Mulch vs compost

Compost improves the soil. Mulch protects the soil. Put compost under mulch and you have given the garden both groceries and a roof.

What compost does

Mature compost adds organic matter and supports aggregation, nutrient cycling, and water management. Spread it over the soil or incorporate it shallowly when building a new bed. It should smell earthy and be cool, mature, and free of recognizable ingredients.

What mulch does

Coarse leaves, straw, pine bark, or wood chips shield soil from sun, pounding rain, and evaporation. Mulch suppresses many weeds and slowly becomes organic matter as organisms work at the soil-mulch boundary.

How to layer them

1. Pull existing mulch aside.
2. Spread mature compost over the root zone.
3. Add castings lightly near active roots if desired.
4. Return 2–4 inches of coarse mulch, keeping stems and trunks clear.
5. Water slowly enough to reach the soil beneath.

Common mistakes

• Mulch volcanoes: piles against trunks hold moisture against bark and hide root-flare problems.
• Fresh unfinished compost: active material may heat, tie up nitrogen, or damage roots.
• Fine mulch mats: dense layers can repel water or restrict air.
• Buried crowns: vegetables and perennials can rot when compost or mulch covers the crown.

What about wood chips and nitrogen?

Wood chips on the surface generally do not rob deep soil nitrogen from established plants. The issue is mixing large amounts of fresh woody material into the root zone, where decomposers compete for available nitrogen. Keep coarse carbon on top and let the boundary decompose naturally.

Compost feeds the system from within. Mulch guards it from above. They do different jobs and work best as a team.