible Container Garden by Michael Guerra (Fireside, 2000)
says the following:

“Galvanized buckets are increasingly popular but don’t use them for
ericaceous or acid composts.” (This would be a compost which is
lime-free. Usually soils in the Pacific Northwest tend to be acidic. I
don’t know from your message what part of the country you live in, but
this might be something to consider as well).

Another issue is that the metal containers will probably heat up quickly,
meaning that your plants might need more attentive watering.

Los Angeles County Cooperative Extension offers the following information, in an
article on trace elements and urban gardens. Excerpt:

“Cadmium is a contaminant of many manufactured products containing zinc.
Any zinc plating or galvanizing operations and galvanized metal
containers sometimes used in horticulture and gardening operations are
potential sources of cadmium.”

I certainly don’t think you want to grow vegetables in a cadmium-laced
container. In situations where there is any doubt about safety, I would
recommend growing ornamental plants in the tubs, and growing edibles in
untreated wood or clay pots.

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using treated wood and other alternatives

We are thinking about putting in a retaining wall and a fence on our property, which is near a lake. Should we avoid using pressure treated wood? If so, what are some alternatives?

There are many reasons not to use treated wood for your fences and/or retaining walls. The chemicals most used to preserve wood—creosote (on railroad ties, among other things) and penta—were banned by the EPA in 1986 for indoor use and for many outdoor uses. The chemical used to pre-treat wood (CCA, a mixture of copper, chromium, and arsenic called chromated copper arsenate) has been shown to leach into the soil and to transfer to human skin through contact.

There are safe paints and preservatives for coating wood; there are safe types of pre-treated wood; some people use stone, cement blocks, or other materials instead of wood.

Below is lots of info about treated wood and alternatives.

Start with the page on the EPA site, which is full of information on treated wood. It includes a section on alternatives and some questions and answers about studies.

If you find this too technical, try the next two links below.
The Natural Handyman website has good information.

Toxic-Free Future (formerly known as Washington Toxics Coalition) has a fact sheet about safe and unsafe paints and wood preservatives. Lots of background information on the toxicity of treated wood is included as well.