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Arboriculture & Urban Forestry 40(6): November 2014
©2014 International Society of Arboriculture
319
Bryant C. Scharenbroch and Gary W. Watson
Wood Chips and Compost Improve Soil Quality
and Increase Growth of Acer rubrum and Betula
nigra in Compacted Urban Soil
Arboriculture & Urban Forestry 2014. 40(6): 319–331
Abstract. Tree growth is negatively impacted by the removal of topsoil and compaction of subsoil associated with site development in
urban landscapes. A research plot with 60 Acer rubrum and 60 Betula nigra was created, mimicking the typical urban landscape distur-
bance. Wood-chip mulch (WC), compost (COMP), inorganic fertilizer (FERT), aerated compost tea (ACT), a commercial biological
product (CBP), and a water control (NULL) were assessed for their impacts on soil quality and tree growth aer ve years. e WC treat-
ment signicantly decreased bulk density and increased soil moisture, organic matter, and microbial respiration. e COMP treatment
increased soil moisture, organic matter, microbial respiration, pH, N, P, and K. Soil P increased with the FERT treatment. Tree growth was
signicantly increased with WC, COMP, and FERT treatments. No signicant changes in soil properties or tree growth were observed with
ACT or CBP compared to NULL; and, compared to background soil levels or other treatments (e.g., COMP and WC) ACT and CBP supply
relatively minimal amounts of microbes and nutrients. is research shows strong evidence that COMP topdressings and WC mulches
are eective and also cost-ecient methods for improving soil quality and stimulating tree growth in compacted urban landscape soils.
Key Words. Acer rubrum; Aerated Compost Tea; Betula nigra; Compost; Inorganic Fertilizer; Organic Materials; Organic Matter; Soil;
Wood-Chip Mulch.
Topsoil scraping and subsoil compaction are oen
necessary to prepare sites for infrastructure. e re-
moved topsoil (O and A horizons) is a substantial
loss of soil organic matter (SOM) and nutrients.
Compaction of the subsoil (B horizons) destroys
the soil structure that is important for soil poros-
ity, aeration, proper drainage, and root extension
(Unger and Kasper 1994). Topsoil removal
and subsoil compaction have severe negative
impacts on soil quality which directly hinder
the establishment and growth of urban trees.
Inorganic fertilizers are oen applied to sup-
plement nutrient cycling in urban landscapes
that have had the topsoil removed. A substantial
body of literature has demonstrated tree growth
improvements with inorganic fertilizers (e.g.,
van de Werken 1981; Watson 1994; Struve 2002;
Percival and Barnes 2005). However, many studies
also show negative eects associated with inorganic
fertilizers, including: ground and surface water con-
tamination (Mitsch et al. 2001; Driscoll et al. 2003;
Soldat and Petrovic 2008), gaseous losses of soil
carbon (C) (Khan et al. 2007; Mulvaney et al. 2009)
and nitrogen (N) (Vitousek et al. 1997; Jenssen
and Kongshaug 2003) and soil salt accumulation
(Follett et al. 1981; Finck 1982). Furthermore, inor-
ganic fertilizers have been found to impact plant
resource allocation and may lead to decreases in
defensive compounds and increased herbivory
(Herms and Mattson 1992). e recommended
annual rates for landscape tree fertilization are 1 to
3 kg N 100 m-2 yr-1 (ANSI 2004; Smiley et al. 2013).
Organic materials used as mulches and amend-
ments are also applied to urban landscapes to
improve soil quality (Finck 1982). Chipped and
tub-ground hardwood mulch and compost are two
of the most highly utilized organic materials in
urban landscapes. Comparatively, wood chips are
coarser, have a wider C/N ratio, and decompose
more slowly. Compost on the other hand, is ne tex-
tured, N rich, and rapidly decomposes. Both types
of organic materials have been found to positively
Scharenbroch and Watson: Wood Chips and Compost Improve Soil Quality and Increase Growth
©2014 International Society of Arboriculture
320
impact soil properties and tree growth [see reviews
by Chalker-Scott (2007) and Scharenbroch (2009)].
Benecial eects of these organic materials include
improved soil moisture, reduced erosion and com-
paction, maintenance of optimal temperature,
reduced salt and pesticide use, increased binding of
heavy metals, reduction of weeds, improved soil fer-
tility, improved plant establishment and growth, and
reduction of disease (Chalker-Scott 2007). Current
standards do not exist for application rates of organic
materials. Wood-based mulch is oen applied annu-
ally to depths of 5 to 15 cm around the tree, but not
against the base of the tree. Saebo and Ferrini (2006)
suggest no more than 1.0 to 1.2 kg of plant avail-
able N 100 m-2 yr-1 to be delivered from compost.
Clients and circumstances oen dictate that turf-
grass remain under urban trees in lieu of mulch.
Furthermore, mulch rings rarely cover the full
extent of the rooting area, which has been esti-
mated to be 38 times the tree diameter (Day et al.
2010). Liquid-based organic products and materials
(e.g., compost tea, humic acids, mycorrhizal
spores) are gaining popularity for applying nutri-
ents and organisms to soils for landscape trees.
Aerated compost tea (ACT) is a mixture of
compost, organisms, additives, and aerated water
(NOSB 2004). According to the National Organic
Program (NOP), the predominant ACT produc-
tion method in the United States involves one part
compost in 10-50 parts water, constant aeration for
12 to 24 hours, and immediate application (NOSB
2004). NOP standards specify that compost used
to make ACT must be made from allowable feed-
stock materials and the entire pile must undergo
an increase in temperature to at least 131ºF for at
least three days (NOSB 2002). ACT additives, such
as molasses, yeast extract, and algal powders are
used to encourage growth of benecial microbes.
