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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.

"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.

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 actual 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.

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.