Decompacting soil around trees: what a five-year study, the National Park Service and a city detail report

This page only reports what the named sources say. How compaction harms roots is set out on root problems: what sources say.

What compaction is, as the sources define it

The University of Maryland Extension sheet, revised in 2019 and updated February 28, 2023, says compaction happens when soil particles are forced closer together and pore space shrinks. It lists heavy foot traffic, construction equipment and vehicles parked under trees as causes, and says clay soils compact more readily than sandy ones. Its symptom list for a tree in decline includes reduced growth, scorch, early fall colour and dieback, usually progressing over a number of years. On fixing it, the sheet says correction is difficult, that cultivating organic matter into soil under established trees is not recommended, and that an arborist may use an air spade to fracture compacted soil, often in small pie slices under the canopy, backfilled with native soil and organic matter.

The five-year trial

Percival, Graham and Franklin published a trial in Arboriculture and Urban Forestry, volume 49, issue 4, July 2023. It ran at Hatfield Forest in Essex, United Kingdom, on four grass paths with clay loam soil. Bulk density before treatment ranged from 1.73 to 1.80 g/cm3, against a growth-limiting value for that soil texture of 1.50 g/cm3, which the authors give. Nine treatments were laid out in 3 by 3 metre plots: a control, wood chip mulch, air spading, vertical mulching, and combinations with biochar at 5 percent by volume. They tracked bulk density, organic matter, ground cover, cotton strip degradation, root dry mass and earthworm counts for five years.

The authors report that air spading with biochar and a wood chip layer was the best treatment for soil quality over the five years, and also the most expensive and time-consuming. Wood chip mulch alone was the most effective single treatment and the most cost-effective. Air spading alone was reasonably effective, and chips added to it improved the result. Vertical mulching, alone or with biochar, had little influence on soil quality, though chips applied after it produced a significant increase on every measure. They also report that bulk density rose gradually from year 1 to year 5 even where it stayed below the control.

The National Park Service account

A National Park Service article, last updated November 8, 2024, describes its staff using an air-powered excavation spade in several national cemeteries. It says the tool loosens soil without damaging roots, and it names two uses: radial trenching in the root zone, and vertical mulching, which means boreholes around the base. The article says the trenches or holes are filled with excavated soil mixed with compost or sand, then mulched, and that archeological consultation and monitoring is part of the process at those sites. It says dieback in the canopy can be an indication of compaction around the root zone.

A city drawing

The City of Charlottesville, Virginia publishes a one-page soil decompaction detail. It shows a 3 to 6 inch modified soil layer made by turning 2 inches of compost into the top 2 to 4 inches of existing soil with an air spade at reduced pressure, topped with 3 to 4 inches of shredded bark mulch or 6 to 8 inches of arborist wood chips. Its notes call for removing sod first, keeping air spade work at least 5 feet from the root flare, and watering the whole critical root zone at the end of each day of work. The drawing is undated.

What each source reports

Source	Kind	What it reports
Arboriculture and Urban Forestry, July 2023	Five-year field trial, United Kingdom	Chips were the most cost-effective single treatment. Air spading with biochar and chips scored best and cost most. Vertical mulching alone had little effect.
National Park Service, updated November 8, 2024	Agency article	Air-tool trenching and boreholes, refilled with soil and compost or sand, then mulched.
City of Charlottesville	Construction detail	A modified soil layer, mulch or chips on top, a 5 foot offset from the root flare, daily watering of the root zone.
University of Maryland Extension, 2019, updated February 28, 2023	Extension sheet	Prevention is the best route. Correction is difficult. An arborist may use an air spade.

One trial on one estate with one soil type is not a forecast for another site. The trial authors say few studies have followed these treatments individually and combined over the long term.