Wednesday, March 16, 2016

Lead remediation in soil

October arugula

Late last year, our soil test results from Cornell University came back. The good: no cadmium, super-low arsenic (that is to say well below the minimum levels that send up red flags). The bad: high lead levels. The lead did not surprise me very much - we live in a highly urban environment, but my heart did sink: I grow plants you can eat. And we had been eating bushelsful of arugula up to that point. Nom-nom, nice lead for dinner. Or so I thought, as I freaked out.

The test also revealed a low soil pH of 5.4 - acidic (pH is short for power of hydrogen, if you've ever wondered). 7 is neutral. Above that the soil is alkaline.

As it turned out, pH is one key to lead remediation:

Low pH makes lead available for absorption by plants, although even then, most plants do not absorb much, if any: root crops the most, leaves next, and fruit - seed bearing parts of the plant - minimal, or none; this 2013 Cornell study was extremely helpful. All my leafy greens? "For lettuce, Pb [lead] levels remained well below the recommended limit even at a soil total Pb concentration of 915mg/kg." That is a lot of lead (we have 560mg/kg. The 'safe' level in NY state is 400mg/kg.)

Still, the lower the pH and the higher the lead levels, the more can be absorbed.

I clung to an early sentence in a less technical article: "Plants generally do not absorb or accumulate lead in quantities that would be of concern." The main risk of having lead in the soil comes in fact not from inside the plant, but from residue on the outside of plants that have not been rinsed very well - dust and grit. Small children crawling or playing in garden dust or soil high in lead, and gardeners who do not wash their hands are also at risk of ingesting lead. 

Thank goodness I wash my leaves well (in a large basin of water, rinse, and repeat)

November fenugreek

After my initial meltdown (when in doubt, panic -it's a great motivator), I read extensively and relaxed a lot. It took some time for me to come down from the very high branches of my panic tree, but now I'm leaning against its trunk, reading a book and sipping a cocktail. No firepersons required. 

Because there is a solution: raising the pH to make it more neutral (7 is neutral) makes lead unavailable to plants. I learned that garden lime was not what I wanted, because it also contains magnesium, and the soil test said the magnesium levels were fine. Excess magnesium stunts and even kills vegetables. So I chose crushed oyster shells, pure calcium, sent over from California by Grow Organic, a company that has the best packing practises I have seen (no plastic, no styrofoam - but they ship slowly, be prepared). 

December arugula and mustard

In November last year I applied 6lbs of crushed oyster shells (it looks like white dust) to our 100 square foot central plot in the garden, which I will be using for edible crops. I dug it in to about six inches. This application rate I gleaned from Steve Masley's Grow it Organically, an unusually helpful gardening site. 

What I do not know is how many points it will raise my soil's pH, which is key. Grow it Organically gives a points-rating for garden lime, but not shells. If my pH was 5.4 before 6lbs of oyster shells, what is it after? Stay tuned. The quality of the soil affects the change: the more organic matter in the soil, the smaller the change, because its acts as a buffer. We have lots of organic matter, says the soil test.

Dry egg shells

Over the winter I also collected egg shells, another excellent source of calcium (after they were dry I stored them in a large mason jar and stomped them down with a wooden spoon handle to make room for more).  

After handcrushing, into the food processor

I pulverized the dry shells in the food processor, and several months' worth yielded 1.5lbs of white powder. It takes several minutes to get the fine powder you want - and open the container outdoors, as there is dust, post whizzing.

Ground eggshells

 Recently I applied those to the sunniest end of the vegetable plot, where planting will begin. 

Eggshells applied, before digging in

Yes, I will have the soil tested again. And I advise anyone curious about what is actually in their soil to do so, too, because the results can be surprising. 

Raised beds are the ideal answer in a heavy metal situation, but that is not an option here, as the cubic feet of potting soil we'd bring in would cost a fortune, which cannot be justified in a rental situation. I could use phytoremediation - using very specific plants like ferns to suck up lead, before disposing (where do you toss them?). I could grow mushrooms - mycoremediation; very effective at removing heavy metals from soil. But I want shorter term results. Hence the sweetening of the soil.

I am immensely relieved to have learned more about soil science, and humbled by how much I still do not know. One could devote a lifetime to it.

Our seeds are arriving, monthly from Grow Journey and a small batch from Botanical Interests. Fava beans and spinach have already been planted in the double-calciumed rows. I am waiting on a fresh batch of oyster shells before planting the rest.

The adventure continues. The show ain't over till the fat possum sings.



  1. My local garden/Farm center sells (alongside fresh eggs) crushed oyster shells for the laying chickens to eat. I regularly mix the fine grit into my herb garden and into the potting soil mix I make up for rosemary and other woody herbs. They like it. I'm sure you could find crushed oyster shells locally--or closer to home. Where they raise chickens.

  2. Very interesting post. Thanks. I read years ago that crushed egg shells (not powdered) will kill slugs, of which we have many. It's hard to tell how well they work, but it's a good way to get rid of the egg shells and adds calcium to the soil. Another thing about slugs, they love to congregate under some solid surface. We discovered this by accident when we put some boards in the beds to stand on while working the soil. Turned them over the next morning and there were dozens of slugs attached to the undersides.

    Ooo, isn't it exciting to be thinking about planting again. Good luck with your soil experiments.

    1. ps. You've probably heard of Fedco Seeds, a seed coop in Maine. They have great variety and prices and sell all kinds of amendments and supplies in addition to seeds. Find them at They also have what they call a depot system (new this year), where they send truck loads of supplies to a local drop-off and customers can pick up their orders there. It saves quite a bit on shipping costs and probably time, too.

    2. Thanks for that source, Nancy. Interestingly, they are not cheaper for shipping at all (but maybe they are faster!). Now I am reading all about oolithic aragonite - their version of crushed oyster shells. Mined in the Bahamas...

    3. Too bad. I thought there might be a depot in NYC that would save you some money. If you like having seed catalogs in hand, they have a great catalog. No color pictures but wonderful illustrations and lots of extra tidbits on natural pest control, information on the seed growers and trials they do on the seeds they offer. Fedco was at the forefront of the fight against the powers that be on the national organic standards that were instituted a few years ago and the GMO fight, as well as the consolidation of the global seed trade. Hard working people and always very helpful.

  3. Replies
    1. Marie, I'm sure that I've read somewhere that the body actually can only absorb five percent of any lead that is ingested. The rest is excreted. Lead must be everywhere as most gardens are close to roads. I worry more about bought organic stuff which here in the UK, increasingly comes from China where twenty percent of agricultural land is said to be contaminated with heavy metals.

    2. Indeed. The "organic" certification requires no information about what your plants are sucking up from existing soil. Heavy metals totally allowed. It's perverse. The more I learn about growing food, the more leery I am of what I buy, food labels, and marketing.


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