No standards exist for application rates of ACT in
agriculture or horticulture. Suggested ACT appli-
cation rates for urban landscape plants range from
500 to 5,000 L ACT 100 m-2 yr-1 (E. Ingham of
Soil Foodweb, Inc., July 2008, pers. comm.), albeit
these rates are not based on scientic evidence.
It is suggested that ACT will increase nutrient
availability and retention via microbial mineraliza-
tion and immobilization, build soil structure and
decrease the eects of compaction, detoxify soil
and water, and suppress disease by inducing com-
petition among disease (anaerobic) and benecial
(aerobic) organisms (e.g., Ingham 2003a; Ingham
2003b; Ingham 2004; Lowenfels and Lewis 2007).
However these claims are unsubstantiated and
few peer-reviewed controlled, replicated scien-
tic studies have been performed on the impacts
of ACT on plants, soil, and the environment
(Scheuerell and Mahaee 2004; Duy et al. 2004;
Scheuerell and Mahaee 2006; Larkin 2008; Segarra
et al. 2009). Even fewer studies have examined
ACT with urban trees (Scharenbroch et al. 2011;
Scharenbroch 2013; Scharenbroch et al. 2013).
e objective of this research was to evaluate
inorganic fertilization (FERT), wood chips (WC),
compost (COMP), aerated compost tea (ACT), a
commercial biological product (CBP), and a water
control (NULL) for impacts on tree growth (Acer
rubrum and Betula nigra) on a disturbed site.
Although a relatively large body of literature sug-
gests benecial impacts of inorganic fertilizers and
organic materials, few studies have examined these
six typical urban soil treatments in isolation for their
eects on soil quality and tree growth in a controlled
experiment. e primary research hypothesis was
that due to more comprehensive and dramatic
impacts on soil quality, t he organic materials (COMP
and WC) will have signicantly greater impacts on
tree growth compared to liquid-based treatments
(FERT, ACT, and CBP). Because COMP is relatively
more labile compared to WC, improvements in soil
quality will be greater with COMP compared to WC.
MATERIALS AND METHODS
Soil and Site Preparation
In spring of 2007, an urban soil research plot (1.5
ha) was created in the research grounds of e
Morton Arboretum in Lisle, DuPage County,
Illinois, U.S. (N 41°48’52.5” and W 88°04’15”). e
native soil (ne illitic, mesic, Oxyaquic Hapludalfs
Ozaukee series) on site is moderately well drained,
slowly permeable, and moderately deep to a densic
contact with till. During site preparation, attempts
were made to mimic the activities of a typical
urban site development. e site preparation was
performed when the soil was near eld capac-
ity in an attempt to maximize the impact of the
disturbance. e topsoil (0 to 20 cm) on the site
was stripped with a bulldozer, and the underlying
Arboriculture & Urban Forestry 40(6): November 2014
©2014 International Society of Arboriculture
321
subsoil compacted (Caterpillar D4H Crawler Dozer,
Caterpillar Inc., Peoria, Illinois, U.S.). A nominal
depth of topsoil (3 cm) was replaced on site using
a wheel loader (John Deere 444H, John Deere,
Moline, Illinois, U.S.) and grader (Fiat Allis 65-B
Grader, New Holland Agriculture, Torino, Italy).
Following the disturbance, soil descriptions
were performed to assess the eect of the site dis-
turbance on the soil. A soil pit (2 m wide × 2.5 m
deep) was dug bisecting the disturbed and adja-
cent undisturbed area and the soils were described.
e most dramatic eects of the disturbance were
concentrated in the top 20 cm. Soil bulk density of
the compacted zone (0 to 20 cm) was increased to
likely limiting values for root growth for this clay
loam soil (1.62 Mg m-3), compared to 1.40 Mg m-3
for the clay loam in the adjacent undisturbed area.
e disturbed prole was wetter throughout, with
increased redoximorphic features deeper in the
prole. e structure of the undisturbed Ap hori-
zon was granular departing to subangular in the
E and Bt horizons. e disturbed prole had platy
and angular blocky soil structure in the top 20 cm.
To assess the uniformity of the disturbance
across the plot, 30 penetration resistance readings
to a depth of 46 cm were measured across the plot
at random locations and compared to ten measure-
ments from the adjacent undisturbed area using a
cone penetrometer (FieldScout SC-900 Soil Pen-
etrometer, Spectrum Technologies, Inc., Plaineld,
Illinois, U.S.). ese measurements were per-
formed within a three-hour period on 04/28/2008.
Mean penetration resistance in the top 46 cm
in the disturbed area was 2.26±0.09 MPa, com-
pared to 1.90±0.07 MPa in the undisturbed area.
Plant Materials
A rectangular experimental grid with 120 square
tree plots (3.05 m × 3.05 m) and 1.53 m inter-plot
space was laid out on the disturbed area. Sixty Betula
nigra and sixty Acer rubrum branched liners
(graed top two-year-old and roots four years old),
1 to 1.5 m height, and 3 to 4 cm diameter, J. Frank
Schmidt & Sons, Co., Boring, Oregon, U.S.) were
randomly assigned to plots. All Betula nigra trees
died aer planting and had to be replaced with
new plantings. Tree planting was performed with a
0.45 m diameter auger mounted on a multi terrain
loader (Caterpillar 277B, Caterpillar Inc., Peoria,
Illinois, U.S.). ese two species were chosen for
their ability to tolerate seasonal wetness on the site.
Both exhibit chlorosis with typical urban conditions,
including extreme soil pH and compaction. Birch
is ectomycorrhizal and maple is endomycorrhizal.
Aer trees were planted, the site was seeded with
Kentucky bluegrass (Poa pratensis). Trees and lawn
were irrigated during dry periods in the establish-
ment year (2007). Following the establishment year
treatment application began in the spring of 2008.
