Soil compaction is a major cause of tree decline in urban areas. Compaction occurs when a force, such as feet, vehicles, and even water from some sprinkler systems, creates pressure on the soil surface and compresses the soil particles. This force causes the soil aggregates to break into smaller particles, reducing the amount of pore space in the soil and increasing the bulk density. The reduced pore space hinders aeration, water infiltration, and root penetration. A lack of soil oxygen and poor water drainage retard root growth, jeopardizing the health of the tree.
The following are some common signs of compaction.
- Hard soil -- If it is hard to penetrate with a shovel, the soil is probably compacted. Earlier cultivation may have produced a layer of hard soil 10-12 inches below the surface.
- Standing water -- Water standing on top of soil for long time may indicate compaction.
- Excessive water runoff -- This is a sign that the soil is saturated, there is low permeability or little movement of water through the soil.
- Loss of vegetation or poor plant growth -- Compacted soil does not allow the necessary air circulation and water infiltration into the root zone. A lack of sufficient water, nutrients, and oxygen to the roots causes declining tree health or death.
- Bulk density -- A high soil bulk density usually indicates compaction. The reduced pore space limits water infiltration, aeration, and root growth.
- Surface crust -- Development of a surface crust sometimes occurs with the compaction of fine-textured soils. This crust limits the infiltration of air and water to the soil and increases runoff and erosion. Surface crusts are usually found on soils subject to heavy foot traffic, such as playgrounds and footpaths.
Prevention During Construction Activities
|Photo Credit: Ed Macie
- Avoid working with wet soils -- Always work with dry or moist soils, never with wet soils.
- Limit travel routes and parking areas -- Limit travel by both people and vehicles to a few paths, and do not park vehicles under trees. For pedestrian traffic, a raised wooden path can prevent compaction. Vibrations from construction equipment can compact the soil.
- Use barriers -- Place barriers, such as fencing, around trees (minimum around the drip line) that you wish to maintain at the site.
- Apply mulch -- Spread a layer, 4-6 inches thick, of coarse mulch, such as wood chips or bark, on soil surface in the area likely to be compacted. The mulch should not touch the tree trunk. A thicker layer of mulch (up to 12 inches) can be used temporarily during construction in areas around the root zone.
- Location of roads and driveways -- Roads and driveways should be located as far from trees as possible.
- Use lightweight vehicles -- Try to use only lightweight vehicles with large, smooth, low-pressure tires.
- Apply surface grading -- Place grates or other artificial surfaces (plywood) over the planting site to prevent people from walking or driving on it.
Solving Compaction Problems
Compaction is difficult to correct; however, there are several techniques that may help solve compaction problems. The best option depends upon the conditions at the site, whether this is a new planting or an existing tree, and the available resources.
On New Sites:
- Soil mixing -- Compacted soil can be mixed with compost or a fully composted organic mulch to improve the soil quality, but up to 50 percent volume of soil is needed to make this technique useful.
- Rototilling or grading -- For a new planting site, the compacted layer of soil may be rototilled, disked, or graded to promote water infiltration.
- Subsoiling or drilling hardpan -- If there is a compacted layer of soil 1 or 2 feet below the surface, subsoiling or deep plowing with a plow or backhoe when the soil is dry may break up this impermeable layer. If subsoiling is impractical, holes can be drilled through the hardpan to provide drainage and better root penetration (Harris et al. 2003). If there are existing trees near the site, care must be taken not to damage the roots.
- Top mulching -- In both new and existing plantings, the organic content of soils can be increased by adding mulch on top of the soil. However, organic material takes many years to break down and combine effectively with the soil.
On Existing Sites:
- Core aeration -- Pore space in compacted soil may be increased by removing small soil cores to a depth of about 3 inches. This is effective in increasing surface permeability, but does not address compaction in deeper soil layers.
- Vertical mulching -- Holes 1 - 2 inches in diameter may be drilled in the compacted soil and filled with perilite, vermiculite, or other amendment material.
- Radial trenching -- Trenches 6 - 8 inches wide and no deeper than the root system or depth of compaction can be dug with trenching equipment. The trenches are dug around the trunk of an existing tree in a bicycle spoke pattern, extending from the trunk and backfilled with a mixture of soil and amendments.
- Air excavation -- Alternating pie-shaped wedges of soil around an existing tree can be decompacted by “tilling” compost into surface soil around roots with an air excavation tool. Air excavation can damage roots if high pressures are used, so soil should be decompacted in no more than 50 percent of the root system at a time.
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By: Ed Macie, Regional Urban Forester, USFS Southern Region