Heavy metals in the garden

Explanation

Before planting, it is better to know what lies beneath your feet. Some urban or formerly industrial soils still carry the trace of past pollution, lead paint from before 1949, leaded petrol until its French ban in 2000, old factories, old orchards treated with lead arsenate. These metals do not disappear: they accumulate. And some of our finest crops, lettuces, carrots, cabbages, are particularly good at concentrating them in their leaves or roots.

Key points

• Where heavy metals in a garden come from

  The sources are of human origin far more often than natural. Lead: white-lead paints (banned for indoor use in France in 1949) flaking off around old houses; leaded petrol (banned in France on 1 January 2000) that deposited lead along major roads for 60 years; community-garden soils on former industrial wasteland. Cadmium: industrial phosphate fertilisers (depending on the origin of the raw phosphate), batteries, former metallurgical activity. Arsenic: CCA-treated wood (chromated copper arsenate, banned for consumer use in Europe in 2004), old arsenical orchard pesticides (until the 1950s). Mercury: old agricultural fungicides. Copper and zinc: repeated Bordeaux-mixture treatments (vine, tomato, potato).

• How they behave in the soil

  Heavy metals are partly fixed by clay and organic matter (a protective effect), and released by soil acidity. General rule: acid soil = more bioavailable metals, therefore more easily taken up by plants. Chalky soil (pH > 7) = less mobile metals. This is why moderate liming is a classic strategy on acid contaminated soil. Sandy soils retain them poorly, clay soils better but may release the metals suddenly if the pH shifts.

• Not all plants accumulate the same way

  Plants are not equal when facing heavy metals. Strong accumulators (to avoid on doubtful soil): lettuce, spinach, chard, cabbage, beetroot, carrot, potato, turnip, radish, parsley, fennel, especially for cadmium, which is very mobile. Weak accumulators (preferable): fleshy fruits (tomato, courgette, cucumber, aubergine), tree fruits (apple, pear, plum), cereal grain (the grain is better protected than the leaf), grain legumes (bean, pea). A special case: hyperaccumulator plants such as Thlaspi caerulescens (cadmium, zinc) or Pteris vittata (arsenic) are studied for phytoremediation, but their effectiveness in the amateur garden is limited.

• The official tools to investigate your ground

  France has several free public databases: Géorisques (georisques.gouv.fr) maps all the risks on a plot; BASOL lists known polluted sites and soils; BASIAS inventories former industrial and service activities that are potentially polluting. Before buying land or starting an urban vegetable garden, these three tools let you check the history of the site in 10 minutes. Complete them with observation: age of neighbouring buildings, distance to roads, presence of wasteland, flaking paint.

• When and where to have your soil tested

  A soil analysis for heavy metals generally costs between €60 and €200 depending on the number of elements tested. In France, laboratories such as SADEF (Aspach-le-Bas), Auréa AgroSciences and INOVALYS offer this kind of analysis. Ask at least for lead, cadmium and arsenic. When should you have one done? Before an urban vegetable-garden project; after buying land whose history is unknown; when you plan to feed children or pregnant women with your produce; when a neighbour has had a test done and found something.