The data are in! It’s been about 2 weeks since I first posted the fertilizer experiments. As you can see, the tomato starts on the fertilized side of the pot are much bigger than on the other side.
What’s really interesting to me though, is the difference in the roots. About a week ago, I thinned out the starts to leave just one plant per pocket. I took pictures of each little start as I pulled them out. At the time, the leaves of the different starts looked about the same, but the roots were extremely different. The fertilized starts had much bigger, healthier root systems. (By the way, in case you’re worried about the welfare of the thinned out starts, I replanted all of them in a new seed starter, and they’re still alive and thriving today.)
The conclusion seems pretty clear: a bit of fertilizer leads to happier baby plants. I’ll be mixing it in from now on. Next experiment: testing various concentrations and types of fertilizer.