Treatments
Treatments were applied May through October,
annually, from 2008 through 2010. Treatments and
application rates included 1) water control (NULL)
at 840 L 100 m-2 yr-1 (split evenly over ve monthly
applications); 2) aerated compost tea (ACT) at 840 L
100 m-2 yr-1 (split evenly and applied with water over
ve monthly applications); 3) a commercial biologi-
cal product (CBP) diluted with water at 840 L 100
m-2 yr-1 (applied in May and September with water
alone in June, July, and August); 4) a NPK fertilizer
(FERT) at 1.95 kg N 100 m-2 yr-1 (applied with wa-
ter in May and September and water alone applied
in June, July, and August) (Smiley et al. 2002; ANSI
2004); 5) compost (COMP) at 2.5 cm as a top dress-
ing (applied in May of each year); and 6) double-
ground hardwood wood chip (WC) mulch to a 15
cm depth (applied in May of each year). Compost
and mulch plots also received water at 840 L 100 m-2
yr-1 when other plots were treated. Treatment charac-
terizations were performed and are listed in Table 1.
Aerated compost tea was made with a 946 L
compost tea brewer (Geotea-250, Greater Earth
Organics, Chilton, Wisconsin, U.S.). e brewer
was lled with water and aerated for 24 hours
prior to adding ingredients to de-gas any chlo-
rine in water. Aer de-gassing, 8 L of compost was
added to a mesh bag and placed in the brewer. e
compost (Purple Cow Organics, Inc., Oconomo-
woc, Wisconsin, U.S.) contained 8,300 mg bacteria
kg-1, 3,547 mg fungi kg-1 (mean hyphae diameter
of 3.0 µm), 1,883 agellates g-1, 1,459 amoebae g-1,
1,134 ciliates g-1, and 10 nematodes g-1 (uores-
cence and light microscopic analyses performed
by Soil Foodweb, Inc., Corvallis, Oregon, U.S.).
Additional ingredients in the brew included 750
ml of liquid humic acid, 700 ml of soluble kelp,
500 ml of sh hydrolysate, and 750 ml of brown
Scharenbroch and Watson: Wood Chips and Compost Improve Soil Quality and Increase Growth
©2014 International Society of Arboriculture
322
rice powder. During the 24-hour brew cycle, dis-
solved oxygen, temperature, pH, and electrical
conductivity were measured every hour. Dissolved
oxygen remained above 6 mg kg-1, with a mean
value of 8.4 mg kg-1 throughout the brew cycle.
Mean temperature, pH, and electrical conductiv-
ity were 24°C, 8.1, and 560 µS cm-1, respectively.
On average (18 brews) the ACT contained only a
fraction of what was in the compost itself: 2,688
mg bacteria kg-1, 50 mg fungi kg-1 (mean hyphae
diameter of 3 µm), 200 agellates g-1, 140 amoe-
bae g-1, 8 ciliates g-1, and 0.1 nematodes g-1. Upon
completion of brewing, the ACT concentrate was
diluted at a ratio of 1:4 (1 part ACT concentrate
to 4 parts water) and immediately applied to plots
as a soil drench. Applications were performed in
the morning and not during periods of full sun.
e commercial biological product (CBP) con-
tained the following microbes: Bacillus azotoxans
(33,250 mg kg-1), B. licheniformis (33,250 mg kg-1),
B. megaterium (33,250 mg kg-1), B. polymyxa
(33,250 mg kg-1), B. subtilis (33,250 mg kg-1), B.
thuringinensis (33,250 mg kg-1), Streptomyces griseo-
viridis (665 mg kg-1), and Trichoderma harzianum
(3,325 mg kg-1). e CBP also contained malto-
dextrin (48%), yeast extract (5%), soluble seaweed
(13%), humic acids derived from leonardite (17%),
precipitated silica (8%), leonardite extract (6%),
and polyethylene glycol (3%). e CBP was applied
as a soil drench with water at 840 L 100 m-2 yr-1.
e NPK fertilizer contained 30% elemental N
(20% water insoluble synthesized N and 10% water-
soluble synthesized N), 4.4% elemental P or 10%
available phosphoric acid (P2O5), and 5.8% elemental
K or 7% soluble potash (K2O). e fertilizer N source
is urea-formaldehyde, P source is monopotassium
phosphate, and K source is monopotassium phos-
phate. e fertilizer was also applied as a soil drench.
e compost (Midwest Organics Recycling,
McHenry, Illinois, U.S.) contained 4,698 mg bacteria
kg-1, 2,670 mg fungi kg-1 (mean hyphae diameter of
3.0 µm), 123,831 agellates g-1, 5,756 amoebae g-1,
123 ciliates g-1, and 2.82 nematodes g-1 (analyses
performed by Soil Foodweb, Inc., Corvallis, Oregon,
U.S.). e mulch was double-ground hardwood
chips. Average chip size was 2 cm in length by 0.5
cm thick. e mulch contained 1,000 mg bacteria
kg-1, 750 mg fungi kg-1 (mean hyphae diameter of
3.0 µm), 1,900 agellates g-1, 600 amoebae g-1, 5 cili-
ates g-1, and 0.5 nematodes g-1. Mulch was applied
annually in May to a 15 cm depth. Compost was
applied annually in May as a 2.5 cm topdressing.
