continuing education unit
Managing Soils That
Support Urban Trees
(part one)
By Bryant C. Scharenbroch, E. Thomas Smiley, and Wes Kocher
Objectives
• Describe effective soil sampling techniques
• Identify important soil parameters for assessing urban soils
• Summarize how to assess soil parameters
• Understand the impacts of various soil parameters on tree health
CEUs for this article apply to Certified Arborist, Utility Specialist,
Municipal Specialist, Tree Worker Climber/Aerial Lift Specialist,
and the BCMA practice category.
Soil Assessment
Soil should be assessed prior to conducting any soil management work. Soil assessment is used to determine soil
characteristics and conditions that may limit tree or
shrub development. Once this is determined, management recommendations can be developed to mitigate the
limiting conditions. Soil and site assessment information
can also be used prior to planting to match plant species
with site conditions.
Assessments can be quick, field-based procedures, or
they may include more in-depth site and laboratory analyses. An in-depth soil analysis may include characterizing
a soil’s physical, chemical, and biological properties. Key
soil parameters that are often assessed include texture,
structure, density, water, organic matter, pH, salinity,
nutrients, and microbial activity.
Soil Sampling
Soil conditions can vary significantly within small, seemingly uniform sites. Tree and/or site size, soil variability,
Tree and/or site size, soil
variability, and the conditions
being assessed should guide
your decision on where to
sample and how many samples
to collect. Soil samples should
be collected from locations and
depth(s) within the soil profile
that provide information necessary to make recommendations.
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and the conditions being assessed should guide your
decision on where to sample and how many samples to
collect. Soil samples should be collected from locations
and depth(s) within the soil profile that provide information necessary to make recommendations.
Collecting Soil Nutrient Samples
When collecting samples for soil nutrient analysis, samples should be collected from within the area where fertilizer will be applied. For individual trees and shrub beds,
this area is typically within the drip line. Prior to planting
larger landscapes, samples can be collected from the area
most likely to see planting.
Where multiple samples are collected, the collection
sites should be randomly selected within the defined area,
or they should be collected in a grid-like pattern so that
the sample will represent the variability present on the
site. On larger sites, it may be beneficial to subdivide the
whole site into smaller segments that share common
characteristics, such as position on a slope, plant species,
or management areas.
Comparative sampling is often used for plant or soil
problem diagnoses. It is the process of sampling from
areas where trees have a problem and comparing the
results to those from nearby areas where trees are nonsymptomatic, unaffected, or undisturbed. For instance, if
salts were thought to be involved in the decline of a tree,
you would want to know the salt levels in both affected
and unaffected locations.
Since the majority of fine tree and shrub roots are in
the top six to eight inches (15–20 cm) of soil, this depth
is considered appropriate for nutrient analysis sampling.
In areas where fine roots are growing at greater or lesser
depths, samples should be collected from those areas.
Soil nutrient sampling is performed with a soil sampling probe or other tool. Soil sample probes are steel
tubes, typically ¾ to 1¼ inches (2–3 cm) in diameter,
with a tip designed for specific soil types, a handle, and
sometimes, a foot peg attached to the tube to drive the
probe into the soil.
When to Sample
The timing of when to sample is largely dependent upon
site conditions and the soil properties to be analyzed.
Generally, stable soil properties can be assessed any time
of the year. Soil properties, like inorganic nitrogen availability, change rapidly in response to moisture and temperature, so sampling is often conducted during periods
of peak availability, such as early in the summer, but not
immediately after major rain events. If trees are showing
signs or symptoms of stress at particular times during the
year, it may be useful to sample during these periods to
investigate whether the tree problem is related to soil
conditions.
Soil sample probes are steel tubes, typically ¾ to 1¼ inches (2–3 cm) in
diameter, with a tip designed for specific soil types, a handle, and sometimes,
a foot peg attached to the tube to drive the probe into the soil.
Soil Analysis Parameters
This section defines and interprets some of the common
parameters evaluated during soil profile analysis. These
parameters would typically be assessed on soils from each
depth sampled.
Texture
Soil texture relates to the size and composition of individual soil particles. The most common field-based approach
for determining soil texture is the “feel method.” This technique involves wetting and kneading the soil, making a
ribbon, measuring its length, and then assessing its relative grittiness and smoothness. The feel method is subjective. However, with practice and calibration, accurate
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continuing education unit
crust development and surface sealing, which impedes
water infiltration and increases runoff and erosion.
