I don't know if they still make them, but you used to be able to get seed starter pucks. They were compressed earth inside a mesh. You'd soak them and they'd swell from a 2" x 1/4" puck to fill something as big as shown.

(Yeah. Here we go: https://www.amazon.com/Count-Jiffy-Pellets-Seeds-Starting/dp/B000EHJN7K)