Ensure seeds get off to the best start with a fine tilth. You could use straight soil or potting mix but they will both contain larger elements that will mean seedling roots lose out on maximum moisture contact for roots.
This sieve or garden riddle has a mesh that makes a tilth the perfect size for planting seeds into trays or pots.
Every cottage garden used to have one and now is the time to bring back the tradition. Your seeds will germinate better as a result.