Soil Sampling and Characterization
Soil bulk density (Db) was determined on
06/02/2011. A 7.24 cm wide by 7.10 cm deep undis-
turbed core was extracted from each plot. Soil was
sieved, homogenized, and dried in an oven for 48
hours at 105°C. Material (roots, rock, etc.) greater
than 2 mm was removed and its volume determined
for bulk density corrections for non-soil material
(Topp et al. 2008). On 10/06/2011, ve 2.5 cm soil
cores (0 to 15 cm depth) were collected at random
points from the 120 plots and returned to the labora-
tory for characterization. In the laboratory, soil sub-
samples were weighed, dried for 24 hours at 105°C,
and reweighed to calculate gravimetric soil mois-
ture (Topp et al. 2008). Soil pH was measured in 1:1
(soil:deionized) water pastes (Model Orion 5-Star,
ermo Fisher Scientic Inc., Waltham, Massachu-
setts, U.S.). Soil samples were extracted with 1 M
NH4OAc (pH 7.0) for determination of potassium
(K) via atomic adsorption spectroscopy (Model
A5000, Perkin Elmer Inc., Waltham, Massachusetts,
U.S.) (Schollenberger and Simon 1945). Phosphorus
(P) was determined with the Bray P-1 extraction
Table 1. Amounts of total N, Bray P, exchangeable K, total C, total bacteria, and total fungi added per year and the mois-
ture, pH, and microbial respiration rate of these treatments: water control (NULL), commercial biological product (CBP),
aerated compost tea (ACT), NPK fertilizer (FERT), compost (COMP), and wood-chip mulch (WC).
Response NULL CBP ACT FERT COMP WC
Moisture (%) n/a n/a n/a n/a 26.2 9.7
pH (1:1) 6.61 7.33 8.10 7.34 7.60 5.91
Total N (kg 100 m-2) 4.8E-5 0.0741 0.0773 1.95 0.385 0.527
Bray P (kg 100 m-2) 6.1E-9 7.0E-5 1.9E-3 0.280 0.0688 0.0560
Exch. K (kg 100 m-2) 1.2E-8 2.7E-5 3.4E-5 0.369 0.0343 0.0410
Total C (kg 100 m-2) 1.1E-3 1.22 1.76 1.63 5.25 20.25
Micr. resp. (mg kg-1 d-1) 1.50 19.6 18.8 11.2 26.6 30.6
Total bact. (kg 100 m-2) 6.1E-10 0.420 0.565 4.8E-5 8.22 4.05
Total fungi (kg 100 m-2) 6.1E-11 7.0E-3 0.011 6.4E-7 4.67 3.04
Arboriculture & Urban Forestry 40(6): November 2014
©2014 International Society of Arboriculture
323
and analyzed colorimetrically at 882 nm on a spec-
trophotometer (Model UV mini 1240, Shidmadzu
Inc., Kyoto, Japan) (Olsen and Sommers 1982).
Total carbon (C) and nitrogen (N) were determined
by automated dry combustion on a CN analyzer
(Vario ELIII, Elementar Analysensysteme, Hanau,
Germany) (Nelson and Sommers 1996). Loss on
ignition at 360°C for six hours was used to deter-
mine the soil organic matter (SOM) content (Nelson
and Sommers 1996). Microbial respiration (RES)
was measured in 10 day dark incubations at 25°C
with soils adjusted to 40% water-holding capacity
with 0.25 M NaOH traps. Carbon dioxide seques-
tered in NaOH was precipitated with 0.5 M BaCl2
followed by 0.25 M HCl (standardized) titration to
a phenolphthalein endpoint (Parkin et al. 1996).
Tree Biomass
Sixty trees were destructively sampled in spring of
2011 and the remaining sixty in the spring of 2013.
During each sampling period, ve blocks of 12 trees
each were sampled, with each block containing the
full complement of treatments and species combi-
nations. e sampling period did not interact with
the main eects of treatment or species for any of
the biomass fractions; consequently growth is pre-
sented by combining data across the two sampling
dates. For simplicity, change in tree biomass is pre-
sented as growth aer ve years of treatment even
though sixty trees had four years of treatments and
sixty trees had six years of treatments (Figure 1).
Trees were removed from the ground using a 0.9 m
diameter tree spade (Optimal 880, Optimal-Vertrieb
Optiz GmbH, Eysoelden, Germany) mounted on a
multi terrain loader (Caterpillar 277B, Caterpillar
Inc., Peoria, Illinois, U.S.). Soil was removed from
rootballs using a pneumatic air tool (X-ST, Super-
sonic Air Knife, Inc., Allison Park, Pennsylvania,
U.S.). Turf roots were removed by hand-sorting. Tree
biomass fractions of ne roots (<2 mm diameter),
medium roots (2–5 mm), coarse roots (>5 mm),
stems and leaves were hand-sorted, washed, dried to
constant moisture (60°C for one week), and weighed.
Statistical Analyses
e experiment was a randomized complete block
design, with 120 total plots, two species, six treat-
ments and ten replications. Data distributions
were checked for normality using the Shapiro-
Wilk W-test. Transformations of non-normal data
were performed when necessary. Main eects and
interactions were assessed with two-way repeated
measures analysis of variance (ANOVA). Mean
separations were carried out with the Tukey’s HSD
test. Pairwise correlations with Pearson product-
moment were used to identify signicant rela-
tionships among variables. Principal component
analyses were used to establish which soil property
explained most variance in the complete data set
(Fox and Metla 2005). Signicant dierences were
determined at the 95% condence level. Statisti-
cal analyses were conducted using SAS JMP 7.0
soware (SAS Inc., Cary, North Carolina, U.S.).
RESULTS AND DISCUSSION
Soil Responses
All soil responses assessed in this research showed
signicant treatment eects and only microbial
respiration (RES) showed a signicant response
Figure 1. Stem, leaf, coarse, medium, and ne root biomass
fractions of Acer rubrum and Betula nigra on a compacted
urban soil plot receiving water control (NULL), commer-
cial biological product (CBP), aerated compost tea (ACT),
NPK fertilizer (FERT), wood chip mulch (WC), and compost
(COMP) treatments for ve consecutive years. Uppercase
letters on top indicate signicant differences among treat-
ments in total biomass using Tukey’s HSD test. Lowercase
letters indicate differences among leaf, stem, and coarse
root fractions. Differences were not signicant for medium
and ne root fractions. Each fraction is a mean of 20 trees.
