I'm not really sure about microgreens as I've never really bothered with them, but from what I've experienced, adding a small amount of worm castings with greatly improve both growth rate and the plants capacity to absorb nutrients.
With an NPK of 1:1:1 it doesn't so much work as a fertilizer, rather a medium for the introduction of beneficial bacteria.
RE: Do You Need to Fertilize Seedlings? [Podcast]