Soil compaction: How it happens

Soils are composed of individual particles (sand, silt, and clay) and aggregates or clumps of particles. Between these soil particles and aggregates are spaces or pores. These “pore spaces” hold air and water. Large-sized soil pore spaces are usually filled with air, and provide good aeration to soils. Smaller soil pore spaces are usually filled with water.

Soils become compacted when heavy equipment, vehicles, and even riding lawn mowers are driven on them. Even concentrated pedestrian and animal traffic can compact soils. In compacted soils, the pore spaces in the soil are crushed, causing them to collapse. With no pore spaces, aeration and water infiltration in soils are greatly reduced.

The negative impact of soil compaction on tree health

Most tree growth occurs from the tips of branches and roots. However, while the leaves of trees are surrounded by open air and can freely exchange gasses, tree roots need a source of water and oxygen in the soil to grow. Roots utilize soil pore spaces to access water and essential elements, such as oxygen.

Compacting soil over a tree’s root system reduces the air spaces between soil particles. The reduced pore space hinders aeration, water infiltration, and root penetration or growth. Tree roots grow very poorly in compacted soil. This can lead to a decline in overall tree health and even tree death.

Soil compaction is a major factor limiting the growth of urban and community trees. Every place where humans and machines exist, and the infrastructures that support them are built, soil compaction will be present.

How you can prevent soil compaction

A few passes under a tree’s canopy with heavy machinery can compact the soil and dramatically change its ability to support root growth. During construction and development projects it is important to restrict site access to a tree’s rooting zone as much as possible by erecting fences prior to site development starting.

Selecting working conditions, such as dry summer months, that minimize compaction is also helpful. Additionally, softening and distributing heavy loads by using a temporary layer of course mulch and/or plywood driving mats around a tree’s rooting area is also helpful. Of course, it is best to restrict all vehicles to designated corridors and not use the area under a tree’s canopy as a parking zone or materials storage area.

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Can soil compaction be corrected once it occurs?

It is difficult to correct or improve compacted soils. However, there are several techniques that can help solve the problem. The options available depend on existing site conditions and whether this is a new planting or an existing tree.

On new sites, dig wide, but shallow, planting holes and mix soil 50/50 with compost to improve soil quality. Rototilling or disking the compacted soil layer prior to planting may be helpful and will improve water infiltration.

In both new and existing plantings, the organic content of soils can be increased by adding mulch on top of the soil. However, it can take years for it to break down and effectively combine with the existing soil.

On existing sites, soil compaction can be reduced in a number of ways, including removing small soil cores to a depth of about 3 inches, similar to aerating a lawn. This method increases surface permeability, but does not address deep soil compaction. Similar to soil cores, a method called vertical mulching can be used. This is where holes 5-6 inches in diameter are drilled into the soil across the compacted area and then filled with organic compost.

Probably the most common method used, and the method of soil decompaction used by Cutting Edge Tree Professionals, is radial trenching. With this method, trenches 5-6 inches wide and no deeper than the depth of compaction are made using an air excavation tool. The trenches are arranged around the trunk in a bicycle spoke pattern, extending from the trunk outwards. The trenches are then filled with a mixture of topsoil and compost. The entire area, out to the tree’s drip line, is then properly mulched.

Contact your local arborist for help

For a free on-site evaluation, contact Cutting Edge Tree Professionals at 814-201-9757 and we’ll send an Arborist Representative to help.


Written by Dave Jackson, Cutting Edge Tree Professionals Sales and Consulting Representative