Mycelarium leaf mulching is the unlock that turns the leaves falling on the forest floor into a renewable food source. Until you have it, every meal in a run comes from salt rocks that run dry, and a colony that runs out of food with no salt left nearby has nothing left to do.
Mycelarium leaf mulching: the unlock that changes the run
Leaves fall and decay into food on the surface. Before leaf mulching is unlocked, that decay is not yours to collect, so the only food in the ground is what the salt rocks and food pockets give you. Players describe the early game as the stretch where you do not have a renewable source, and one player stuck at 3 food with nothing else to do named leaf mulching as the way out of that soft-lock.
Once it is unlocked, food no longer has to be found to be food. Where food comes from in general is in how to get more food.
Where the leaves have to land
Leaves fall on the forest floor, not underground. The way to use them is to run the mycelium up close to the surface, under the leaf fall, so that leaves land on it. The practical version of this is to push a network up toward the grass line and let it sit there, because a network that stays deep in the soil never sees a leaf.
Growing a lot of mycelium near the surface for the leaves is the approach players describe, and it doubles as a shortcut to the surface mushrooms that drift spores. Which mushrooms are worth putting up there is in building mushrooms on the surface.
What to do while you wait
Leaf food arrives on the surface cycle, so the network under the leaf fall is not a fire-and-forget unlock — you keep it alive and keep it exposed. During the wait, the useful work is elsewhere: the points you earn from every hypha you grow are what buy the next branch of the tree, and those points come from growth rather than from food. How that works is in the unlock points guide.