I'm using a flower-box that has a ~5l water reservoir. When it's time to use fertiliser, should I use liquid fertiliser and put it into the reservoir or should I apply it traditionally by watering it with diluted fertiliser onto the soil?
It probably won't matter but if there's a risk that the insoluble components of the fertiliser precipitate out, it would be better to apply the fertiliser from the top, and use the reservoir for water.
Those reservoirs normally rely on dry soil wicking the water up and that should carry the fertiliser salts with it. But you've got more certainty the other way.