I have never owned a chicken either, but I used to have two cockatiels. Chicken grit is basically just very very fine gravel. Though with the small quantity I would need I would probably just pick up what ever they sell in the pet section for birds, as opposed to buying a 50lb bag from the farm store here. Larger seedlings could easily push right through it. I am less sure of the tiny ones. It might need to be added after germination for some of them.
I'll have to experiment.
