I would also really appreciate any information about this. Lead is a huge issue in urban and in-town gardening. Any information about techniques to lower or stop bioavailability would also be very useful.
I've been scanning a lot of scientific papers on the web about using these plants. Although they work well for some other heavy metals, with lead, they take up so little that they have little effect on the lead in the ground. On the other hand, once they have teken up the lead, they have to be diposed of as toxic waste, althoug some people seem to be extracting the lead from them to be used again. I've also read about using various kinds of phosphates (rock phosphate, bone meal, etc.) to bond with the lead and lower its bioavailability. But as far as I can tell, this is used for making soils safe for children to play, rather than gardens, and I wonder how the phosphates would affect growth or if the action of the plants might change the effect of the phosphate bonding with the lead. There's a big EPA project in Oakland in progress now using fish bone meal. Seems they are using about a ton of the meal, or more, per yard! I've been trying to get contact various people doing this kind of work by email and phone to ask about it use for gardening, but so far, no luck.
I would not rely solely on plant remediation if I suspected that lead or other heavy metals were an issue.
Soil pH is a very important factor in determining how soluble lead is. If the pH is below 5, then solubility starts to go way up, and that lead will be absorbed into plants. Liming is the way to go, IMO... unless the lead levels are very high, in which case, it may not be possible to use it. In such cases, think about capping it with clean soil, or having it dug up and removed.
I'm not Geoff, but I do have some thoughts/ideas on this. I am an environmental toxicologist/environmental health scientist as well as a permaculturist, and I live in the Inland Northwest, where lead and other heavy metal contamination can be a huge problem.
Basically, there are three main issues here that I think about; growing food, preventing exposure from other sources like air and water, and regenerating the soil/healing the earth.
1. Growing food: I would recommend growing your food in containers, with soil you make from non-contaminated sources, especially if you are feeding kids. You can grow just about anything in containers, including fruit and some nut trees, and you can make some really nice polycultures in containers. (I do my gardening in containers on the porches of my house because I have a wildlife-rich site and it is the only way I can get to eat what I grow.) There are some good videos on Youtube for making self-watering containers, which are a very efficient use of water. If you have a problem with blowing dust that is contaminated with lead, I would recommend putting your container garden in a poly tunnel or greenhouse to protect it.
2. Preventing exposure from air and water: Blowing contaminated dust is a big problem in some parts of the west. If that is a problem for you, as I mentioned above, I would keep my food plants in a poly tunnel so they don't collect a lot of dust ( and wash them well before eating them). I would also plant windbreaks and put up wind barriers, making sure to design them so they didn't cause dirt to be deposited where I didn't want it, like in my yard. I would also make sure that I didn't have any bare ground that could turn to dust. Depending on the source of your contamination, I would also check the lead levels in my water supply (particularly important if you have kids). And, of course. houses with old plumbing can have lead in the water from old lead solder. I remember when I was a small child our plumber giving me a lump of lead to play with when he was soldering pipes. I think I still have it somewhere.
3. Regenerating the soil and healing the earth: Lead in the soil doesn't break down, so our choices are to try to remove it, or to sequester it so it can't move around and cause trouble. Removing it usually involves fairly heroic measures like bulldozing the contaminated soil up and trucking it away and putting it in a special landfill somewhere and then bringing in new uncontaminated topsoil. Some plants and fungi are dynamic accumulators of lead and other heavy metals but the process of using them to remove/reduce the lead in soils is relatively slow, and you have to dispose of them offsite, which means you are mining carbon and other things from your soil as well as lead. However, lead will form stable complexes with soil organic matter, and I think that most of the time that is the best way to deal with it. ( It is worth remembering that lead can be released from being bound in the soil if the pH changes, like when you get a really acid rain.)
If I had a plot of land that was contaminated with lead, I would start by laying down a layer of biochar. This would help to keep the lead from migrating into upper soil levels as you build soils, and it also can provide habitat for soil life, which may need to be jumpstarted in contaminated soils. I would put down layers of organic matter in an optimum C-N ratio for composting ( I'd try to include some animal manures, as they are rich in soil-building microbes), making sure that the materials were from uncontaminated sources (test if there is any doubt), and I would put in at least one other layer of biochar as I went along. (Or you could make compost and spread it rather than composting in place, or a combination of the two.)
I would plant with cover crops and green manures that can be slashed (not tilled in), and I would look for ones that are relatively shallow-rooted because you want to try to avoid bringing up lead from the subsoil. Grasses are excellent soil builders, and will prune their roots in proportion to the size of their tops, so this is one place where close mowing of grass could be helpful. I would water my my soil-building plants with compost tea for additional microbial power.
If I wanted to plant some larger things like shrubs or trees to make a perennial system that will build the soil, I would do it in a hugelkultur, the bigger the better. Sooner or later things are going to get their roots down into the contaminated subsoil, but by the time they do you should have a good layer of topsoil to work with that is very rich in organic matter to bind any lead you do get into it. I would periodically test for lead in the leaves that are falling from the trees, and in the new topsoil you have built. A good lab should be able to tell you not only how much lead you have but also what form it is in and how stable that form is. Over time, by building the soil, you should be able to dilute the lead in the soil and to bind it with organic matter so it isn't a problem. (As they used to say, "Dilution is the solution to pollution".)
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So what is the strain that I worked with in my Mycolab to do this? Pluerotus Djamor!!! The Pink Oyster!!! Isolate the rhizomorphic pattern and this strain will fly through any plastic!!!
want to start a thread in the fungus forum? sounds real interesting to me and I would like to hear more details, but I don't want to derail this thread.
* they were saying the sunflower remediation study was flawed
* they said it's complicated, so hard to determine what will truly remediate lead, so many factors, and it could vary from one part of your garden to another
* suggested capping the soil (mainly to cover dust) with fibrous mulch and then do raised beds of some kind on top of that. Not necessarily a plastic sheet, just mulch.
They were being more conservative in their recommendations--A, they're professionals and have to make sure they don't give a recommendation that hurts someone and B they're not urban farmers, they're more focused on remediation of brownfields and other things, like spilled pesticide containers or dry cleaners.
The rest of the talk focused on how plants can be successful at these other kinds of remediation, rather than on lead.
So, wanted to pass on this resource and also ask for any thoughts?
At the moment we're figuring we'll keep gardening here, that we've probably handled most of the dust problem with a few years of piling compost on the bed, and since the plant tissue in a (supposedly dynamic accumulator) brassica was 0, we should be fine with what we are eating from there. But still there seem to be a lot more unanswered questions, and possible need to test for more metals, such as arsenic, and other toxins.
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Do heavy metals like lead leach down into the soil over time? In other words, in the same way that nitrogen or other good elements can wash through soils and out of the root zone of most plants, does lead eventually do the same, safely taking it out of range?
If the concern with lead is that it blows in with dust from lead polluted sites, it would seem that the particles are pretty small. Thus, a heavy layer of mulch on top of the soil, and then regular rain and watering would wash it down further with each successive year. Or am I missing something?
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