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I am facing an unusual problem. The soil concentration of heavy metal cadmium (Cd) in my garden is high. Getting cadmium out of the soil is a tricky and long term process, and the quick solution is to make it unavailable to plants by using zinc fertilizer. Plant roots do not know the difference between zinc and cadmium due to similar size of hydrated ions, and they uptake any of the two.

I was hoping I could save the garlic plants I am growing by applying a zinc fertilizer (ZnSO4). Unfortunately, zinc is immobile in soils and I don't think broadcasting it between the garlic rows would make it available to plant roots.

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The alternative is to inject a solution of ZnSO4 directly into root system of the garlic plants using a syringe. The question is, what volume and concentration of Zn is large enough to make cadmium unavailable and at the same time is not toxic to the garlics?

Garlic cloves typical have 1-3mg of Zn per 100g, total soil concentration of Zn is 60mg/kg dry weight and total soil cadmium concentration is 4mg/kg dry weight. Any tips are welcomed. Even if you don't know the answer, please share your thoughts.

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  • I think you may be too late - Cadmium may already be in your plants. May 13 at 8:42
  • The photo looks like injecting into the soil. Where did cadmium come from ? Maybe you can stop it at the source . May 16 at 14:12
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Please take caution, cadmium is a very toxic pollutant both for the environment and human health. I have done (PhD) research on soil toxicity, and cadmium was one of the most toxic compound I analyzed. I would suggest not to consume any of the vegetables grown in this ground. Your zinc treatment sounds like a bad plan, I would advice not to take the risk of jeopardizing your own (and your family's) health.

Please remediate the soil properly (by professionals), or grow your vegetables in containers in the future.

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  • Thanks benn for taking your time. I agree with the cadmium toxicity. Fortunately the measured levels are pretty low (still over the limit). Unrelated question: The EU Cd limit for wheat is 0.2mg/kg fresh weight and for garlic it is 0.05mg/kg. A four time difference. It is very easy to eat 0.5kg of wheat in a day. On the other hand, it is very difficult to eat 0.05kg of garlic. What is your opinion on this regulation? Why is eating wheat with 4 times higher dosage of Cadmium considered ok?
    – sanjihan
    May 17 at 13:21
  • I am extensively looking for remediation techniques, phytoremediation to be more precise. As someone with PhD on soil toxicity, you must be aware of the existing plant hyperaccumulators (and extremely long time needed) for bringing total Cd levels to normal 1mg/kg dw. Can you suggest a few commercially available cultivars, suitable for removing Cd from the soil and their performance?
    – sanjihan
    May 17 at 13:32
  • Thanks for commenting @sanjihan The regulations (REACH) are always far below the real toxicity limits (just to be safe), so they should be considered with a grain of salt so to say. However, the nasty thing with Cd (and also for Hg) is that it is not an essential element used in the body, and it accumulates. That means it stays in your body and builds up over time. So therefore I would not chronically eat Cd containing food. Maybe once won't harm, but overtime (every year) is really something to avoid.
    – benn
    May 17 at 14:44
  • Phytoremediation is worth trying, @Colin has some nice suggestions. However, taking away the top layer of soil and replacing it with clean soil would of course be the quickest way. But might be expensive, I understand that. In time Cd will also leach into deeper soil, so in the meantime maybe using containers?
    – benn
    May 17 at 14:48
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The expert answer by @benn is interesting and authoritative. When combining garden plants with ingestion and animal health clearly there is a problem for the gardener in the presence of cadmium. However not all plants in a garden are destined to be eaten; some are destined for other uses not involving deliberately putting cadmium-rich plants in contact with digestive juices.

One example in a temperate context is flax which produces a fibre useful for clothing, household wares and cordage, or hemp which is used for production of manila rope. As such any cadmium taken up would be locked in for a period. In a tropical context we might think of sisal or one of the fibre-producing banana family (Musa spp.). Again the idea is to get the cadmium out of the ground and into something non-digestable.

In this case adding the suggested zinc would just delay the removal of the cadmium; better not add zinc so that the plants go straight for the cadmium.

Even better than clothing or rope would be incorporation into lumber destined for furniture or buildings. See if there is a market for composite boards using some kind of renewable resource. As always check with local authorities to ensure that what you propose to use the product for complies with any rules in place.

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