Soil Compaction: “Number One Urban Tree Stressor.”
Trees are vital to the health of our neighborhoods, providing beauty and shade in the summer months and reducing energy costs. However, these trees can only reach their full potential if they have access to adequate soil moisture and nutrients. Unfortunately, soil compaction is one of the most common constraints on their growth.
The adverse effects of compaction on trees include physical damage to roots and reduced growth. In addition, compacted soil can impede water infiltration, leading to drought stress and increased susceptibility to pest and disease problems. Trees in compacted soil are also more likely to be uprooted during storms. It is essential to take steps to reduce compaction in the landscape. Proper management minimizes the negative impacts of soil compaction, allowing landscape trees to thrive.
Trees in parking areas, near driveways, or along sidewalks are subject to the heaviest vehicular traffic and are most likely to become compacted. In these areas, choosing tree species that are tolerant of compacted soils is vital, and designing the tree planting zone with enough room for root growth.
There are several ways to prevent soil compaction:
Avoid driving or parking on lawns whenever possible. If you must drive on grass, use a vehicle with low ground pressure, such as a car with front-wheel drive.
Limit foot traffic in areas where you want trees to grow. Use paths or walkways instead of walking directly on the ground.
Aerate your soil regularly to help loosen compacted soil and improve drainage.
Reversing compaction is challenging; however, several techniques may help solve compaction problems. Several of the following methods involve an experienced practitioner; a certified arborist is ideally suited to assist you in this process.
New Planting Sites:
Soil mixing — you can mix compacted soil with compost or a fully composted organic mulch to improve the soil quality, but up to 50 percent of the soil volume is needed to make this helpful technique.
Rototilling or grading — For a new planting site, the compacted layer of soil may be rototilled, disked, or graded to promote water infiltration.
Top mulching — In both new and existing plantings, the organic content of soils can be increased by adding mulch on top of the ground. However, organic material takes many years to break down and combine effectively with the soil.
Existing Planted Sites:
Core aeration — Pore space in compacted soil may be increased by removing small soil cores to a depth of about 3 inches. Core aeration effectively increases 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 perlite, 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. Trenches dug around an existing tree trunk 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 the soil should be decompacted in no more than 50 percent of the root system at a time.
The following information is from Dr. Kim Coder, Professor, Community Forestry, Tree Biology and Health Care, Warnell School of Forestry and Natural Resources at the University of Georgia.
Soil Compaction – Number One Urban Tree Stressor
Soil compaction is the most prevalent of all soil constraints on shade and street tree growth.
Every place where humans and machines exist, and the infrastructures which support them are built, soil compaction is present. There are few soil areas we see without some degree of soil compaction. Soil compaction is a fact of life for trees and for tree health care providers.
There are many environmental constraints on tree survival and growth. All limitations for trees have impacts on daily and seasonal growth which can be measured and prioritized. Many people become obsessed by small constraints on trees while major life-altering impacts are ignored. Soil compaction is one of those major problems causing significant tree stress and strain, and whose impacts are usually blamed on other things.
Figure 1 (Below) shows the individual items causing the greatest growth limitations for tree growth. The top three things (by far!) are soil water availability, soil aeration, and soil drainage -- all three greatly disrupted by site compaction. Tree care providers must help people understand soil compaction influences on tree growth and the need for soil renovation.
