In my experience emulsion works well, just tends to be a little light on nutrients (they are usually in the neighborhood of 2-2-2). Some brands smell pretty bad. Kelp supposedly has other compounds that promote root growth, not sure if there is any solid science on that.
IMO, Espoma products are great, I usually use them.
Other ways to boost soil would be compost, worms, and mycorhyzzal fungi.
Martin - That's what Native Americans used to do: bury a fish under the corn/squash/bean combo. If it worked for them, it'll work for us.