Elisabeth C. Miller Library

Gardening Answers Knowledgebase

Knowledgebase record #697

PAL Question

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.

Keywords: soil contamination, Soil testing, Plant-soil relationships, Wood preservatives
Date: 2009-03-28

Need an answer to your gardening question? Ask us directly!

Browse keywords or Search:

Keyword Search