Best Practices for Gardening in Contaminated Soil

File this ad under "bad ideas."

File this ad under “bad ideas.”

The benefits of growing food in cities (nutrition, exposure to nature) outweigh the very low risks of lead contamination. This is the conclusion of a recent University of Washington study:

It is highly unlikely that urban agriculture will increase incidences of elevated blood Pb for children in urban areas. This is due to the high likelihood that agriculture will improve soils in urban areas, resulting in reduced bioavailability of soil Pb and reduced fugitive dust. Plant uptake of Pb is also typically very low. The exceptions are low-growing leafy crops where soil-splash particle contamination is more likely and expaneded hypocotyl root vegetalbes (e.g. carrot). However even with higher bioaccumulation factors, it is not clear that the Pb in root vegetables or any other crop will be absorbed after eating.

The paper outlines a set of best practices for dealing with contaminated soils. You should always:

  • Wash fruits and vegetables from your garden. Also wash your hands and don’t wear shoes in the house.
  • Compost, compost, compost! Compost dilutes the overall amount of lead in soil and encourages healthy plant growth which also dilutes the (usually small) amount of lead a plant will uptake. Apply compost annually since it breaks down over time.
  • Plant away from painted surfaces of old (lead paint era) buildings.

Some other things to consider especially if you garden with children under 5 years old:

  • Don’t eat a lot of carrots, radishes, redbeets or turnips grown in contaminated soil. The edible portion of these plants consist of xylem tissue that traps lead (potatoes are OK since they are phloem fed). That said, you’d have to eat a lot of contaminated root vegetables to elevate lead levels in your blood.
  • Adding phosphorus fertilizers will decrease the bioavailability of lead.
  • In general root vegetables uptake the most lead, leafy greens less and fruit almost none.
  • The most conservative approach is to grow in raised beds.

The main concern is for children under 5 years old. If your soil tests high in lead and you have young ones you should consider doing your edible gardening in raised beds and limit their exposure to garden soils to a few times a month. Mulch and ground covers will also decrease lead exposure in the other parts of your yard.

The paper concludes that for older children and adults, “There is little indication that growing or eating food from urban gardens will result in high Pb (lead) exposure.”

Is Lead Poisoning a Risk in Urban Gardens?

82-Lead-Tile

Those of you who have followed our blog over the years know that our yard is contaminated with lead. This is, most likely, due to some combination of lead paint residue and car exhaust. For years we’ve grown vegetables in raised beds because of this issue.

But if the results of a University of Washington study on lead and urban agriculture are to be believed, we might not need to be as concerned. The researchers note that most vegetables don’t take up lead and that improving soil with compost greatly reduces the bioavailability of lead.

You can read a summary of the results of this research paper here.

Thanks to Joanne Poyourow of Environmental Change Makers for tipping me off to this research.

Phytoremediation of Heavy Metals

ArtemisiaVulgaris

Mugwort (Artemisia vulgaris)

Us city slickers have fouled the sandboxes we play in. Find an open field in a big city like Los Angeles or New York and the odds are that it’s a former toxic waste dump. Here in our neighborhood we’ve got a lot of lead and zinc–lead from paint and gasoline and zinc from brake linings.

These heavy metals don’t magically go away. They are elements, and short of an alchemical transformation you have to physically remove them, cover them up, or apply phosphate so that plants don’t take them up as readily.

One promising strategy is phytoremediation, the use of plants to uptake heavy metals. City Atlas youarethecity, in New York, is experimenting with Indian mustard, mugwort, basket willow and sunflowers to remediate a contaminated garden. The results are promising with some metals down 50% in a year. Mugwort (Artimesia vulgaris) did an especially good job with a wide range of contaminants.

I should note that Garm Wallace, who runs Wallace Labs, a soil testing service, says that phytoremediation can take many years to get heavy metals down to a safe level. That being said, breeding plants specifically for heavy metal hyperaccumulation is a technology that could make up for our past transgressions.

Thanks to Michael Tortorello for the tip.

Urban Chickens and Lead

From the One More Thing To Worry About department, the New York Times has an article on lead levels in eggs laid by urban chickens “Worries About Lead for New York City’s Garden-Fresh Eggs.” According to the article, the lead levels found in New York City’s home grown eggs ranged from none to over a 100 parts per billion. Since the FDA does not have an acceptable lead level in eggs it’s difficult to interpret the results. And I have to wonder what unknown problems lurk in industrial eggs.

It’s a reminder that those of us who live in older cities and grow food need to confront the lead problem. Personally, I’d also like to see the Real Estate industry come clean on this issue beyond boiler plate disclosures buried in sales documents. But I’m not holding my breath.

Five Lessons We Learned About Lead in Soil

As regular followers of this blog may recall, we did some soil tests last year that revealed elevated levels of lead and zinc in our backyard. The cause? Most likely, paint from our 92 year old house and nearly a hundred years of auto exhaust and dust from brake linings.

Applying a little alchemy to turn lead to gold, I think the most productive thing I can do is to help get the word out about lead soil and how common this problem is in urban areas. Towards that end I though I would share five things that we learned from our backyard lead crisis:

    1. Buyer Beware. When you are shopping for a house do multiple soil tests. Once you buy the house it’s too late. Real estate contracts in California (and I suspect elsewhere) have been loaded up with disclaimers about lead and old houses. I’m no legal expert, but I suspect it would be difficult to go after the seller given the lead clauses we signed in our ignorance.
    2. The dirt on soil labs. Choose a soil lab that gives detailed results and is willing to do some phone consultation. We used to recommend the cheap tests form UMass, but their results were significantly different than Timberleaf Soil Testing and Wallace Labs, which were more in line with each other. Both Wallace and Timberleaf give you more detailed reports and are both willing to chat on the phone. They cost more but are worth it.
    3. Raised beds.  Not much else to say other than those two words if you want to grow vegetables and have a lead problem. Right now I have a big compost pile going that I made with hay, straw and horse manure. I’ll use this compost along with imported soil to fill the raised beds that I’m going to build this winter. And you can bet that I’m going to test that imported soil first.
    4. Philosophical lesson. The future health of the human species requires us to be a lot more conservative in the use of chemicals. Lead was known to be a problem since the Romans, but we went ahead and used it anyways in paint and in gasoline and the results have been tragic. Nassim Taleb has written eloquently about the need to approach complex systems like nature and the economy with an attitude of humility and admit our ignorance. When we fool around with complex systems we’re asking for trouble.
    5. Rhetorical lesson. There should be a logical fallacy called the “appeal to technology.” It’s the idea that there just has to be a technological solution to every problem. This is a very common fallacy in our age. Some people have suggested that I try phytoremediation, the process of growing plants like sunflowers that accumulate lead. At the end of their growing cycle you pull the sunflowers and send them to a toxic waste dump.  Sadly, this is just not practical in a residential yard. I would have to pull every living thing, including a mature avocado tree, and grow nothing but sunflowers for the next 20 years. Others have suggested mushrooms as a remediation technique, but they don’t grow well in this dry climate.  Lead is an element and it simply doesn’t go away. Once it’s in the ground it’s in the ground. The best way to deal with it is not to use it in the first place. The city of Oakland is trying a more practical solution to deal with lead: applying fish bone meal to lead contaminated soil. Fish bones contain phosphates which bind up lead and make it less bioavailable to plants. Our soil tests indicate that we already have lots of phosphates in our soil, so I’m not sure if adding more would help.

    Of all of these points I think the first is the most important. I’ll repeat it again, if you are shopping for property get a soil test.

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