The following is an excerpt from “Arboricultural Practices, A Scientific Approach,” a new book by Lindsey Purcell, with illustrations by Jeff Harris. It is tentatively scheduled for release in January 2024. This excerpt has been edited for style and length.
There are numerous challenges, below ground as well as above, that imperil tree growth in urban and suburban zones. Trees need freedom to grow naturally, above and especially below ground, to develop a sustainable root system supporting long-term survival. Soil characteristics affecting tree survival include pH, drainage, depth, salinity and physical impediments to root growth, including common hardscapes. Many plantings fail because these factors are improperly evaluated or ignored completely with the mistaken belief there won’t be a problem. A thorough site analysis will identify soil conditions and provisions necessary to improve circumstances.
Adequate, quality soil is a valuable commodity and should not be overlooked. In the event of new installations, preconstruction planning gives the consulting arborist the opportunity to work with contractors to prevent soil compression, which can lead to compaction in areas where trees will be planted. Tree installation on sites with poor conditions found in urban areas, such as inner-city lawn strips and suburban rights of way, often experience similar issues with inherited, poor- performing soil conditions and challenging below-ground issues.
Most soils encountered in urban and suburban landscapes are compacted and poorly drained. (Photo 1) Construction and building activities compress soils if precautions are not taken to preserve pore space. Test for compaction and drainage before planting to determine if the planting strategy needs to be adjusted. The testing can be accomplished by digging several holes or an individual hole at least 12 inches deep and checking results.
If the soil is difficult to dig with a shovel, it is likely compacted. If soil is fairly easy to penetrate with a shovel, it is likely more acceptable. Consider soil moisture when digging, since dry soils are much more difficult to excavate. Soil moisture is critically important to the process of compaction. Dry soils are difficult to compact, because the friction between particles prevents them from moving.
Next, check for drainage. The ability of the soil to allow water to drain away from tree roots can be determined by filling these holes with water and measuring percolation rates. (Photo 2) If the planting pit is empty within a few hours, drainage is acceptable. If water is still standing for several hours or more, it’s trouble for the tree. This is an indicator the soil is compacted all the way to the bottom of the planting hole – adjustments will be needed.
One option may be a change in tree selection. Consider trees that tolerate poor drainage. A soil probe or Dutch auger is a handy and minimally invasive tool to investigate soil texture and level of compaction. (Photo 3: 6.8) These instruments can provide quick, field-level determinations on soil moisture, compression and texture.
More detailed metrics can be obtained in the lab with various tests. One important test is determining bulk density of soils. Bulk density is defined as the mass of soil particles divided by the total volume they would occupy. This measurement is an indicator of compaction, a situation often encountered by plant-health professionals. The extent of compaction in soils can be determined in a few ways.
A Proctor test is most often used to determine whether a soil is compacted to the minimum level to support structures (such as sidewalks, walls or buildings). For engineering applications, 90% is standard for many residential sidewalk applications and 95% and greater for (larger) structures. Eighty-five percent is often quoted as the maximum density for planting soil, yet research indicates this value is root limiting for many plants. A measurement below 80% is a better standard for planting soil. The Proctor test is a very reliable test, but requires specialized equipment and geotechnical lab facilities for testing. This makes it a less-practical tool for consulting arborists to apply in the field.
Penetration resistance is another method of testing compaction. This requires the proper tool, called a penetrometer, which reflects the difficulty of pushing the tool’s probe into the soil, measuring resistance. (Photo 4) A penetrometer is most useful to check the relative compaction of planting soil where the soil type and moisture are relatively consistent.
A penetrometer is a pointed steel rod with a gauge that records the pressure needed to penetrate the soil. It provides a specific reading. However, it requires some experience and knowledge of the soil and process. Measures of penetration resistance are highly dependent on soil moisture and the speed it is pushed into the soil. A dry soil will have greater resistance than a soil at field capacity, which is the ideal moisture condition for a given soil. Therefore, caution should be exercised when using a penetrometer – it is most reliable when the soil profile is moist.
Compaction measurement using bulk-density values is simpler than the Proctor test and more accurate than penetration testing. This requires samples to be excavated from the site and sent to a soil lab for analysis. It is most effective and informative during the soil-analysis phase on construction sites and those that have been mechanically disturbed. Calculating bulk density is the best diagnostic tool for existing soil, and should be included as part of an overall soil analysis.
It is worth noting that plant roots do not respond directly to bulk density. The rate of root extension and diameter increase is determined by soil mechanical impedance. This is conditional on particle size, pore space and moisture levels. As bulk density increases, soil moisture increases. However, moisture in the soil can act as a lubricant, allowing roots to grow even in soils with very small pore spaces.
Effects of compaction
Compaction affects infiltration, rooting depth, water-holding capacity, porosity, the availability of plant nutrients and the activity of soil microbes. The total volume includes particle volume, inter-particle-void volume and internal-pore volume.
