Well, I made an important discovery: the type of potting soil you use is extremely important.
The plants in all of these pictures were started by seed at the same time, yet some of the plants took off while others remained slow-growing. The obvious factor? The potting soil.
I used 3 different kinds and had the best results - hands down - with the first bag that I purchased. I got it from Sixteen Acres Garden Center. I do not have the name of it right now, but I am going to go back and get another bag of it. It was a "slow-release fertilizer" type. Anyway, I can see from these pictures that all the seedlings that I transplanted into 12-oz cups with this fertilizer did the best.
One final experiment: Two weeks ago, I was about to throw out a bunch of tiny tomato seedlings that just never seemed to grow. However, I noticed that I didn't have any Red Grape Tomato plants or Lee Fanelli Heirloom plants, so I decided to do a little experiment. Suspecting that the reason they never grew was due to the lousy potting soil, I replanted them in the Fox Farm Ocean Forest (FFOF)potting soil. These pictures show what a difference the potting sol made over just 2 weeks. The first picture shows the seedlings that I was about to throw out:
These next two pictures show the seedlings after just 2 weeks: