Heavy metals in fertilizer? Is fertilizer toxic to humans?

I recently saw a youtube video by “Learn Organic Gardening at GrowingYourGreens” called “Top 6 Worst and 6 Best Garden Fertilizers” (posted in October 2015). In this video, the presenter, John, says:
“In 2002 the EPA started allowing companies to basically put in toxic heavy metals into fertilizers without telling you guys and without any kind of labelling.”

Until seeing this video, I’d never considered that fertilizer could contain heavy metals which are known to be toxic to humans, dogs, cats and lots of animals.

I wanted to investigate this for myself, and to find out if in the UK and EU where I live, there is risk of heavy metals in fertiliser.

I discovered that there are heavy metal limits in fertilizer in the UK and EU. The EU Fertiliser Regulation from 2019 were transposed into UK Law.

The allowed limits are:

Contaminants in an organic fertiliser must not exceed the following limit values:
(a) cadmium (Cd): 1,5 mg/kg dry matter,
(b) hexavalent chromium (Cr VI): 2 mg/kg dry matter,
(c) mercury (Hg): 1 mg/kg dry matter,
(d) nickel (Ni): 50 mg/kg dry matter,
(e) lead (Pb): 120 mg/kg dry matter, and
(f) inorganic arsenic (As): 40 mg/kg dry matter

(For those wanting to see it in the original source, you can see the original document at this link, and this specific data can be found under the heading “PFC 1(A): ORGANIC FERTILISER” within Annex I.)

At first glance, I thought “120mg lead is allowed in 1kg fertilizer! I don’t want to grow my food in this much lead!” But if you think about it, you don’t grow your plants in 1 kg of pure fertilize: you grow them in soil and only add a handful or two of fertilizer.

Say you use 2 handfuls of fertilizer in a big pot: they say 1 handful is about 35g, so 2 handfuls equates to about 70g of fertilizer.

70g of fertilizer, containing the maxmimum allowed toxic elements, would contain:

(a) cadmium (Cd): 0.1mg / 70g fertilizer
(b) hexavalent chromium (Cr VI): 0.14 mg/ 70g fertilizer
(c) mercury (Hg): 0.07 mg/ 70g fertilizer
(d) nickel (Ni): 3.5 mg/ 70g fertilizer
(e) lead (Pb): 8.4 mg/ 70g fertilizer
(f) inorganic arsenic (As): 2.8mg/ 70g fertilizer

Then, you would be spreading this amount in your big pot, diluting it in, say, 30kg of soil, so the amount per kg of dry matter of soil would be quite small.

Say, 70g fertilizer in 30kg soil, so you’ now have 8.4mg lead, in 30.07kg of soil, which works out to 0.28mg lead per 1kg soil.

Then the plants don’t take up all the amount of heavy metal in the soil so the total amount getting into the plant is a small fraction of the total amount of heavy metals in the soil.

I looked up studies checking how much heavy metals potato plants take up from the soil and deposit in the edible tuber and found one study (Setiyo et al 2020) that the following uptake levels:

 Extrapolated from source: Setiyo et al. 2020, The concentration of heavy metals in the potato tubers of the basic seed groups examined by the variation of fertilizers, pesticides and the period of cultivation,  AIMS Agriculture and Food 2020, Volume 5, Issue 4: 882-895. https://www.aimspress.com/article/doi/10.3934/agrfood.2020.4.882?viewType=HTML

In another study by Zeliha et al 2017, the absorption levels were as follows:

Extrapolated from Source: Leblebici Zeliha et al. 2017, Accumulation and effects of heavy metals on potatoes (Solanum tuberosum L.) in the Nevsehir, Turkey, Fresenius Environmental Bulletin 26(12):7083-7090 https://www.researchgate.net/publication/320558572_Accumulation_and_effects_of_heavy_metals_on_potatoes_Solanum_tuberosum_L_in_the_Nevsehir_Turkey

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So, say you use 70g fertilizer containing the maxmimum allowed toxic elements in your 30kg of soil ; Based on the above absorption percentages, your tuber would at most absorb the following amounts of heavy metals from the fertilizer used:

• cadmium (Cd): 0.1mg (per 70g fertilizer) x 11,3% absorption = 0.01mg cadmium
• lead (Pb): 8.4 mg (per 70g fertilizer) x 17.1% absorption = 1.4mg lead.

And that’s the amount spread out between all the plant and all its tubers, so I’m not sure how much would be found in a single tuber. Maybe if one plant has 10 tubers, it would be 1.4 / 10, so that’s 0.14mg per tuber? or 140mcg.

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Now, let’s see if these levels are safe and to determine: is fertilizer toxic to humans?

For lead (Pb), the FDA suggests a limit of 3 µg per day for children and 12.5 µg per day for adults. (Source: https://www.fda.gov/food/metals-and-your-food/lead-food-foodwares-and-dietary-supplements under heading: “FDA Monitoring and Testing of Lead in Food”)

So 1.4mg lead in potatoes (grown with 70g fertilizer) would contain 1,400µg of lead, (or 140mcg per single potato) which would be pretty high and well above the recommended 3-12.5mcg limit.

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So for Lead: unless I made an error in my calculations (which is entirely possible since math is not my strongest subject, and please let me know in the comments if there’s something I miscalculated), based on this, it seems that it probably is NOT safe to add fertilizers to the soil if they are at the maximum allowed lead levels for UK & EU fertilizers :/

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I should mention that different plants have different tendencies of uptake of heavy metals.

One study I saw showed that tomatoes seemed to exclude uptake of heavy metals much more than other vegetables.  For example, Arsenic concentrations were highest in lettuce and green beans, lower in carrots, and much lower in tomato fruit, in this study: McBride 2013, Arsenic and Lead Uptake by Vegetable Crops Grown on Historically Contaminated Orchard Soils, Appl Environ Soil Sci.2013; 2013: 283472.  (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4776765/)  

In another study by Zhou et al 2016, they concluded that  leafy vegetables  are most susceptible to heavy metals and contaminated soils whereas melon vegetables absorb the least. (ref: Zhou et al 2016, Accumulation of Heavy Metals in Vegetable Species Planted in Contaminated Soils and the Health Risk Assessment,  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4808952/)

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I’m not certain, but it’s possible that the safest fertilizers to go for, less likely to have high levels of hevy metals, are ones made entirely from decomposed plants rather than those made from artifcial chemicals. One such example is the Organic Bronte range of fertilisers (Link: https://www.bronteheritage.com/) (Not sponsored to name them by the way).