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I would advise you check with the manufacturer on the method of how to fill your irrigation system.
Fertilizing amounts and NKP can be tree and plant specific.

Yes, you can mix the Epsom Salt into a solution of water to feed your plants;
but I also drop handfuls of Epsom Salt right at the base of my plants or sprinkle around my garden!
I've done it for years, as I learned from my Grandmother!

Your flowers will love it!

This article will help you.

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/what-is-foliar-spray.htm

It is best to have your soil tested before adding fertilizers.

The NPK you mention is an extremely high ratio of phosphorous.

Most flowering plants can do well with a balanced fertilizer, 5-5-5 or 10-10-10.

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/fertilizer-numbers-npk.htm

What are you fertilizing? What is the nutrient level in your soil?
Have you had a soil test done on your soil? Are you trying to raise the nitrogen levels in your garden?

Nitrogen is certainly beneficial to your garden and necessary for your plants to grow healthy, but excess nitrogen can be a detriment inhibiting formation of flowers, fruits and vegetables.
Excess nitrogen can burn and even kill your plants.

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/understanding-nitrogen-requirements-for-plants.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/blood-meal-fertilizer.htm

In my opinion, alway apply nitrogen in lighter formulations, you can always reapply! You can't take it back--very easily!

Your plant actually look green and healthy. The spent flowers can be dead headed from the plant.
You can pinch back to a healthy set of leaves to promote a bushier plant.

https://www.gardeningknowhow.com/ornamental/flowers/million-bells/calibrachoa-million-bells.htm