I started these from seed indoors under a grow light. About two weeks ago I potted a few that were doing well, and now it looks like they're getting sick. The first image is arugula / rocket, and its leaves are getting gray and dry at the tips, almost like they're mineralized. They crumble to the touch. The sage in the second picture looks like it's starting the same thing.
My suspicion right now is the soil. It's about a year old, and I checked here beforehand and people said it should be fine and come back to life once rehydrated. Maybe something nasty set up shop and now it's attacking the plants?
My only secondary hypothesis is that the lawn rocks I put on top might have leached something into the soil after the first watering.
I'd like to know what's going on before the rest of my seedlings need potting.
edit: these plants are now outdoors on a table, with direct sun for about 8 hours per day. The soil was kept in its original bag, but it was outdoors in the shade.