Soil structure is one of the quickest and easiest assessments of soil quality, and can be performed with minimal
training. Structure is visually assessed in the field to classify the aggregate type (shape), class (size), and grade (distinctness and cohesion). In the surface horizons, granular
structure that is loose, crumbly, and porous is preferable
to platy, massive, or blocky structures. Granular or blocky
aggregates that remain stable in water are considered ideal
for most plants. Granular structure is indicative of a soil with
adequate porosity, organic matter, and biological activity.
Soil structure is best assessed by excavating soil from
large holes or soil pits. Augers and core samplers are not
suitable tools for assessing soil structure due to the limited sample volume and the destruction of aggregates
during extraction.
Density and Porosity
The most common field-based approach for determining soil texture is the “feel
method.” This technique involves wetting and kneading the soil, making a ribbon,
measuring its length, and then assessing its relative grittiness and smoothness.
assessments can be made with minimal training or expertise. Soil texture can also be determined in a laboratory
when a more precise assessment is needed.
The majority of urban trees will do well in soils that
have balanced portions of sand, silt, and clay (i.e., loams)
because they have better water and nutrient supply capacities compared to sandy soils. Well-structured loams are
preferred to clay soils because they have better drainage
and are less prone to compaction-related problems. These
preferred soil textures have clay contents of less than 40
percent and sand contents of less than 80 percent.
Structure
Soil structure is the arrangement of soil particles into
larger groups called aggregates. A soil with good structure
allows air and water to readily penetrate, and allows for
root and microbial development. Loss of soil structure
hinders root growth and other biological activity, and
alters water and air movement into and through the soil
profile. Breakdown of soil aggregates is the first step to
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Soil is not a solid mass. It is a matrix of particles (sand,
silt, clay, organic matter) with spaces in between called
pores. The pores are filled with air and water. Soil compaction is the process of applying force to a soil, which
increases the density by decreasing pore space. Due to
vehicular and pedestrian traffic in urban areas, compaction is common and may limit plant development by
reducing root penetration, soil aeration, and water
infiltration.
Bulk density is the weight of soil (mass) per a unit volume, usually expressed in kg/m3 or pounds per cubic foot
of soil. Porosity is the measure of pore space in the soil,
which is inversely related to density. Soil bulk density is
most commonly measured by removing an intact soil
core of known volume, drying the soil, and weighing it.
Since gravelly or rocky soils do not remain intact when
removed from a coring device, a different method should
be used to calculate their density. To assess these soils, dig
a hole, determine the volume of the hole, and calculate
the dry weight of the excavated material. The volume of the
hole is obtained by lining and filling the cavity with a known
volume of sand, water, plastic foam, or other material.
As plants grow, their roots must penetrate the soil. If
the soil is too dense for roots to penetrate, plant growth
may be limited. To estimate how much resistance roots
are facing, a solid soil probe or soil penetrometer can be
used. Penetration resistance is highly dependent on moisture and texture, so care should be used when interpreting readings. Often, penetrometer readings are taken
along a line, from an area that is expected to have high
density to a nearby area that is expected to have low density (such as from a heavily used pedestrian trail under a
tree to a mulch bed). This can be used to determine where
the soil is compacted or where to collect samples for bulk
density analysis.
Ranges of bulk density and penetration resistance that
limit tree growth vary with soil texture. Root growth in
sandy soils is limited at higher density and resistance values than clayey soils. Across soil textures, soil bulk density
values less than 1.4 Mg/m3 are ideal for most urban trees.
Water
Water deficits and excesses strongly influence urban tree
growth and survival. Water and air occupy soil pores and
must be balanced for optimal tree growth. Excess water
due to poor drainage, compaction, surface crusting, and
overwatering are common problems in urban soils. Water
movement depends on texture, structure, and moisture
conditions. Water movement is most rapid in well-aggregated soils with large continuous pores in the surface.
Knowing the soil’s water content is useful in identifying plants stressed from too much or too little water.
Basic information on soil water can be obtained in the
field by observing physical indicators. Water deficits are
usually widespread in sandy soils, arid regions, during tree
establishment, or during periods of prolonged drought
and/or unusually high temperatures.
As soil approaches wilting point (the point at which most
plants begin to wilt), it becomes lighter in color and dusty
when crushed. As soil approaches field capacity (the point at
which gravity water has drained), it becomes more plastic
and leaves a muddy impression when squeezed in the hand.
The amount of water present in the soil can be measured
in a number of ways.
1.Electric soil moisture meters determine electrical
conductivity, which is related to water and salt
content. They are more accurate than visual assessments, quick, and relatively inexpensive, but the
readouts are comparable only to other soil of the
same texture and salt content.
