Compost vs. Arsenic: And the winner is–compost!

Several posts back I mentioned in passing that compost can help fix arsenic in soil, and it seems reasonable to explain what the hell I was on about there. To "fix" arsenic is get it to bond with other, stable molecules so that it can neither leach from the soil into the water supply, nor migrate into your vegetables, and thence into you.

I got onto this originally because the news articles about the great Tennessee coal ash spill of '08 kept mentioning arsenic. Arsenic, I learned, occurs in several different forms, but the one that turns up most commonly in coal ash is arsenate(V), the same form that leaches into soil from telephone poles and fences treated with copper chromate arsenate (CCA).

And yes, at least one recent study conducted at the University of Florida found that when carrots and lettuce are grown in soil that's contaminated by CCA, both vegetables absorbed less arsenic when the soil received plentiful treatments of compost. 

It turns out that pH is also important–hardly a surprise, since it affects just about everything. Plants absorb more arsenic at lower pHs, so don't plant blueberries or other acid-loving plants near the treated fence that's leaching arsenic into your soil. If your pH is low, add lime or wood ashes; at pHs of 6-7, arsenic uptake will decrease significantly.

One more factor–phosphorous–plays an important role in how readily plants take up arsenic. Several studies (including the one mentioned above) have found that adding phosphates to arsenic-contaminated soil increases arsenic uptake by plants, because phosphates and arsenic are close enough chemically that they compete for the same attachment sites on soil aggregates. (Arsenic is directly below phosphorus in the periodic table.) When phosphorus is added to soil, it displaces arsenic, which is then free to travel—and some of it travels into the roots (and leaves) of plants. 

If you suspect your soil may contain arsenic:

  • Add compost.
  • Raise soil pH above 6, if it's lower.
  • Avoid high-phosphate fertilizers.

The organization safe2use offers several arsenic test kits for home use. I have no idea how accurate they are, but I suspect that high values on a home kit would be a pretty clear indication that it's time to call in the experts for a real soil test.

15 Responses to Compost vs. Arsenic: And the winner is–compost!

  1. Very useful info. I have an old dog-run fence that I inherited when we moved into our house. The posts are treated, so I’ve always avoided planted food crops anywhere near them, worrying that arsenic might have leached into the soil. The info about compost is helpful. Our soil is limey already, but I’ll add some compost there anyway…

  2. Susan, compost helps balance pH; it doesn’t push it solely in one direction. So if your soil is alkaline, compost won’t make it more so; if anything, compost will make it trend slightly towards a lower pH. Sounds magical, I know, but it has to do with cation exchange capacity and other soil chemistry. At any rate, compost away.
    Not planting food crops too near that fence is still probably a good idea, but the compost does help keep the the arsenic in the soil and not in the plants.
    –Kate

  3. I remember the termite treatment man, who put pesticide around the walls of my childhood home, telling my parents: Don’t grow vegetables next to the house for ten years. To this day I feel uneasy about planting edibles in the beds by my 85 year old house. Glad to know compost helps.

  4. Compost is what godiva is to humans. Or, compost is like falling in love. No. No. Wait. I’ll come up with a good one. I am itching for my 8 cubic yard delivery in March. And for no more snow and highs in single digits.

  5. Lucky for me I do not have arsenic in any of my gardens. However I found the parts about the PH levels rather interesting and may look for a simple test that I can use to investigate the levels in different areas of my gardens.

  6. Holy H., Mary, EIGHTY-FIVE YEARS? But at least he told them. Are you gardening by the same house?
    Have you any idea what the Terrible Termite Man was using?
    Benjamin: Godiva, my foot. I’ll give you the benefit of the doubt and assume that you’re saving the good stuff for your real writing. I sure hope so.
    Judy–not having arsenic in one’s garden is a good thing.
    –Kate

  7. As a gardening novice, I place a small veggie garden next to an old telephone pole in my backyard. It then dawned on me that maybe the soil could be contaminated with whatever they treated the pole with. The house is 60 years old, so I don’t know, but now I’m tempted to give up, for fear of contaminating my family! Advice?

