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I had the soil in one of my garden beds tested for a wide spectrum of minerals. This bed was originally filled with bought topsoil and mushroom compost, then it was cultivated with parsnips and spinach, finally I have added some homemade compost. The results show an excess of Zinc, with potential toxicity.

(1) What could this be due to?

(2) How do I avoid problems for future crops? I am still eating the parsnips from that bed, and have seen no problems whatsoever so far. I am planning to plant tomatoes and courgettes in May.

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"Toxicity" is about plant. Some will not growth well. It is not about human toxicity (which depends on plants).

Mushroom could have more zinc, in general compost could have more metals (and if you used also ash you get more). Compost concentrate stuffs (but water and carbon). Because metals are used on very few quantities, it could add. Topsoil could be also a problem (what were the previous use). Are you eating a lot of food rich on zinc? As you see from your analyses, you have a very fertile soil (too much?). Maybe you are using too much compost.

You may check if zinc is frequent in your region.

It is difficult to find the cause.

I would not really worry about food, if you keep a variegated diet (and you do not do mono-culture). Plants should not store too much of it (mushroom are worse, but it depends on species).

About plant toxicity. Possibly your have not the extreme of toxicity. From a short search, it seems tomatoes could have some problems. But I would expect not more than just tip of leaves being yellow (I would expect nothing), and not dead tomato plants.

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  • May the use of pressure treated softwood have something to do with this?
    – usumdelphini
    Mar 16, 2019 at 15:18
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Zinc is tricky, soil can contain a higher ppm than what you might want (10 ppm or so) without it being a problem because a good portion of the zinc is being held within the iron and oxides which is actually not available to the plants. Soil pH will contribute to dictating zincs accessibility/solubility where a higher pH equates to a lower solubility. For instance, bed 1 could be more likely to have a zinc uptake problem than bed 5 because bed 1 has a lower pH than bed 5. You may be able to bring down the zinc concentration by switching to a phosphorus based fertilizer.

To address your first question. It might be possible that the mushroom compost is comprised of mushrooms which wear heavily treated with a zinc based fertilizer? It is hard to say for sure.

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The front bed looks unusual; high pH, high metals Zn, Cu and Fe. Looks like some contribution from domestic scrap metals. High pH and phosphorus could be TSP used to clean something. Maybe grow something like annual rye grass for a season to let it rest or mellow. I use TSP as a fertilizer for P but I have a very acidic soil so the alkalinity is no problem for me.

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    There is a lot of calcium, so probably the soil is limestone, so high pH.
    – Giacomo Catenazzi
    Mar 13, 2019 at 20:51
  • And potassium and magnesium would likely add to the high pH. Strange mix because K is so water soluble, I doubt that K hurts anything but it should wash out with rain.
    – blacksmith37
    Mar 14, 2019 at 19:16
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One possibility given the high iron content as well is that lots of galvanised metal has ended up there. This could be a patch that's been used as a dumping ground in the past (high copper as well), perhaps only for short periods such as during renovation work indoors, and perhaps quite a long time ago.

High iron and zinc could also have been someone burying nails to try to try to get blue hydangeas (that needs aluminium not iron, but people used to try iron). Fencing wire can be galvanised steel; if buried that would add both iron and zinc to the soil (e.g. a chicken run with chciken wire buried to try to stop foxes).

I'm not an expert on soil modification, so I'd just remove any obvious bits as I went along, and add organic matter. I might avoid growing large quantities of staple foods, especially root crops, in that bed just in case - but even then I'd be more worried about quality than safety.

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  • That is actually very likely, given the site. We have found some scrap metal digging. Not much, but enough to corroborate your theory. Any suggestions on how to tackle this? If any is needed, obviously.
    – usumdelphini
    Mar 14, 2019 at 20:21
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    I'm not an expert there, I'd just remove any obvious bits as I went along, and add organic matter. I might avoid growing large quantities of staple foods, especially root crops, there just in case - but even then I'd be more worried about quality than safety.
    – Chris H
    Mar 14, 2019 at 20:28
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Just remove any obvious metals from your garden and don't worry overmuch about the soil test result. It does not indicate plant available zinc. I farm next to a steel mill and my soil test results are double yours, a known source. Never been a problem as the natural biome handles it OK. Just keep adding that compost each year to keep the balance underground at the correct level. Happy gardening!

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