Jake, I dont know about foliar feeding nitrogen, but I have sidedressed some before with leftovers from corn. No reaction, like water.

First thing to keep in mind: Base materials = growth        Acid materials = fruiting

Second: Find a bunch of materials you would like to try and spot spray them with a hand sprayer. About the size of a houlahoop. Take the brix reading with a refractomenter before and after spraying them. If the brix does not go up within a 1/2 to an hour, then you're wasting your time and money. Pick the material that will raise the brix the most. Brix is the content of sugar in the sap.

 Another thing to keep in mind is that too much sodium will starve the plant of water even if your soil is saturated.