By under watering, I'm going to assume you mean what we usually refer to as bottom watering, as in watering the plans by setting them in standing water and allowing to wick up, rather than under watering meaning an insufficient quantity of water. I fertilize by adding fertilizer to the water in the tray when bottom watering. Pour some in,give a little stir, set plants in.

Thank you, that is what I meant by bottom watering.

I assumed this was the method, but I just need some clarification.

To be sure my fertilizer used in bottom watering is mix to the correct strength, I mix it in another measured container and add it to the tray used for bottom watering as needed. Al

When fertilizing seedlings, you generally want to dilute the fertilizer, rather than using it at full strength. Seedlings are small, and generally don't need a lot of fertilizer, especially their first dose.

As they grow larger, their demands will increase, and you slowly build them up to getting full doses of fertilizer.

For my first fertilization, after second leaves appear, how much liquids should I be using? I realize I need to use the diluted strength of the fertilizer. I have the standard 72 cell black flats that come from Lowes.

Do I need to wait until the soil is ready for a drink? Does all the existing water under the tray need to be soaked in before I fertilize?

In general, it's better to underwater (meaning to provide less water than is necessary) than to overwater (meaning to drown your seedlings in massive amounts of water). Because it's easier to correct the dry plant problem (provided it's not too late), than it is to correct the soaked soil problems (damping off, mold growth, etc).

The cells should start to seem a little on the dry side, before watering, and therefore before fertilizing with a diluted solution. The amount depends on the size of the plants, and their need. In general, make too much dilution, and water sparingly at first, and if your plants soak it up quickly, and the top hasn't been moistened yet, add some more. Eventually you will figure out how much you are adding.

Agree with all eaglesgarden said above.

An easy way to do it if all you have is 1 tray is just mix up a jug of the liquid fertilizer you are using and water - a 1/2 gallon milk jug works fine - and follow the directions on the label to make it 1/2 strength or weaker (I often use 1/4 strength). Then pour about 3/4-1 inch in the bottom tray, wait a few minutes until the top of the soil in the cells appears damp and then tip the tray and pour what is left in it back into the jug. Cap and save it till next time.

Dave