2.Soil water content can also be determined by comparing the weight of water to the weight of the soil
(gravimetric measurement) by weighing a collected
soil sample, drying it at 221°F (105°C) for 24 hours,
and weighing it again to determine the mass of the
water lost.
3.Determining, usually with a tensiometer, how
tightly water is held to the soil (tension).
4.Determining how quickly water moves into and
through the soil (infiltration and percolation).
When soil is saturated or inundated with water, and
there is microbial activity in the soil, there will be a depletion of oxygen, leading to anaerobic conditions. Anaerobic soils stop root development for most tree species and
affect nutrient availability. If soil has a gleyed (or grayish)
appearance or a rotten egg smell, it is likely to have been
saturated for an extended period of time.
As plants grow, their roots must penetrate the soil. If the soil is too dense for
roots to penetrate, plant growth may be limited. To estimate how much resistance roots are facing, a solid soil probe or soil penetrometer can be used.
As soil approaches wilting point (the point at which most plants begin to wilt),
it becomes lighter in color and dusty when crushed (left). As soil approaches
field capacity (the point at which gravity water has drained), it becomes more
plastic and leaves a muddy impression when squeezed in the hand (right).
Infiltration rates can be assessed either by digging a
hole or by placing a ring firmly in soil and measuring the
time it takes for water to move into the soil. Using a sixinch (15 cm) hole or ring, add water (1 inch = 0.5 qt or
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continuing education unit
2.5 cm = 444 mL) and record the amount of time it takes
to infiltrate into the soil.
Infiltration rates are used to determine an appropriate
irrigation rate (frequency and amount), and a basis to
recommend other soil treatments. Irrigation rates should
be lower than infiltration rates to prevent ponding, runoff, surface sealing, and degradation of soil structure. Ideally, infiltration rates for urban landscape soils should be
at or exceed one inch (25 mm) per hour.
pH
Soil pH is the measurement of hydrogen ion activity in
the soil solution. The pH scale is logarithmic, so each
whole number is ten times greater or lesser in hydrogen
ion activity (more basic or acidic) than the previous value.
For example, a soil with pH 4 is 10 times more acidic
than pH 5, but 1,000 times more acidic than a neutral
soil (pH 7).
For precise measurements of soil pH, it is best to send
samples to a soil testing laboratory. Soil pH can be
assessed in the field using a pen-like glass electrode meter
and a well-mixed solution of one part deionized water to
one part soil. Metal electrode soil pH probes, litmus
paper, or mixed indicator kits can also be used for field
assessment of soil pH, but are not very accurate.
Alkaline soils (pH > 7) are common in urban areas
due to cement, construction debris, and alkaline irrigation waters. Alkaline soils are also found in areas with low
rainfall and where soil parent materials contain carbonates and high clay concentrations. Acid soils are more
common in highly weathered and leached soils in tropical regions, in soils receiving acidic litter from surrounding vegetation, in soils with acidic parent material, in
areas with high atmospheric nitrogen deposition (acid rain),
or in soils that have received continuous applications of
some fertilizers.
The optimum pH depends on the tree species. In
areas within the temperate zone, a slightly acid pH of
5.5–6.5 is considered optimal for many tree and shrub
species. As pH drops below 5 or rises to about 7, some
nutrients become less available for root uptake. Low pH
(<5.5) may result in deficiencies of calcium (Ca), magnesium (Mg), phosphorus (P), or molybdenum (Mo), and
perhaps excesses of manganese (Mn), iron (Fe), aluminum (Al), copper (Cu), nickel (Ni), and zinc (Zn). High
pH (>7.5) may immobilize Mn, Zn, Fe, or Cu, and result
in excesses of Mo, arsenic (As), and selenium (Se).
Nutrients and Contaminants
Trees require and acquire essential plant macronutrients
[P, nitrogen (N), and potassium (K)], secondary nutrients [Ca, Mg, and sulphur (S)], and micronutrients [Fe,
Mn, Zn, Ni, Cu, Mo, boron (B), and chlorine (Cl)] from
the soil. Visual tree assessments are a good first step in
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detecting nutrient deficiencies or toxicities. Trees with
nutrient imbalances often exhibit symptoms, such as
chlorosis, wilting, stunting, and scorching of leaf margins. The pattern of symptoms on the plant provides
important clues to identifying a nutrient deficiency. If
symptoms are found only on older or lower leaves, the
problematic nutrient is likely to be mobile (easily translocated), such as N, P, K, Mg, Mo, or Cl. Symptoms on
younger leaves are usually associated with immobile
nutrients, such as Ca, S, Fe, Mn, B, Cu, or Zn.
