Certified GKH Gardening Expert
You will need to follow the instructions on the packaging, as well as know the feeding requirements of the specific plant in question.
Feeding rates can differ from brand to brand, depending on purity and proper storage.
As a general rule, you should start with half the recommended strength and work your way up, starting about 2 or 3 weeks before a plant starts to flower into maturity. Usually, this is mixed into water, as per the instructions. It is not usually safe to apply raw phosphate powders directly to the soil, like it is with a granular.
Basically, it all boils down to packaging instructions, and the individual plant's feeding habit.
Here is an article that will help you with superphosphate use: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/what-is-superphosphate.htm