Scharenbroch and Watson: Wood Chips and Compost Improve Soil Quality and Increase Growth
©2014 International Society of Arboriculture
324
to tree species (Table 2). Microbial respiration was
signicantly greater with Betula nigra (106.4±31.6
mg kg-1 d-1) compared to Acer rubrum (91.8±28.2
mg kg-1 d-1). e increase in microbial respiration
is likely due to substrate decomposability. Nitro-
gen content of Betula nigra leaves (2.10±0.47) was
signicantly greater than in Acer rubrum (1.67±0.35)
leaves. No signicant treatment by species eects
were detected for any of the soil responses.
Soil bulk density was signicantly lower under
WC compared to NULL (Figure 2). Soil moisture
content was signicantly greater with WC com-
pared to FERT, ACT, CBP, and NULL. Soil moisture
was also greater with COMP compared to FERT and
NULL. Soil organic matter was signicantly greater
with COMP compared to all other treatments. Soil
organic matter under WC was greater than FERT,
ACT, CBP, and NULL. Microbial respiration was
signicantly greater with COMP and WC com-
pared to ACT, CBP, and NULL. Soil pH was signi-
cantly greater with COMP compared to FERT, ACT,
CBP, and NULL. Soil N, P, and K was signicantly
greater with COMP compared to all other treat-
ments. Soil P was signicantly greater with FERT
compared to ACT, CBP, and NULL. Soil K was
signicantly greater with WC compared to CBP.
e overall eects of WC treatment on soil qual-
ity were to reduce bulk density by 10%, increase soil
Figure 2. Soil bulk density, gravimetric soil moisture, soil organic matter, microbial respiration, pH, and N, P, and K on a compacted
urban soil plot receiving water control (NULL), commercial biological product (CBP), aerated compost tea (ACT), NPK fertilizer
(FERT), wood-chip mulch (WC), and compost (COMP) treatments for ve consecutive years. Lowercase letters indicate signicant
differences among treatments using Tukey’s HSD test. Each bar is a mean of 20 plots.
Arboriculture & Urban Forestry 40(6): November 2014
©2014 International Society of Arboriculture
325
moisture by 26%, increase SOM by 25%, increase
RES by 33%, and increase K by 24%. e COMP
treatment increased soil moisture +13%, SOM +57%,
RES 39%, N +68%, P +1544%, and K +64%. Relative
to the control, the FERT treatment only impacted
soil P, which increased 286%. No dierences were
observed for these eight soil properties between
the NULL treatment and ACT or CBP treatment.
Both WC and COMP appear to be the most eec-
tive treatments at improving soil quality. e WC
treatment tended to be more eective at improv-
ing soil physical condition (density and moisture),
whereas the COMP treatment better improved
soil biochemistry (SOM, RES, N, P, and K). It
was expected that soil moisture would be great-
est with the WC treatment. Wood-chip mulches
would limit evaporation and also limit water
uptake by competing vegetation (Watson 1988).
Compost is a source of labile organic mat-
ter and nutrients. It was expected and found
that the COMP treatment would most increase
organic matter and stimulate microbial activity.
ese increases in SOM and microbial activity
appeared to, in-turn increase soil nutrient sup-
plies of N, P, and K (Scharenbroch et al. 2013).
e increases in soil P with COMP are concern-
ing given the potential for P pollution of aquatic
ecosystems (Carpenter et al. 1998). Precautions
should be taken when using composts in urban
landscapes to minimize runo and erosion, which
are major pathways of P loss to surface waters.
e observed decrease in bulk density with
WC treatment, and not with COMP treatment,
was somewhat surprising. Biological activity is an
important driver of soil aggregation, which would
work to reduce bulk density and alleviate soil com-
paction. e WC treatment increased microbial
respiration but also had the most signicant impact
on tree growth. Increases in soil moisture with the
WC treatment may also have been important for
facilitating root growth through these compacted
urban soils and thereby increasing the extent of soil
that roots and microbes would access. Together,
increased microbial and tree root growth likely
contributed to the observed decreased bulk density.
Soil pH was expected to decrease with COMP
and FERT treatments due to the pH-lower-
ing eects of respiration and N mineralization
(Sikora and Yakovchenko 1996). However, soil
pH increased with compost and it may be that the
compost increased base saturation by increasing
Al complexation (Van den Berghe and Hue 1999).
Tree Growth Responses
Signicant treatment eects were detected for total
tree biomass, total shoot biomass, total root bio-
mass, stem biomass, leaf biomass, and coarse root
biomass (Table 3). Treatment eects were not sig-
nicant for medium and ne root biomass. Spe-
cies eects were signicant for all tree responses
except ne root biomass, and the treatment by
species eect was only signicant for leaf biomass.
Given the minimal treatment by species eects,
main treatment eects were examined by combin-
ing both species. Total tree biomass and total stem
biomass were signicantly greater with WC, COMP,
and FERT compared to ACT, CBP, and NULL (Fig-
ure 1). Total stem biomass was also signicantly
greater with WC compared to FERT. Leaf biomass
was signicantly greater with WC compared all other
treatments, and leaf biomass was greater with FERT
and COMP compared to CBP and NULL. Stem
biomass was signicantly greater with WC, COMP,
and FERT compared to ACT, CBP, and NULL.
Coarse root biomass was signicantly greater with
Table 2. Prob > F values for effect tests of treatment, spe-
cies, and treatment*species using ANOVA standard least
squares models for soil bulk density, soil moisture, pH, N,
P, K, soil organic matter, and microbial respiration.