Soil and/or leaf samples should be collected and sent
to laboratories for nutrient analyses to confirm suspected
deficiencies and toxicities. Interpretations of nutrient
analyses and recommendations for amendment are often
included in laboratory reports, mostly by request and
additional fees. However, sometimes these recommendations are not based on tree nutrient requirements, so it
may be necessary to consult an expert for more a specific
interpretation. Additional information on soil nutrients
and fertilization is provided in the ISA publication Best
Management Practices—Tree and Shrub Fertilization (2013).
Salt contamination is a common problem for trees in
urban soils. Salts in soils may hinder plant growth by
making it more difficult for plants to take up water from
the soil. Trees growing in salt-affected soils often exhibit
symptoms resembling water stress (e.g., wilting, browning, leaf scorch, leaf drop, and ‘witch’s broom’). Visual
clues of excessive soil salinity include salt crusts on the
soil surface, loss of soil aggregation, reduced water infiltration, and water ponding. Soil salt levels can be measured in the field or at a laboratory with an electrical
conductivity meter. A laboratory analyses is recommended to determine the amount of sodium and chlorine in the soil, along with indicators, such as sodium
adsorption ratio or exchangeable sodium percentage.
Soil salts are weathered from primary minerals or
added in fertilizers, irrigation water, or other amendments. High salinity is also common in coastal soils due
to salt spray or contaminated groundwater, and in urban
soils in temperate regions where de-icing salt is applied.
Salt-affected soils are also common in arid regions, where
capillary action brings water and salts to the surface and
evaporation removes water, leaving salts on the surface.
Furthermore, arid soils are often irrigated with water that
is high in salts.
Contaminants may also produce plant problems.
Some common soil contaminants in the urban environment are heavy metals (from vehicles and industrial activities), organic pollutants, and pesticides. Assessing soil for
contamination is often very difficult and expensive.
Researching the site’s previous uses and understanding
local conditions that could contribute hazardous materials to the soil is a good beginning point to determine
whether soil contamination analysis is needed. Soils suspected to contain toxic levels of hazardous pollutants
should be assessed by a professional soil or environmental
scientist.
Organic Matter
Soil organic matter (SOM) is often described as the single most important indicator of soil quality. Soil organic
matter supports tree and ecosystem health, is dynamic,
and is highly responsive to management practices. Soil
organic matter includes anything that is currently, or
once was, living in the soil. It is derived from plants and
animals and is found in various states of decay and
decomposability. Decomposition also releases (mineralizes) nutrients, such as N, making them available for tree
uptake. Soil organic matter contains up to four times as
much readily available water and nutrients as clay,
increases soil resistance to compaction, fuels microbial
activity, improves aggregation, buffers pH, and sequesters
contaminants. If the topsoil has been removed or
degraded, management should focus on SOM restoration to facilitate tree health and growth.
Rough approximations of SOM can be made in the field
by assessing soil color. In most cases, the darker the soil, the
higher the SOM content. Quantitative determination of
SOM is a standard analysis performed by most commercial
laboratories, and is necessary when a precise determination
of organic matter content is required. Ideally, SOM contents of topsoil should be three to ten percent by weight.
Bryant C. Scharenbroch, Ph.D., heads The Morton
Arboretum Soil Science laboratory. E. Thomas Smiley,
Ph.D., is an arboricultural researcher with Bartlett Tree
Research Laboratories. Wes Kocher is an ISA educational
development manager.
Best Management Practices
Soil Management
for Urban Trees
ISA has published a new volume in its Best Management Practices series: soil management for urban trees.
This BMP describes the recommended practices for
assessing, modifying, and conserving soils that support trees and shrubs in the urban environment. This
publication serves as a companion for ANSI A300—
Tree, Shrub and Other Woody Plant Management—
Standard Practices (Soil Management a. Modification,
b. Fertilization, and c. Drainage). #P1552
Order online at www.isa-arbor.com/store
or call +1 217.355.9411
Soil organic matter (SOM) is often described as the single most important
indicator of soil quality. Rough approximations of SOM can be made in the
field by assessing soil color. In most cases, the darker the soil, the higher the
SOM content.
CEU Questions
Did you know that ISA members can take Arborist News quizzes online for
free? Simply go online to the ISA web store (www.isa-arbor.com/store), log into
your personal account, and select “Online CEU Quizzes” on the store’s main
page. If you need a username and password, send us an e-mail (isa@isa-arbor.
com). CEUs for quizzes submitted online are processed within 24 to 48 hours
(one business day).
If you prefer to use the paper answer sheet for home study of this article,
darken the appropriate circles on the answer form of the insert card in this
issue of the magazine. A photocopy of the answer form is not acceptable. A
passing score for this test requires sixteen correct answers. ISA will be phasing
out the magazine’s paper answer sheets in 2016.
