Get the soil right & the plants will look after themselves.
Most plants will either work in a position or they won’t. If it’s too cold, dry, shady etc. they’ll just struggle & eventually die. If all other conditions are OK, they’ll mostly do reasonably well in even poor soils without much help.
If you want to help them, think about improving the soil not attending to the plants. If the soil’s good, many plants will thrive without any care whatsoever – if it isn’t thriving, then it’s probably better to try something else.
My approach to the soil is to make it as attractive as possible for the worms and bacteria living under the soil – they do all the work in making nutrients available to plants – nutrients that end up on our tables. They also improve the structure of your soil making it easy for your plant’s roots to obtain air and water.
My worms seem to like compost, animal manure, mulch I’ve grown myself (especially Comfrey), seed-free garden waste, hay or lucerne mulch, mushroom compost, seaweed drenches, blood and bone and even newspaper and cardboard!. Rather than use the same “worm-food” every time, I prefer to vary it as much as possible, thinking that will make a wider range of nutrients available to the plants (and me).
I get what I can locally (living in a rural area helps) and I go out of my way to try and get organic products. If you don’t have access to fresh manure, try the animal manure pellets that are available in garden shops – especially if they have added trace elements – good for the garden – good for you.
Even better – grow your own worm farm and use the juice on your garden for great results.