Soil response Treatment Species Tr*Sp
Bulk density (Mg m-3) 0.0116 0.7174 0.4962
Gravimetric soil moisture (%) <0.0001 0.0590 0.9538
pH (1:1) <0.0001 0.8827 0.3997
Total N (%) <0.0001 0.3304 0.9330
Bray P (mg kg-1) <0.0001 0.9667 0.9917
Exchangeable K (mg kg-1) <0.0001 0.8466 0.8702
Soil organic matter (%) <0.0001 0.9151 0.1367
Microbial respiration (mg kg-1 d-1) <0.0001 0.0025 0.6508
Table 3. Prob > F values for effect tests of treatment,
species, and treatment*species using ANOVA standard
least squares models for tree biomass fractions.
Tree response Treatment Species Tr*Sp
Total biomass (g) <0.0001 <0.0001 0.1401
Total shoot biomass (g) <0.0001 <0.0001 0.0319
Leaf biomass (g) <0.0001 <0.0001 <0.0001
Stem biomass (g) <0.0001 <0.0001 0.1917
Total root biomass (g) <0.0001 <0.0001 0.8981
Coarse root biomass (>5 mm) (g) 0.0006 <0.0001 0.7934
Medium root biomass (2–5 mm) (g) 0.1544 <0.0001 0.6990
Fine root biomass (<2 mm) (g) 0.2393 0.6841 0.2955
Scharenbroch and Watson: Wood Chips and Compost Improve Soil Quality and Increase Growth
©2014 International Society of Arboriculture
326
WC compared to NULL. No signicant dierences
were detected for medium and ne root biomass.
Aer ve years in this compacted urban soil, total
tree biomass was 170% greater with WC compared
to control trees. Trees that received COMP and
FERT grew 82% and 69%, respectively, more total
biomass compared to control trees. ese results
provide strong evidence for the benecial impacts of
WC, COMP, and FERT for increasing tree growth in
compacted urban soils. Many studies have identied
positive improvements in tree growth with organic
amendments and these ndings have been summa-
rized in two relatively recent literature reviews by
Chalker-Scott (2007) and Scharenbroch (2009). e
potential mechanisms for these observed increases
in tree growth are presented and discussed in the
preceding section of this paper. Trees that received
ACT and CBP treatment did not dier in total bio-
mass or any specic biomass fraction compared to
control trees. ese results provide evidence for
the ineectiveness of these treatments to stimulate
tree growth in this compacted urban soil. Similar
ineectiveness of compost teas to stimulate tree
growth in urban soils has been reported previously
(Scharenbroch 2013; Scharenbroch et al. 2013).
Researchers unexpectedly found that medium
and ne root biomass did not respond in a similar
fashion to coarse root biomass. Coarse roots grow
by producing wood, like stem tissue; whereas, ne
root growth is largely primary growth and more
inuenced by soil conditions (Shigo 1999). It is pos-
sible that these treatments did not impact ne and
medium root growth, but we feel this is unlikely
given the signicant responses in soil properties
we observed. Soil responses would suggest that
ne and medium root growth are equally or more
responsive to these treatments than coarse roots.
Relative to coarse roots, ne roots have a very
low dry density. It is possible that these small dif-
ferences in mass with ne and medium roots were
not detected in statistical testing. In addition, roots
outside of the tree-spaded hole were not included
in this assessment. A subsampling of the soil out-
side of the hole on 13 plots revealed that on average
0.054±0.041 kg of root biomass was not sampled,
which is only 1%–2% of the total biomass mea-
sured on the trees. It is recognized that the esti-
mates of root biomass may be conservative due to
sampling methodology; however, this underes-
timation is not likely to impact the main focus of
the research, which was to examine the eects of
these treatments on tree growth and soil quality.
Another plausible explanation is that the sam-
pling methodology masked treatment responses by
damaging and removing ne and medium roots.
To determine root biomass, trees were dug with
a tree spade, and an air knife was used to remove
the soil from the root balls. In removing the soil
with the air knife, the ne and medium roots may
have been removed. Attempts were made to mini-
mize root damage during this process but ne and
medium roots were visibly removed and it was not
possible to quantify this eect. However, research-
ers have no reason to suspect that the damage
was unequal across treatments and/or species.
Tree Growth Modeling
Linear regressions among soil and tree responses
showed signicant and positive relationships with
tree biomass and SOM, GSM, RES, N, P, and K
(Table 4). A signicant and negative relationship
existed between bulk density and tree biomass.
Soil pH was not signicantly correlated with
tree biomass. Although signicant, correla-
tions among soil and tree responses were gener-
ally weak (r2 < 0.18). Of the soil responses, SOM
was most strongly correlated with tree biomass.
Stepwise regression modeling did not improve
correlation with tree biomass. Forward, back-
ward, and mixed modeling approaches all iden-
tied SOM as the sole and most important
predictor of tree biomass. A principal component
analysis revealed that the rst principal component
explained 51.4% of the variation in soil responses.
e rst principal component was strongly cor-
related with SOM (Eigenvector score of 0.4051).
Soil organic matter was also signicantly cor-
related with all other soil properties (Table 5).
All models suggest SOM to be the most impor-
tant soil indicator of tree growth. Soil organic
matter is a critical component of soil quality as
it integrates physical, chemical, and biological
properties (Doran and Parkin 1994). Soil organic
matter is composed of living and dead ora and
fauna. Soil organic matter is relatively lightweight,
has a high surface area, and contains nutrients and
microbes. Soil organic matter is known to improve
biological condition of soil by increasing microbial
Arboriculture & Urban Forestry 40(6): November 2014
©2014 International Society of Arboriculture
327
activity, which in turn leads to an increase in nutri-
ent availability and also biological aggregation
(Tisdall and Oades 1982; Carter 2002). Soil organic
matter is also known to impact chemical properties
such as pH (Gerritse and Van Driel 1984). Reduc-
tions in compactibility and soil bulk densities
(Soane 1990) and increases in soil water availability
(Hudson 1994) are known improvements in
soil physical properties with soil organic matter.
