The Walden Effect: Farming, simple living, permaculture, and invention.

Heavy metals in garden soil

Lead contaminationMost problems with garden soil can be remedied with judicious application of organic matter or other supplements, but heavy metals are more troubling.  Although humans, plants, and soil microorganisms need small amounts of many heavy metals, high concentrations can be toxic.  To decide whether you should be concerned, look for these possible sources of contamination near your garden:

The table below gives information on the seven heavy metals found in soil that are regulated by the EPA.  Of these, lead is the most likely to be found in your soil and is the one you should be most concerned about.

Heavy metal
Natural levels (ppm)
Unsafe for vegetable gardening (ppm)
Unsafe for children to play (ppm)
Arsenic (As)
3 - 12
more than 50
more than 200
Cadmium (Cd)
0.1 - 1.0
more than 10
more than 50
Copper (Cu)
1 - 50
more than 200
more than 500
Lead (Pb)
10 - 70
more than 500
more than 1,000
Nickel (Ni)
0.5 - 50
more than 200
more than 500
Selenium (Se)
0.1 - 3.9
more than 50
more than 200
Zinc (Zn)
9 - 125
more than 200
more than 500

You have to ingest heavy metals to get sick, which generally means eating plants that have sucked those heavy metals up out of the soil.  Luckily, plants don't tend to accumulate lead the way they do some other heavy metals, so you can garden in soil with moderately elevated lead levels as long as you don't eat much dirt.  At a lead concentration of 100 ppm, you'd need to eat two teaspoonsful of soil per week to create any problem; at 300 ppm, you'd need to eat 3/4 of a teaspoonful per week.
Urban garden
I wasn't particularly concerned about heavy metals in my garden, but the lab I sent our samples off to tested all of the problematic metals except for arsenic and selenium as a matter of course.  Mom was more worried since her front yard is right beside a busy city street and her backyard (home of her vegetable garden) isn't all that far away from car exhaust and road runoff.

Mule (1) Mule (2) CP3 (3) Back (4) Front (5) Mom front Mom back
Zn 8.3 7.8 12.5 7.5 8.5 12.5 28.9
Pb 1 1 1 1 1 3 7
Cd 0 0 0.1 0 0 0.1 0.3
Ni 0.1 0.1 0.1 0.1 0.1 0.1 0.1
Cu 1 1 0.9 1 1 1.1 1.2
Est. total Pb 37 38 40 37 37 64 108

Luckily, levels of all of the metals except lead were far too low to be of any concern for either of us.  In my garden, estimated total lead (different and quite a bit higher from the lab's values for measured lead) ranged from 37 to 40 ppm.  Mom's front garden --- right beside the street --- showed only a slight increase, with 64 ppm lead.  Strangely, her back yard had higher levels --- 108 ppm.  Since lead and other contaminants can flow downhill in water, I suspect that the lower ground level of her backyard concentrates any industrial pollutants even though it's further from the source.  Even so, it looks like she's pretty safe to keep growing her food in town.

Our chicken waterer keeps your flock healthy with clean water.

This post is part of our Holistic Soil Test Analysis lunchtime series.  Read all of the entries:

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About us: Anna Hess and Mark Hamilton spent over a decade living self-sufficiently in the mountains of Virginia before moving north to start over from scratch in the foothills of Ohio. They've experimented with permaculture, no-till gardening, trailersteading, home-based microbusinesses and much more, writing about their adventures in both blogs and books.

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Hey, the one thing you did not test for is aluminum in the soil. This is still a new test most places don't think about doing. Here are some links about this in our soil:

Comment by Ruthlynn Fri Nov 11 14:18:04 2011

We actually did test for aluminum, but I didn't have a clue what I should do with that data, so I ignored it. :-) Thanks for posting the links!

