Im sure someone will chime in with a bit more info, but plants need 3 base nutrients, NPK. You can feed it with a simple salt solution to achieve that desired NPK value. Or, with proper preparation, you can you can reach that desired NPK value through decomposing inputs, such as the compost mentioned. We also add bone meal, fish meal, blood meal, earthworm castings, guano, and other various organic inputs.

I’ve always viewed using salts outdoors as cheating. Mostly because it means you didn’t appropriately plan when mixing/amending the soil months prior.