Gardening Q&A: testing gardens for toxicity

Horticultural advice from the experts at the RHS
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Question: I have inherited a garden with the vegetable patch framed with train sleepers. A small area on the outer edge is leaking or has visible tar exposed. The rest has nothing visible, however the soil covers some of the wood.

I'm unsure how serious it is to continue to grow veges in the soil, especially because I have young children. Is there a way of testing the soil for toxicity or contaminants? Do you think the soil is subject to contaminants? If so do you have a solution to this problem?

Answer: This is a frequently asked question to our office at Wisley, and very understandably with both edible crops and small children around. It would be safest to say not to use such sleepers at all.

Since 2003, harmful chemicals have been banned in the production of railway sleepers, so without knowing how old these are, it’s impossible to say whether yours pose a risk. There is little evidence that contaminants leach outwards into soil, but to be on the safe side, replacement with an alternative would be best, along with replacing 10cm soil adjacent to the inner edge of the wood.

More modern wood preservatives are safe and normally bind strongly to the structure of the wood, so it is doubtful that they can leach into the soil. The tar that you describe is almost impossible to remove from clothes and hands; reason enough to replace with new.

Although testing samples of soil for contaminants is possible, it is rather costly and technically difficult. Certainly more expensive than several chunky new pieces of wood.

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