  8. Well, Jen, this answer has gotten so long, I’m going to make it a post. It should be up in a few days. Let me know if it’s useful.
    And don’t give up yet.
    –Kate

  9. Kate, I have cca fence posts in my back yard, 14 new ones. I started reading about them and now I hate myself for using them. They still sell them at the farm supply here. They have been in the ground about a year. I sealed them with Zinnser oiled based sealer primer and followed with two coats of 100% Acrylic Latex porch paint. I am worried about what is in the dirt around them. I love my big dog but he likes to dig. Should I put flagstone or something around the posts or dig up the dirt around them to keep him, and us, safe.

  10. Kate, P.S.,The fence posts were not sealed or painted for the year before. Thanks

  11. Yogi, if you bought them within the past year or so, you’re probably okay, though the real question is when they were actually made.
    If they were made afer 2003, they should be free of arsenic. Given the way inventory moves these days, (quickly:fewer and fewer companies stockpile much inventory any more) chances are that these are the next generation, containing CC but not CCA. The chromium and copper leach at such low rates they’re not usually serious concerns.
    The primer and paint you used would also slow any leaching, even if they were only applied after a year.
    If you’ve got flagstone and think it would look good by the fence, you could certainly lay it down for extra peace of mind. But all in all, I wouldn’t worry.
    Here’s hoping you get it sorted out to your satisfaction. Let me know what you decide to do–
    –Kate

  12. Thanks Kate,
    You are the only person I have talked to that makes any sense. The tiny tags on top of the posts have a warning ( in itty-bitty letters ) that says “Warning Contains Arsenic”, and give the number to a CCA information line. I am REALLY beating myself up for buying what I thought were good fence posts that were in a huge pile out in the open at The Tractor Supply with nothing but the prices on the racks. I did not know anything about CCA until I started reading information sites such as yours. I feel like I have poisoned my family and our property. I gotta be the world’s biggest jerk. I thought about digging up the dirt around them but that might make thing worse. So I guess I’ll put something on top around them to keep my best friend ingesting the stuff. It makes me sick.

  13. Whoa, whoa, Yogi, don’t be so quick to claim that world’s biggest jerk title for yourself. I can think of several other candidates right off, and at the least we should have an American Idol type show to decide who gets the Ultimate Jerk crown–maybe call it American Anti-Hero.
    Back to busines.
    Can you afford a soil test? If so, do it, because there’s a good chance you could stop worrying.
    Bear in mind that your fence posts are not nearly the size of a telephone pole. The soil at the foot of a telephone pole can be seriously contaminated, but I’m not at all sure that the soil by your little posts would be. I know, there are 14 posts, but if each one raises the arsenic level near it by a few points, you don’t have a problem.
    When did you put these posts in? It sounds as if it was at least two years ago. So they leached for the first year, but then you painted them, which drastically slows leaching. Meantime, the soil microbes start working on whatever did leach that first year.
    Because arsenic is quite immobile, the biggest problem is in soil within a foot of the posts.
    So, don’t grow vegetables right beside them. If you can, get hold of one of the ferns I mention in the Phytoremediation post. Pile on the compost (don’t dig it in; just lay it on top of the soil). Grow a fern right next to the fence, and cut off and trash the foliage every fall for five years. (The amount of arsenic in the foliage isn’t a problem in a landfill.)
    The biggest issue in the meantime, it seems to me, is to keep your dog from digging near these posts. Got any ideas? If the flagstones would do it, go for it.
    Finally, do you have kids? Or are there kids on the other side of this fence? I was amazed to see a comment from a carpenter who said he’d made it a policy always to wear gloves when handling CCA treated wood. Keeping children away from the fence should be a priority.
    My bet is that if you got your soil analyzed, you’d get a pleasant surprise. Let me know how it goes.
    And Yogi–Thanks for the compliment. I’m glad I can help.
    –Kate

  14. Our utility company is digging to put in a new pole today. I have been researching the info on contaminants entering the soil. I have an orange tree, and a fig within approx 8 to 10 feet. and a compost pile right next to the pole. I haven’t used the compost soil. I am not sure if we should eat the fruit from the trees… It’s nice to see others are also questioning these issues, I would appreciate any feedback!

  15. This blog really provides readers with lots of knowledge about compost and arsenic! Very well done!

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