If you are talking about a container potting mix, there is no need to test. Very few mixes have any sort of nutrient load at all (unless you are using something like the MG soils that have added ferts) and so any and ALL nutrients are provided by the gardener with routine fertilization. It has been determined that most plants consume nutrients in a 3-1-2 NPK ratio so any liquid or water soluble fert that offers that ratio will work well for just about anything. That is the reason most here favor Dyna Gro's Foliage Pro, as it provides the correct NPK ratio and offers a full range of micronutrients as well. No need to over think.......it is a very simple and efficient approach ;-))

If you are talking about fertilizing plants growing in the ground, that is an entirely different kettle of fish and a whole lot more complicated. Happy to explain further if you wish.......but won't bother now unless that is the sort of info you are looking for.

@gardengal48 Okay, that makes life a little bit clearer. Some of my plants are azaleas and rhododendrons in containers, planted in either pure bark or bark and turface. Should I prefer a fertilizer like Espoma Holly-Tone for just those plants? That will cover the micronutrients as well? I will use Foliage Pro for succulents/cactus and misc plants. I had been using a little Osmocote just to provide at least some basic nutrients in most of my succulent/cactus pots.

For those "infamous" Photinia and Pittosporum hedges I have been posting about - that are both in-ground plantings in clay soil - should I think about testing the soil?

The Holly Tone won't work well, if at all in the container mix, so don't bother to waste your money on that :-) Just stick with the Foliage Pro. This is why container mix pH is not near as critical to proper growth as is the pH of the same plants grown in the ground......since you are providing all the nutrients and pretty frequently, there is less of an issue of an improper pH binding or holding on to necessary nutes.

And yes, testing the soil for the inground plants is advisable. But not with any at-home test kit. There just aren't any that will provide an accurate reading. Get a professional soil test done.