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.