|Young seedlings benefit greatly from water-soluble fertilizer mixed at half strength|
When a seed sprouts, the first set of leaves that unfold are called cotyledons. All the food that's needed to sustain these first leaves is contained right inside the seed. But once the second set of "true" leaves appear, you should begin feeding your seedlings with a dilute fertilizer solution. Mix the fertilizer at half the recommended rate and apply it twice a week. You can continue this feeding program until the seedlings go into the garden, or switch to less frequent full strength feedings, every 7-10 days.
What should you feed your seedlings? An all-purpose, water-soluble fertilizer is best. Ideally it will contain trace nutrients and minerals and other organic components such as humic acid. Plant Health Care for Seedlings is specially formulated for seedlings and mixes easily in water. A seaweed/fish emulsion fertilizer is also good, though you will want to find an unscented version for use indoors.