This wasn't an experiment, so I'm just guessing what's going on here. I don't usually bother too much about what I sow my seeds into. In theory, seeds don't mind much where they germinate, as they have their own little store of food to get them started. It's only after they've got going that the soil nutrients make a difference, or so I've heard. If you prick the seedlings out into new homes as soon as possible, the seed tray compost/soil shouldn't matter much. Of course, that's a big
Since I don't pay much attention to what I put in my seed trays, I can't be sure what I've got here, but I do know that for some of the trays I used some garden compost from the year-before-last heap, which by now has a fine texture and was conveniently available. These two trays of basil were planted at the same time, using seeds from the same packet, and kept side by side in the greenhouse, so the number of variables that could account for the difference is fairly small. I'm guessing that the tray on the left got old soil, which might have been used to germinate other seeds already, whereas the tray on the right is presumably full of garden compost.
Of course I knew that soil nutrients are important, but seeing the contrast in these seedlings really brought it home to me. I shall make more effort now to ensure that my plants are well fed.