I use it when transplanting. Other than that I'm not sure how effective it'd be as a general amendment to the medium. Are you organic gardening or is your feed synthetic? It wouldn't have too much of an effect if you're mainly synthetic, to my knowledge at least.
If you're interested in nutrient cycling and uptake a great product is Mammoth P or amend your medium with alfalfa/kelp meals and add in a compost/worm tea every once in a while