Soil Compaction Can Threaten Trees

When you started the hobby of gardening, little did you suspect that you would have to play doctor. It has no doubt become obvious to long time gardeners that part of your commitment involves diagnosing problems with plants. It is essential that you know or discover what certain signs mean so that you know how to treat unhealthy plants.

Most signs that a plant is not healthy appear where you can see them. However, some plant problems occur deep within the soil where you can’t.

Trees, like any other plant, can suffer an assortment of diseases or problems due to insects. The signs are clear to you because you can see them.

However, trees can be under threat due to things that are occurring underground near their roots. If you are observant, then you might be able to see tell tale signs of a problem. For example, the tree’s leaves are yellowing and its canopy is thinning. These signs may mean that a tree may be suffering soil and root problems.

These symptoms also occur when a tree is experiencing pest issues and as a result many gardeners may turn to chemical sprays or fertilizers as a remedy. However, sprays or fertilizers won’t help if the tree is experiencing root problems due to soil compaction.

What Causes Soil Compaction?

Believe it or not, soil compaction is one of the biggest health problems trees can confront. It occurs due to excessive traffic or activity on the ground that affects the soil around the root of the tree. Also referred to as compression, compaction occurs because the excessive activity on the ground above the soil force soil particles to come tightly together and prevents air and water from reaching roots.

You should be aware that soil compaction leads to drought and reduces nutrient and organic matter. The reduction of organic matter is due to less ground cover that lessens the accumulation of plant litter and the development of fewer roots beneath the soil surface. This results in slower buildup or a net loss of organic matter in the soil.

Organic matter is essential for healthy soil because it serves as a “glue” that hold soil particles together in granules. Called peds, the granules are beneficial in resisting compaction. Organic matter is also a source of nutrients.

In forests, many nutrients develop from the fallen leaves from the trees. The leaves serve as natural mulch that gathers nutrients in precipitation.

Specific activity that can cause soil compaction include lawn maintenance, vehicles parked in an area near a tree, driving a vehicle over the tree root area, construction work, flooding, and lack of proper drainage.

The obvious way to limit compaction near trees is to limit the traffic in the area. This can be achieved with paths that direct foot traffic away from trees and preventing anyone from parking or driving a vehicle near trees.

If you suspect that a tree on your property is suffering issues due to soil compaction, it is advised that you contact an International Society of Arboriculture (ISA) Certified Arborist and ask him or her to inspect the condition of the site in question, evaluate the tree’s health, and examine the area around the tree.

Don’t fret if you discover that your tree or trees are suffering problems due to soil compaction. The issue can be treated with good cultural practices including keeping people and heavy things off a tree’s root zone, spreading mulch around the tree, and extra irrigation. If conditions are extreme, then air excavation tools can be used. Professional arborists use special tools to loosen soil and add soil replacements to improve aeration and water infiltration.

Treating A Tree In Compacted Soil

To treat the problem one needs to not only deal with the compaction, but also with the lack of organic matter and nutrients.

Treatments include:

  • Drill-hole soil replacement
  • Compressed-air treatments
  • Radial-trench soil replacement
  • Soil cultivation
  • Mulching
  • Irrigation

A traditional method for treating trees suffering issues due to compaction, drill-hole replacement involves drilling holes 2-inches to 4-inches in diameter and 8-inches to 10 inches deep. The holes are drilled in spacing of 1.5-feet by 1.5-feet around the tree. Depending on the analysis of the soil, the holes are filled with a combination of fertilizer and organic matter including compost or peat moss. This provides much needed nutrients to the roots. Roots grow within the drill holes within weeks after they’re created.

The knock to Drill-hole replacement is that it affectively treats a small area of soil. The use of 2-inch-diamater holes replace only about 5-cubic feet per 1,000 square feet down to a depth of just 6-inches. Using a 4-inch-diamater hole increases the amount of area treated to about 20-cubic feet. Although it is more, it still is inadequate for treating a larger area. It’s been shown that this method offers little improvement to trees.

