I have a big piece of Homasote. What's your opinion of using it to start a no-till garden bed?
Since it's mainly paper pulp & glue, it would probably disintegrate rapidly after a few rains & turn to mush. On the website they warn against "saturation", LOL.
Here's a list of ingredients, you may want to spend some time researching them before you do anything:
And since it's made from recycled post consumer paper products, they probably can't say for sure what kind of inks are in it.
Just from a quick look at the ingredient list, I'd be a bit careful--it's got both aluminum sulfate and alumina in it, and too much aluminum can be bad for plants. It's all a matter of quantity of course, having some aluminum around is fine so if it's just a small amount it wouldn't be an issue, but if there's a lot of it that could cause problems. Since they're using the hydrated alumina as a fire retardant it seems like there might be a decent quantity of it in there so I would definitely do some research on that before you consider using it in the garden.