Management Implications
Soil management for urban trees most oen
focuses on fertilization. Increased tree growth
was observed with FERT, but no changes to soil
properties were observed with this treatment.
Nutrients in the FERT treatment may have been
directly taken up by the tree and/or turf grass, but
may also have been lost to environment through
volatilization or leaching (Follett 1981; Finck
1982). is research found no evidence that the
FERT treatment worked to improve soil quality
on this compacted plot. Current best manage-
ment practices for urban trees describe the
appropriate rates and application methods for N
fertilization but do not consider overall soil quality
as a management target (Smiley et al. 2013).
ese results suggest that fertility is only a com-
ponent of soil quality with inuence on tree growth
in compacted urban soils. ese results point
toward managing soil organic matter as an eec-
tive strategy in remediating urban soil quality for
tree growth. Furthermore, if soil organic matter is
a management target, these results show that eec-
tive treatments for increasing soil organic matter
are COMP and WC. Management goals may oen
dictate the type of organic amendment to use. For
instance, if the goal is to build SOM quickly and
supply nutrients, relatively labile organic materials
(i.e., C/N ratio <25), like the compost used in
this study, might be the preferred amendment.
On the other hand if the goal is to increase soil
water and improve soil structure, more recal-
citrant organic materials (i.e., C/N ratio >25),
such as wood chips, may be more appropriate.
Similar to inorganic fertilization, management
recommendations can be developed for organic
materials to attain target nutrient release levels.
Many agriculture extension stations have devel-
oped rates and calculators for applying organic
materials to meet plant nutrient demands (e.g.,
Organic Fertilizer and Cover Crop Calculator
2014). e potential N release from compost can
easily be calculated with dry weight, the N con-
tent and an estimate of the plant available N (PAN)
released annually from the compost (Equation 1).
[1] kg N yr-1 = [mass of compost (kg) * total N (%)
* PAN (%)]
For example, the ANSI Standard’s clause for
tree fertilization suggests a rate of 1 kg N 100 m-2
y-1. Total N contents of composted materials tend
to range from 1%–3% with PAN ranging from
5%–20%. Compost (500 kg) with 2% total N and
Table 4. Slope, intercept, r2, and Prob > F values for linear fit models of soil properties to total tree biomass.
Fit to tree biomass (g) Slope Intercept r2 Prob > F
Bulk density (Mg m-3) -5.03 10.49 0.0904 0.0008
Gravimetric soil moisture (%) 0.23 -2.02 0.1216 <0.0001
pH (1:1) 2.47 -13.98 0.0213 0.1119
Total N (%) 10.91 1.19 0.0899 0.0009
Bray P (mg kg-1) 0.04 3.36 0.0505 0.0136
Exchangeable K (mg kg-1) 0.03 1.59 0.1077 0.0003
Soil organic matter (%) 0.71 -0.30 0.1780 <0.0001
Microbial respiration (mg kg-1 d-1) 0.02 1.85 0.0644 0.0052
Table 5. Slope, intercept, r2, and Prob > F values for linear fit models of soil properties to soil organic matter.
Fit to soil organic matter (%) Slope Intercept r2 Prob > F
Bulk density (Mg m-3) -4.31 11.48 0.1887 <0.0001
Gravimetric soil moisture (%) 0.18 1.04 0.2252 <0.0001
pH (1:1) 2.50 -12.23 0.0617 0.0062
Total N (%) 15.12 2.15 0.4896 <0.0001
Bray P (mg kg-1) 0.07 4.98 0.5023 <0.0001
Exchangeable K (mg kg-1) 0.03 3.40 0.3436 <0.0001
Microbial respiration (mg kg-1 d-1) 0.02 4.01 0.1308 <0.0001
Scharenbroch and Watson: Wood Chips and Compost Improve Soil Quality and Increase Growth
©2014 International Society of Arboriculture
328
10% PAN would release 1 kg of N yr-1, so this com-
post could be applied over 100 m2 to meet the ANSI
clause for N application rate. Assuming a compost
density of 500 kg m-3, this would be 1 m3 of compost.
In addition to the improvements in overall soil
quality and tree growth observed in this research
with organic materials, there are many environ-
mental reasons to favor organic materials to inor-
ganic fertilizers (Follett et al. 1981; Finck 1982).
Nutrients in organic amendments are less likely to
leach, run o, or volatilize compared to nutrients
in inorganic fertilizers. Because organic materials
tend to release nutrients more slowly, less consid-
eration is required for matching application to
plant demands. Organic amendments also provide
additional nutrients (e.g., manganese, zinc, sulfur)
that are oen not included in inorganic fertilizers.
Many organic amendments are created from resid-
uals that would potentially enter landlls. Fossil
fuel consumption is required to make and trans-
port inorganic fertilizers—albeit, transportation of
organic materials also requires fossil fuels (Jenssen
and Kongshaug 2003). Long-term fertilization with
inorganic N has been found to decrease soil C stor-
age, whereas, organic materials tend to increase soil
organic matter and C storage (Khan et al. 2007).
