Is it safe for the garden to sprinkle/mix human cremation remains?
First, please accept my condolences on the passing of your loved one.
Thank you for contacting the Toronto Master Gardeners about this question. Cremated human remains are sterile, organic matter that are safe to handle because they have been heated to temperatures between 760 C and 980 C, and this kills any micro-organisms they may contain. However, cremains are also high alkaline (high pH) and high in sodium, which can be damaging to and kill plants, especially young ones. Spreading them around plants and in a garden should be done only around established trees and shrubs, and very thinly, to avoid killing plants.
In researching this answer for you, I learned that you can obtain a soil cremation mixture that will neutralize the cremains to make them safe to add to your garden. There are several companies in Ontario that I found online that offer this service.