Something water soluble that you can mix and apply as a liquid when watering. (risk of burning roots, applying too much using a dry product in containers) And please keep mind that some companies will indicate you need more than you could find necessary...I mix half strength of what is on the label directions and rarely any stronger. 1/4 strength for seedlings and young plants.

If you have none and are shopping, try this one - Tapla/Al is about as informed and experienced with growing in containers as anyone you might find and a great resource (Container Gardening Forum)
Posted by tapla z5b-6a MI (My Page) on Sun, Sep 6, 09 at 13:25

As noted, for beginners and anyone who wants to be reasonably sure they can supply adequate nutrition w/o having to micro-manage, I would suggest Dyna-Gro's Foliage-Pro 9-3-6. It supplies nutrients in the ratio in which plants use them, and supplies all macros, secondary macros, and all the micro-nutrients necessary for normal growth. It also supplies a large portion of its N in nitrate form, which at times can be a significant advantage.
Al

*My note - others are Miracle Grow (I've never used their products), Peters, Shultz....I have a large quantity of Peters on hand, found it offered at closeout price (literally, the store was going out of business). It isn't quite complete but close (may lack some calcium and possibly magnesium which I can add for no more than I paid ).

I use a potting soil that has season long fertilizer in it....but then I only grow annuals in pots.
Also some nurseries recommend Osmocote as a slow release can't burn fertilizer.
Linda C