Our extractable aluminum levels were 6 and 7 ppm everywhere except in the chicken pasture where it was elevated to 14. The soil test had a note "Soil range 10-250 ppm", which I assume means that's the normal amount of aluminum in non-troubled soil. Sounds like we're in good shape (although the slightly elevated levels in the chicken pasture are probably due to the feed we've brought in for the flock. We need to grow more of our own chicken feed!)

Comment by anna Fri Nov 11 14:26:48 2011
A tempest in a tea pot. Thirty yrs ago, pseudo-science was used to convince the govt to remove Pb from paint & gas, thus decreasing our fuel mpg and causing us to use more fuel. While airborne Pb levels have plummetted to near zero, IQs of economically deprived kids haven't gone up even one point since then. The minimal toxic dose of As, for instance, for a young kid ( is about 6 mg. At the maximum safe level you noted above, about 20x the natural levels, a kid would have to eat about 66 lb of dirt to ingest that amount. Govt regulators have to come up with something they can regulate whether or not it really needs regulation or they're out of a job. Unless you're farming a toxic waste dump, I wouldn't worry too much about heavy metals. Keep things in perspective.
Comment by doc Mon Nov 14 07:26:41 2011
I haven't looked into the science of heavy metal poisoning, so I can't speak to that one way or another. However, I do know that the maximum safe level is vastly below the levels found in many gardens near industrial centers. And you have to keep in mind that heavy metals can build up in your body, so if a kid is out there playing in the dirt every day and eating a couple of tablespoons of soil, it would really add up. I think it can't hurt to test your garden soil for heavy metals at least once in the garden's lifetime and act accordingly.
Comment by anna Mon Nov 14 08:16:43 2011

The toxicity of lead has been well known since ancient times. Greek and Roman authors like Dioscorides and Vetruvius wrote about it. Even today there is no blood-level of lead that is considered safe.

The international chemical safety card for tetraethyllead (the well-known anti-knock agent) is not a fun read. I'd definitely put it on my list of stuff to avoid! If you want to know how dangerous this stuff is, check out "of dead bodies and dirty streets"; in 1924, 32 of 49 workers in a Standard oil refinery working with TEL were hospitalized and 5 died. Also of interest is an article on the research into anti-knock agents presented to the SAE in 1994.

Have a read through the references of the lead poisoning article on Wikipedia. Dismissing all the evidence for the toxicity of lead as "pseudo-science" boggles the mind.

We owe a great debt of gratitude to Clair Patterson, a geologist who was trying to determine the age of the earth by looking at uranium/lead ratios in rocks and found all of his samples contaminated with lots of lead. You can read the transcript of the Caltech oral history interview with Pattersone here. Note that he wasn't initially interested in environmental effects. He was researching the age of the earth. The number that he came up with in the 1956 still stands today.

Comment by Roland_Smith Mon Nov 14 15:34:19 2011
I was really hoping you would weigh in --- thanks, Roland! I didn't have time to do real research, but my gut reaction was that lead toxicity was extremely well documented and nowhere near pseudoscience. Thanks for doing the research!
Comment by anna Mon Nov 14 15:57:00 2011
The problem of leaded gasoline is interesting: Pb chelates four short-chained HCs not completely oxidized during combustion in an ICE. That forms a rather large molecule essentially too big to be absorbed very well into the blood stream when inhaled. It's just exhaled again without causing any damage. Unchelated when burning unleaded gas, the four short HCs now can be absorbed by lung tissue and potentially act as carcinogens. Lead poisoning in the US is found in less than 2% of all kids <6y/o, but is defined by an arbitrary blood level, not by the appearance of symptoms or recognizable pathology. Symptomatic lead poisoning is almost non-existant in the US. And let's not extrapolate breathing tetraethyl gasoline 40 hrs a week for years on end by industrial workers to eating a few home grown tomatoes for two months of the year. The HC exposure in the workers was surely more damaging than the lead exposure anyways.
Comment by doc Mon Nov 14 20:37:54 2011
Maybe you can give us some insight on this?
Comment by anna Tue Nov 15 17:18:19 2011

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