Adding Miracle-Gro to an EarthBox?

[Greatgardens]

I've always been a bit skittish about adding blue water to my EB reservoirs, and as a result, I probably significantly under-fertilize. For those of you that do add MG, how much do to add, and do you use liquid or powder? And does anyone ever add a bit of iron? (I have a couple of plants that are a little pale.) On the bright side, most all of my plants are doing great, including a Celebrity that has an EB all to itself and looks awesome!