The Loam Ranger – Verge veges

Loam Ranger logoSESL Australia’s friend Gil Teague, who runs the Florilegium Bookshop, wrote:

I’ve wondered for some time now as to whether food grown on street verges gets contaminated by car fumes.

Have you ever been asked to run soil tests on food grown thus? Are there other studies available? I guess now that lead has been removed as an additive from petrol, that lowers the possibility of lead contamination, but it’s possible there is remnant lead in the soil being used, and that there are other contaminants. I’d appreciate any light you can throw on this matter.

We come across this issue all the time. Certainly advocates of food grown on verges should have the contaminant levels in their soil measured for a range of heavy metals before planting. Some elevation above “natural background” levels is acceptable, and there are guidelines to help here (see “Further reading” below).

When levels become “borderline”, however, my advice is not to grow and eat root vegetables or specific accumulators such as silver beet and spinach. (Accumulators are plants that, for one reason or another, take up very large amounts of normally toxic elements from the soil.)

Apart from lead (from paint and petrol fumes), inner city sites can be contaminated with arsenic, cadmium, mercury, zinc and copper. These last two are very interesting as they are not toxic to humans in virtually any conceivable amount coming from soil but they can be toxic to plants, more so in fact than lead! Zinc toxicity is the most widespread thing we see in inner city soils: plants simply cannot grow on account of the elevated zinc content.

The other potential issue is hydrocarbons, particularly polycyclic aromatic hydrocarbons (PAHs). Hydrocarbons are very difficult to measure in plant materials as all plants contain natural hydrocarbons. PAHs come from diesel exhaust and can be measured. They are very insoluble, however, and if properly washed, even leaf vegetables should be OK. Vegetables that are peeled present no problems.

We’ve covered a similar question before:

There is a general rule that heavy metals (HMs) do not get into fruiting structures easily, and hence grains and fruit are nearly always protected.

The second rule is that phytotoxicity prevents ecotoxicity. This means that if a plant were to take up enough HMs to cause harm to grazing animals or any animal that ingests the foliage, the plant would be dead and hence the grazer would be protected anyway. This is not quite true with chronic high-level intake (eating the same contaminated food every day for months or years).

These two mechanisms explain why HM toxicity in humans from eating food grown on contaminated soils is uncommon. The only exception is where leaf vegetables like spinach or silver beet (and root crops like potato) grown on lead-contaminated soils can contribute to the overall dietary input of a young child. This is secondary, however, as pica (the direct eating of soil) is the main dietary input in infants.

Fertile Minds readers might also have been following Costa Georgiadis on the ABC’s Gardening Australia as he has turned the verge in his street into a productive garden. In episode 16, he recommends “An important precaution is to have your soil tested for heavy metals or chemical residue, particularly if you want to plant food.”

Further reading

Australia New Zealand Food Standards Code – Standard 1.4.1 – Contaminants and Natural Toxicants (lists maximum permissible limits of heavy metals in foods).

National Environment Protection Council. 1999. Assessment of Site Contamination NEPM [National Environment Protection Measure] (includes “Investigation Levels for Soil and Groundwater”).