Quick answer: Mulch and Soil Health — Mulch is the cheapest thing that happens to a tree: a flat layer over the root zone holds moisture through the dry season, keeps the soil temperature steady and feeds the soil life the tree depends on. Almost every mulch problem is a depth problem or a contact problem — too thick, or piled against the trunk where the bark has to stay dry.

Typical depth: Two to four inches, flat, from the drip line inward — never a cone against the trunk.
Typical timing: After planting or after a clean-up; refreshed in autumn so winter rain carries it down.
What it involves: Keeping the root flare visible, keeping mulch off the bark, and choosing material that breaks down rather than mats.
The standard: ISA and university extension guidance on arboricultural mulching and the root flare.
Our own record: 503 pages and 291 stories published on this site; Uncategorized (109) is the rail this page files under. Counted from our own archive, not a stock table.

Field note. What we see most on this coast is not too little mulch but too much, in the wrong place: the volcano against the trunk that keeps the bark wet through the fog season and invites rot where the tree cannot show it until the canopy thins.

On the other hand, a thin ring of the wrong material is worse than none at all — matted, water-repellent mulch sheds the rain it was meant to store.

Standards and sources: Arboricultural standards (ISA — International Society of Arboriculture) · Soil and root health (UC IPM — University of California pest management)
Prepared by Cambria Pines Tree Service Inc. — ISA-certified arborists working on the Central Coast.

Mulch is usually discussed in terms of what it does to water and weeds. The more interesting thing it does is select which organisms live in the soil beneath it, and that selection is largely determined by the carbon chemistry of the material.

Two food webs

Soil biology is often described on a spectrum from bacterial-dominated to fungal-dominated. Annual and disturbed systems — vegetable beds, lawns, weedy ground — tend toward bacterial dominance. Perennial systems — forest, orchard, grassland, shrubland — tend toward fungal dominance.

The organisms differ in what they eat. Bacteria consume simple, soluble carbon: sugars, root exudates, fresh green material. Fungi consume complex, lignified carbon: wood, bark, the structural tissue of plants. Which means the material on the surface is a substrate that favours one group or the other.

What woody mulch does

Coarse arborist chip is a fungal substrate. Its lignin content selects for the white-rot and brown-rot fungi that can break down wood, and those fungi build hyphal networks that extend far beyond the chip layer into the soil, transporting water and nutrients between particles that roots could not otherwise reach.

That network is the reason a mulched tree survives a dry summer better than an unmulched one of the same size on the same soil, and it is not fully explained by evaporation alone. The hyphae are a distributed water delivery system.

What compost does

Compost is a bacterial substrate. It is already largely decomposed, its carbon is simple, and applying it selects for bacterial dominance — which is exactly right for a vegetable bed and wrong for a shrub border.

Which is why compost applied thickly around trees is a mistake even though it is sold as a soil improver. It pushes the biology in the wrong direction for the system that is there.

How to tell which you have

Two field tests, neither requiring a lab. Push a spade into the soil and lift a slice. A fungal soil holds together in a crumb structure with visible white threads and a smell of leaf litter. A bacterial soil is either powdery or compacted, with a sour or ammonia smell and no visible mycelium.

The second test is the litter layer itself. Lift the mulch in a bed that has been mulched for two years. If the underside is bound together with white mycelium and the material beneath has darkened and aggregated, the fungal component is working. If the underside is slimy and the soil beneath is grey and compacted, it is not.

The practice that follows

For a tree, a shrub border, a hedgerow or an orchard: coarse woody chip, on the surface, two to four inches, renewed as it breaks down, never dug in. That is a fungal programme.

For a vegetable bed: compost at one to two inches, incorporated or left on the surface at the end of the season, plus a straw or leaf mulch during the crop. That is a bacterial programme.

Mixing the two — compost around the trees because it is “better”, chip in the vegetable bed because it is free — works against both. The material is a tool, and the tool has a target.

The time scale

None of this happens in a season. The shift in fungal biomass in a mulched bed takes three to five years to show in a spade test, and the structure that develops is the visible evidence. Which is the argument for starting now and for not skipping years: the layer has to be maintained continuously for the biology to establish, and a bed that is mulched for one season and left for three loses most of the gain.

What a healthy fungal soil then does for the plant is described in the chemistry side.