I use up leaf lettuces (anything I have left over from the previous season) and Arugula and Spinach and Beets (for the spicy greens).
I do a standard flat filled with seed starting mix (I like the ‘Fertilome’ brand) and after the plants start leafing out, I fertilize with a really light concoction of any organic liquid fertilizer on hand.
Of course, these are under grow lights, on for 16 off for 8, because up here there is not enough sunlight from a window to grow anything this time of year.
OK. Thanks.