Please also complete the registration information on your answer form,
including your certification number, and mail the form to: ISA, Certification, P.O. Box 3129, Champaign, Illinois, 61826-3129, U.S. Answer forms for
this test, Managing Soils That Support Urban Trees (part one), may be sent
for the next twelve months to obtain one continuing education unit (1 CEU).
If you do not pass the quiz, ISA will send you a retake answer sheet. You may
take the quiz as often as is necessary to pass. If you pass, you will not be notified; rather, you will see it credited on your CEU report (available online).
Processing CEUs manually takes four to six weeks.
CEUs for this article apply to Certified Arborist, Utility Specialist,
Municipal Specialist, Tree Worker Climber/Aerial Lift Specialist, and the
BCMA practice category.
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continuing education unit
1.What is the first step in soil
management?
a.modification
b.implementation
c.assessment
d.amendment
2.In temperate regions, a common
suitable range for soil pH that
supports trees is
a.3.5–4.5
b.4.5–5.5
c.5.5–6.5
d.6.5–7.5
3.When sampling on large sites, it may
be beneficial to subdivide the site into
separate areas that share the same
a. plant species
b.characteristics
c. management objectives
d.all of the above
4.Soil nutrient sampling should be
conducted
a. where buttress roots meet the
ground
b.along the perimeter of the tree’s
drip line
c. at the depth of the tree’s fine roots
d.as early in the spring as possible
5.Factors to consider for the timing of
when to take soil samples include the
a. tree species
b.typical soil pH in the area
c. soil properties being assessed
d.depth of structural roots
6.Which of the following should be
considered when determining site
irrigation rates?
a. water infiltration rates
b.soil sampling frequency
c. cation exchange capacity
d.all of the above
7.Considerations for how many soil
samples to collect should include all
of the following except
a. conditions being assessed
b.soil variability
c. site size
d.time of day
8.Soil structure can be assessed by
a. mixing the soil with water and
shaking it
b.excavating soil from a large hole
c. kneading moistened soil in your
palm
d.a soil penetrometer
9.A soil with good structure readily
allows the movement of
a.water
b.air
c.organisms
d.all of the above
10.Site and soil information should be
used when selecting
a. appropriate soil textures
b.a well suited tree species
c. with which clients to work
d.when to sample soil
11.Soil compaction is common in
urban areas due to
a. vehicle traffic
b. pedestrians
c. construction activities
d. all of the above
12.High soil salinity is common in
a. areas where de-icing salt is used
b.coastal areas
c. arid regions
d.all of the above
13.Comparative soil sampling is accomplished by comparing
a. samples taken from areas with
healthy trees to samples taken from
areas where trees have problems
b.samples taken in the winter to
samples taken in the summer
c. the vigor of trees in areas with low
soil pH to the vigor of trees in areas
with high soil pH
d.soil nitrogen availability to the soil’s
bulk density
14.Soil bulk density is defined as the
a. size of soil aggregates per unit area
b.mass of water per unit volume
c. volume of soil per kilogram
d.mass of soil per unit volume
15.Water movement is most rapid
through soil possessing which of the
following characteristics?
a. high salt concentrations
b.well-compacted silty clay
c. surface crusting, due to breakdown
of aggregates
d.large continuous pores at the
surface
16.Which of the following is not an
example of a micronutrient needed
by trees?
a. nickel (Ni)
b.zinc (Zn)
c. potassium (K)
d.boron (B)
17.Which of the following soil characteristics is very responsive to management practices?
a. soil organic matter
b.cation exchange capacity
c. soil pH
d.salt concentration
18.A common field-based approach for
assessing soil texture is the
a. classification key
b. texturometer
c. feel method
d. clay:sand ratio
19.Soil bulk density can help an arborist
estimate
a. how much resistance plant roots
are facing as they grow
b.the water holding capacity of
different soil textures
c. the volume of soil needed to
sustain a vigorous tree
d.all of the above
20.Ideally, soil organic matter should
make up about how much of the soil?
a. 3%–10% by volume
b.3%–10% by weight
c. 15%–20% by volume
d.15%–20% by weight
I n t r od u c t i o n to A r b o r i c u l t u r e
Soil & Water CD-ROM
Earn
four
CEUs!
This training CD explains the relationship between tree root systems and soils as the single greatest
influence on tree health; provides valuable insight into the effects of water availability in the root zone;
and discusses conscientious irrigation practices. #CD1014
To order, call +1 217.355.9411 or visit us online at www.isa-arbor.com/store
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