A protective top layer of carbon-rich organic material (straw, wood chips, leaves, spent mushroom substrate) laid on the living soil surface to insulate soil, retain moisture, house beneficial biology and slowly decompose into new compost.
Jeremy describes the mulch layer as doing three jobs at once: protecting the soil surface from powerful lights and wind, providing a protected home for predator mites, rove beetles, and red wigglers, and decomposing into future compost. It's the final step in finishing a living-soil bed.
Discussed in: Ep 006 · Ep 007 · Ep 008 · Ep 015 · Ep 017 · Ep 020 · Ep 024