I'm not aware of any toxins in plaster which would prohibit you from recycling it into soil (but then again, this is not my area of expertise).
However, many older plasters had asbestos fibers added as a binder to the plaster to enhance stength and reduce cracking. Traditionally, the asbestos content was quite low (1-5%) but removing the plaster can cause the asbestos fibers to go airborne if they are present(sampling is the only method to confirm).
On the same note, if asbestos fibers are present, adding them to your soil could pose a low risk of inhalation hazard to persons tilling or disturbing the soil.
All provinces have regulations which govern asbestos and many have regulations concerning construction waste and disposal.
Good luck with your project.