We have a mushroom plant nearby that practically gives away it's spent mushroom compost to gardeners. Even the organic folk like to use it. But I've heard that some tests on the compost itself and on the soil it's used on reveal a high level of arsenic. It's been pasteurized before inoculated with the mushroom spore and once it's heaved back to the composting area, it heats up again there, too. Unless the original horse manure (deworming, medications, etc) they use has arsenic, I don't know how it would be appearing in the compost.
But, if it's in the compost they grew in, wouldn't it stand to reason it's probably in the mushrooms also?
