I didn't read all of the responses, so I apologize if this was mentioned already....
Sounds like you're having trouble with the medium drying out too quick, and concerned with nutrient availability? Do you have access to worm castings or compost? If so, you could lay down some dry organic amendments.. I go with about a 1/2 cup per cf of soil. Scratch that in a bit, and then cover with a layer of worm castings/compost. You can then either plant a cover crop (preferably a nitrogen fixing plant like clover or legumes) or use a mulch like wood chips, discarded marijuana fan leaves, hay, etc. The microbes in the compost will mineralize the organic amendments over a period of time giving you a nice slow release of nutrients, and the cover crop/mulch will help retain moisture in your medium.
If you need a quick fix you can bubble a nutrient tea, or pick up a little bottle of something at the hydro shop. Here in Michigan we have a local company called Organically Done that has some nice products. I've used their fish hydrolysate
http://www.organicallydone.com/
.