"I have several seedling plants (broccoli, spinach, carrots, lettuce) that are about 2 weeks old and have one set of leaves. When is a good time to give liquid fertilizer? What is the best mix?"
John 76 wisely asked her if she had used a potting soil that already had fertilizer in it. Some do, some don't. If the potting mix has fertilizer in it, you shouldn't need to add more. I'd opt for a soil less mix without fertilizer. First, it's cheaper. But more importantly, your seedlings don't need fertilizer until they develop their true leaves and you can control how much and what type they get.
Those first oval leaves aren't really leaves at all. They are cotyledons, part of the embryo of the seed. Cotyledons contain the remainder of the stored food reserves of the seed and keep the seed fed until the true leaves sprout and the plant can begin photosynthesis. Usually the cotyledons disappear shortly after the true leaves appear.
So to answer MDLady45, you begin fertilizing your seedlings once it has developed true leaves. Seedlings tend to need a fertilizer high in phosphorous, like a 1-2-1, N-P-K ratio.
- Synthetic Fertilizer: If you are using a synthetic fertilizer, feed your seedlings weekly, but it's wise to dilute the label's recommendation by at least half. Tender seedlings can easily be burned by too much fertilizer. Really young seedlings can get away with 1/4 of what the label recommends for full grown plants.
- Organic Fertilizer: Luckily there are more and more liquid organic fertilizers available, although they can still be hard to locate. A mix of fish emulsion and kelp has always kept my seedlings growing. (Thankfully, they've figured out how to make fish emulsion a little less potent smelling.)
Another option is to mix a granular organic fertilizer into the soil, when your seedlings are ready to be moved from their started cell packs to larger pots.