Yes it is slow release as the dusty granules are gradually “activated” by microbes in soil and compost, but I don’t know how long the benefits continue, a few years I reckon. I usually spread it then compost on top but surface application is fine as worms soon find it, they like it for digestion I think!
Incidentally there is a new feature now, you can post photos using the “Choose File” button below the box you write in.