In compressed-air treatments, several machines are used to inject 100 psi to 300 psi of air into the soil. The influx of air lifts the soil surface by up to a foot. Fill material including peat, Styrofoam or porous ceramic along with fertilizer is then injected into the soil to ensure that some cracks stay open.

Compressed-air treatments affect a large area of up to 50-square feet per injection. Using spacing of 10-feet by 10-feet improves 500 of every 1,000 square feet of area. However, the aeration machines create one saucer-shaped crack in fine soil. The fill material is hardly able to keep the crack open. So only about 0.5-inches is affected around the fracture. Treatment of 1,000-square feet of surface enhances only about 30-cubic feet of area. At first thought, one would think that treating 50 percent of an area would cause dramatic improvement. However, it doesn’t because only 4-percent of the volume is treated. It has been shown that this technique is ineffective. Positive results do occur due to the injection of fertilizer, not the aeration.

Used by arborists in Europe, radial-trench soil replacement involves vacuuming compacted soil from around trees and replacing it with a soil mix. It has been shown that this treatment works well, but is expensive.

Soil compaction can threaten trees as much as plants.
(Courtsey: David Ohmer at

A more conservative approach of this method involves replacing soil from selected areas including a trenches radiating out from the trunk of the tree. When treating larger trees with this method it is suggested stretching the trenches at or beyond the drip line of the tree and trenching toward the trunk no closer than 6-inches for each inch of trunk diameter. It is recommended that roots that are greater than 1-inch in diameter should not be cut if encountered.

The ideal trench width has not yet been determined. It is shown that radial-trench soil replacement provides benefits for trees for two or three consecutive years. However, you can’t trench the same line twice.

It has been discovered that trees that have been recently transplanted, but fail to establish a root system due to compacted soil will benefit with the Soil cultivation treatment. However, before performing this treatment it is suggested that dig sample holes inside and outside the root ball to ensure that there are live roots in the ball or backfill. Soil cultivation with this method is fast and simple. However it is appropriate to use if a few tree roots extend into the soil. This treatment may be harmful to large, established trees.

Soil cultivation treatment involves the spreading of organic matter including composted yard waste or wood chips to a depth of 2-inches. The material should be applied as a circle around the root ball. The radius of the circle depends on how long you intend the treatment to benefit root growth. Healthy root growth averages about 18-inches a year in temperate climates. This treatment has been shown to be effective for two to three years if the width of the circle is about 3-feet to 4.5-feet. It is suggested that you add granular slow-released fertilizer on top of mulch if soil tests show it is necessary.

Till the fertilizer/mulch mix into the soil to a depth of 4-inches to 6-inches. If the level of organic matter in the soil is less than 3-percent, then till two or three times to work sufficient levels of material into the soil. Mulch the area last with 2-inches to 4-inches of wood chips. This method affects 100 percent of the soil volume and there is a good probability of success. However, you can’t repeat the treatment because roots will grow into the treated area.

Applying mulch to trees on compacted land can be beneficial. This is the least expensive treatment available. Apply the mulch in 2-inch to 4-inch layers from near the trunk to out as far as the drip line if possible. Don’t apply mulch directly to the trunk. This method conserves soil moisture in the short run. Reapply the mulch yearly if necessary to ensure adequate thickness. In the long term, the mulch will guard against additional compaction and introduce organic matter to the soil.

Some who have used this treatment claim that using only mulch has proven to be very successful in helping trees survive compacted soil.

Irrigation has been shown to be a successful method for trees planted in compacted soil. Proper soil-moisture levels are essential to growing root. This treatment and the use of a moisture-sensor can assist trees to overcome the effects of compaction. The use of tension meters to time manual watering is recommended to enhance the chance of success of this treatment.

Compacted soil can be a threat to trees of all sizes. However, there are treatments that will ensure the tree’s survival.

(Sources: and

About Robert Janis

Written by Robert Janis for LawnEq - Your specialists for Lawn Mower Parts and Small Engine Parts. We offer genuine premium OEM parts for Land Pride, Toro and many more dependable manufacturers.