Bulk density is not a fundamental property of the soil; it can change depending on how the soil is treated or manipulated. For example, a soil can have a particular bulk density measurement of 1.3 g/cm3 (gram per cubic centimeter), as measured by the lab, which is an acceptable metric for a soil with about 50% pore space. Later, the soil may be compressed by construction equipment in preparation for a new sidewalk or driveway. The arborist determines that the bulk density has increased to 2.1 g/cm3 from laboratory measurements, which is highly compressed with very limited pore space. This difference demonstrates that compression has moved the soil particles more closely together, so they are occupying the once-open pore space, displacing air and water space with solids.
Note the differences in soil, air and soil water in Figure 1, which illustrates the preferred composition of a healthy soil. Also, it illustrates the composition of a soil that has experienced compression by heavy equipment or vehicles. As soil particles are squeezed or compressed together, pore space for air and water is minimized. The reduced pore space creates an unfavorable growing medium for any plant, including trees.
A key indicator of soil compaction is expressed by symptoms of decline, usually in the upper crown. When soil is compacted, there isn’t enough room for water, nutrients and air to occupy the space of the roots (pore space). As conditions worsen, plants begin to express nutrient-deficiency symptoms such as yellowing leaves, early leaf drop and reduced growth.
When soil is compacted, there isn’t enough room for water to easily penetrate the ground. This means that, even if there has been lots of rain, the moisture likely hasn’t been able to reach the roots and the pore space. This level of compaction will cause trees to start showing drought symptoms, such as wilting and curling leaves, yellowing leaves, decline in vigor and leaf scorch (brown outside edges or browning between veins). When symptoms are expressed in the upper portion of the crown, rooting and soil issues are prime suspects for compromising tree health. This can be seen in Figure 1.
Soil health is important for tree health. Bulk density is a useful metric to help evaluate soil conditions and diagnose tree-health issues. The greater the bulk density, the more restrictive the root growth and function. Recognizing soil texture can help interpret bulk-density measurements and provide an idea of what to expect for the situation.
Mitigating compressed soils
Compaction is a killer when it comes to sustainable tree plantings. Often it requires specialized procedures to mitigate the compressed soils to improve rooting. First, when planting in compacted soil, break down and pulverize the substrate as much as possible to allow roots to anchor into the surrounding soil, outside the planting pit. Roots are likely to remain shallow in compacted soil, then begin to form dysfunctional systems. The tree may grow poorly if this preparation is inadequate. Drilling holes in hardpan below the root ball might help encourage some deep roots that could help secure the tree in place, as well as possibly improve drainage.
Because roots grow poorly in compacted soil, it should be tilled or broken up with specialized equipment prior to planting. It has been demonstrated that trees benefit from larger, wider planting holes in these situations. This means breaking up or disrupting the soil around the planting pit, about 6 inches deep, to facilitate root growth. Avoid doing this beneath the canopy of existing trees. Significant root damage could occur, causing decline and perhaps compromised stability.
Radial trenches can help with compacted soils and encourage rooting beyond the planting hole as well. (Photo 5) Air-excavation tools can create trenches that can be filled with amended soil media or organic products, improving structure and air exchange.
Also, a coring machine can be used to enhance growth in compacted soil. This machine pulls cores out of the soil, creating openings for water and air penetration. The soil cores are left on the surface of the soil to gradually break down. This is sometimes referred to as vertical mulching.
Modified original soil or amended soils can be placed back into the trenches or holes, although there is little evidence amended soil increases root growth more than backfilling with original soil. Although this process may not provide all the benefits of loosening the soil around the entire planting hole, it may be less expensive, and roots should be able to grow better in the loose, aerated soil in the trenches. This technique also can increase root growth on existing trees in many situations.
Trees need to thrive, not just survive in harsh urban conditions. Our responsibility as urban foresters and arborists is to find ways to overcome these constraints to create more sustainable tree canopy. This will require creative planting strategies and critical thinking to develop longer-lived trees. As tree and landscape professionals, we must focus on not just planting trees for the future, but rather planting trees with a future.
A note from the author: This book provides an overview of various arboricultural skills and best practices based on current research. As an educator in the industry for more than 30 years, training tree care professionals and teaching undergraduate and graduate students, I realized during the writing process that this was the manuscript I would always have wanted as a textbook. Tree selection, installation, pruning and tree appraisal are just a few of the chapters included in this book, which is comprised of more than 300 pages and illustrations.
– Lindsey Purcell
Lindsey Purcell is an ISA Board Certified Master Arborist (BCMA), an American Society of Consulting Arborists (ASCA) Registered Consulting Arborist (RCA) and principal with LP Consulting Group LLC in Franklin, Indiana. He spent many years as an urban-forestry specialist and teacher in the Department of Forestry and Natural Resources at Purdue University, and serves as the executive director of the Indiana Chapter of the International Society of Arboriculture.