What are the little blackspecks on my petunia, about the size of salt?
Signs of Bugs
Certified GKH Gardening Expert
Although it could be a number of insect pests, I have a treatment that I use for a broad range of pests. You can ignore the soil preparation part, but it wouldn't hurt to do that as well.
My recipe is as follows: First, you will want to prime your soil with wettable sulfur (for fungus control) and dolomitic lime. (for balancing pH, supplementing magnesium, and calcium.) Then, you will mix the following in a spray bottle with warm water, and a spoonful of coconut oil- Neem oil, rosemary oil, garlic extract, thyme oil, cinnamon extract, and for heavier insect infestations, Clove oil.
Spray as often as needed.
Here is an article for more natural pest remedies: https://www.gardeningknowhow.com/special/organic/natural-home-pesticides-organic-garden-pest-control.htm