Growing on contaminated or doubtful soil

What to understand before starting

• Ideally, a soil analysis to guide the strategies.
• A budget proportionate to the desired level of works.
• Time: the structural solutions (beds) require a few days of work at the start.

Detailed steps

1. Step 1

   Favour low-accumulating crops

   Prefer fleshy fruit (tomato, courgette, cucumber, aubergine, pepper, squash), tree fruit (apple, pear, plum, cherry), grain cereals, grain legumes (bean, pea). Avoid or reduce: lettuce, spinach, chard, cabbage, beetroot, carrot, turnip, radish, potato (high accumulators, especially for cadmium). Special case of the potato: if eaten peeled, the exposure drops significantly.

2. Step 2

   Raise the pH if the soil is acid

   On acid soil (pH < 6), moderate liming (maerl, dolomitic lime, controlled wood ash) raises the pH towards neutral and reduces the bioavailability of cadmium, lead and zinc. Expect 100 to 300 g/m² of dolomitic lime depending on the initial pH. Repeat every 2-3 years. Do not overdose: too alkaline a pH (> 8) creates other problems (iron chlorosis).

3. Step 3

   Add organic matter massively

   Mature compost, RCW, composted manure, a thick permanent mulch: organic matter binds heavy metals by complexation. A humus-rich soil releases fewer metals to the roots. Target 3 to 5 % organic matter (an agronomic soil analysis measures it). Add 3 to 5 cm of compost to the surface each autumn.

4. Step 4

   Build raised beds on geotextile

   The most reliable structural solution: beds at least 30-40 cm high, the bottom lined with a heavy geotextile that isolates from the original soil (and stops deep roots going down into it). Fill with a loam / compost / vermiculite or perlite mix. Untreated wood for the sides (oak, chestnut, Douglas fir), definitely not CCA-treated wood. Over 100 € per 1 m² bed but durable for years.

5. Step 5

   Mulch permanently

   Mulch limits contaminated-dust splash from the soil onto the leaves by rain or wind. It also avoids contact between low fruit (courgette, strawberry) and the soil. Straw, hay, RCW, dry clippings, dead leaves: all valid. Renew as soon as the layer drops below 5 cm.

6. Step 6

   Wash and prepare the vegetables correctly

   For leaf vegetables eaten raw: soak for 5 minutes in water + 1 tablespoon of baking soda or vinegar per litre, then rinse. This removes the adhering dust. For root vegetables: scrub with a stiff brush under water, peel. The skin often concentrates more lead than the flesh.

7. Step 7

   Know when to give up

   Some productions are to be avoided on heavily contaminated soil even with adaptations: lettuce and spinach on soil above 200 mg/kg of lead; any crop for children under 6 if lead is above 300 mg/kg; food intended for sale. Ornamental flowers, potted aromatics with imported soil, and fruit trees almost always remain possible.

Concrete examples

• A small 50 m² urban garden with 250 mg/kg Pb: 4 raised beds for leaf vegetables + tomatoes in the open ground + gentle annual liming + permanent mulch.
• A 500 m² industrial wasteland with 600 mg/kg Pb in spots: geotextile over 30 cm of clean fill, planting of fruit trees and vines (low accumulators).
• A family home with 80 mg/kg Pb (below the vigilance threshold): diversified cultivation kept up, careful washing, no particular works.

Can hyperaccumulator plants clean up my soil?

Phytoremediation has been studied for 30 years (Thlaspi caerulescens for cadmium-zinc, Pteris vittata for arsenic, poplar and willow for various metals). Laboratory results are promising, but the time needed to significantly decontaminate a garden remains long (often several decades) and the effectiveness in an amateur garden is limited. To be seen as a complement to a strategy rather than a standalone solution.

Can biochar help?

Yes, moderately. Biochar (plant charcoal) holds back part of the metals, lead above all, then copper and cadmium, and working it in raises the soil pH, which holds metals more firmly on the soil particles (Rees, Simonnot and Morel, 2013, European Journal of Soil Science, INRA and Université de Lorraine). To be incorporated at a moderate rate. It is not decontamination, but a reduction of bioavailability.

Can I trust municipal compost on urban soil?

It varies. Waste-collection sites and green-waste collections in urban areas sometimes concentrate contaminated plants (roadsides, leaves with atmospheric dust). Ask the provider for an analysis sheet or favour home composting from controlled inputs.

And the earthworms, are they affected?

Worms accumulate metals but are relatively resistant to them. Their presence often indicates a living, functional soil, which is positive. The raptors and hedgehogs that eat them, however, can bioaccumulate in turn. That is one more reason to aim for low bioavailability through liming and organic matter.