Soil and tree properties were not signicantly
impacted by ACT or CBP compared to NULL con-
trols. Nutrients added per year by ACT and CBP
were <80 g N 100 m-2, <2 g P 100 m-2, and <0.02 g
K 100 m-2 (Table 1). Conversely, amounts of N, P,
and K added with COMP and WC were 0.4 and 0.5
kg N 100 m-2 y-1, 0.07 and 0.06 kg P 100 m-2 y-1, and
0.03 to 0.04 kg K 100 m-2 y-1, respectively. Nutrients
in COMP and WC were magnitudes greater than
added in ACT and CBP. Total microbial biomass
added per year with CBP and ACT was 0.427 and
0.576 kg 100 m-2 for CBP and ACT, respectively. On
average, the total soil microbial biomass levels for
these soils are 33.5 kg 100 m-2, which is 79 times and
58 times greater than the microbial biomass added
per year in CBP and ACT. To match the existing
soil microbial biomass levels, concentrated ACT
would have to be applied at 12,213 L 100 m-2 y-1.
In comparison, the total microbial biomass added
per year in COMP treatment was 12.9 kg 100 m-2,
or 40% of the existing soil microbial biomass level.
e annual cost of these treatments on a per tree
basis was computed (Table 6). Labor costs com-
prised the largest portion of the budget for all treat-
ments. Total costs were greatest for ACT followed
by WC, FERT, COMP, CBP, and lastly NULL. e
amount of biomass gained per tree per year was
greatest for WC and lowest for the NULL treat-
ment. e most ecient ($ g-1) treatments were WC,
COMP, and NULL. As a result of the large initial
investment in a compost tea brewer, large amount of
labor, and relatively low growth response, ACT was
highly inecient compared to the other treatments.
CONCLUSION
Compost topdressings and wood chip mulches
should be used as soil management techniques
for trees growing in compacted urban soils. ese
results conrm the hypothesis that greater im-
provements in soil quality and tree growth would
be observed with solid organic materials (COMP
and WC) compared to liquid-based treatments
(ACT and CBP). e research demonstrates that
COMP and WC are eective and cost-ecient
alternatives to inorganic fertilizer for improving soil
quality and increasing tree growth in compacted
urban soil. It is reasonable to expect that combin-
ing wood chips and composts may have even greater
benet for improving soil quality for urban trees.
Future research should examine the eectiveness
of combining these and other organic materials
in attempts to mimic the organic and A-horizons
Table 6. Annual costs for materials, equipment, and labor, and growth and efficiency per tree for water control (NULL),
commercial biological product (CBP), aerated compost tea (ACT), NPK fertilizer (FERT), compost (COMP), and wood-chip
mulch (WC).
Treatment NULL CBP ACT FERT COMP WC
Materials ($ tree-1) 0.0 3.5 7.5 6.5 3.0 10.0
Equipment ($ tree-1) 0.5 3.5 28.5 3.5 1.0 1.0
Labor ($ tree-1) 7.5 11.3 30.0 11.3 15.0 15.0
Total cost ($ tree-1) 8.0 18.3 66.0 21.3 19.0 26.0
Growth (g tree-1) 22.8 23.9 24.1 38.7 41.5 61.3
Eciency ($ g-1) 0.35 0.77 2.74 0.55 0.46 0.42
Arboriculture & Urban Forestry 40(6): November 2014
©2014 International Society of Arboriculture
329
found in temperate forest soils. Future research
must also consider the impacts of these organic
materials on tree health, which does not necessar-
ily have a positive linear relationship to tree growth.
Acknowledgments. Funding for this
research was provided by the Tree
Research and Education Endowment
(TREE) Fund Hyland R. Johns grant
(07-HJ-01) and e Morton Arbo-
retum Endowment. e trees in this
research were donated by J. Frank
Schmidt & Sons. We acknowledge
and thank Research Assistants (Michelle Catania, Angela Hewitt,
Marvin Lo and Doug Johnston) and also the many volunteers and
student interns in e Morton Arboretum Soil Science Laboratory
and Root Biology Laboratory for their work on this research.
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Bryant C. Scharenbroch (corresponding author)
e Morton Arboretum
4100 Illinois Route 53
Lisle, Illinois 60532-1293, U.S.
BScharenbroch@mortonarb.org
Gary W. Watson
e Morton Arboretum
4100 Illinois Route 53
Lisle, Illinois 60532-1293, U.S.
Arboriculture & Urban Forestry 40(6): November 2014
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Zusammenfassung. Das Baumwachstum wird negativ durch
die Entfernung von Oberboden und die Verdichtung des Unter-
bodens im Verbindung mit der Standortentwicklung in urbanen
Landschaen beeinusst. Es wurde ein Forschungsstandort mit 60
Acer rubrum und 60 Betula nigra eingerichtet, der typische Störun-
gen durch urbane Landschasentwicklung simuliert. Holzhack-
schnitzelmulch (WC), Kompost (COMP), anorganischer Dünger
(FERT), belüeter Komposttee (ACT), ein kommerzielles biolo-
gisches Produkt (CBP) und eine Kontrolle nur mit Wasser (NULL)
wurden hinsichtlich ihres Einusses auf Bodenqualität und Baum-
wachstum nach fünf Jahren untersucht. Die WC-Behandlung re-
duzierte signikant die Körperdichte und die Bodenfeuchte, den
organischen Anteil sowie die mikrobielle Atmung. Die COMP-
Behandlung steigerte die Bodenfeuchte, organische Masse, mikro-
bielle Atmung, pH, N, P und K. Der Bodengehalt an P wurde durch
die FERT-Behandlung vergrößert. Das Baumwachstum wurde sig-
nikant durch WC, COMP, und FERT-Behandlungen gesteigert.
Es wurden keine signikanten Veränderungen der Bodeneigen-
schaen und des Baumwachstums bei der Behandlung mit ACT
oder CBP im Vergleich zu NULL beobachtet und verglichen mit
hintergründigen Bodenebnen oder anderen Behandlungen (z. B.
COMP und WC) liefern ACT und CBP relativ geringe Mengen an
Mikroben und Nährstoen. Diese Forschung liefert einen starken
Beweis, dass COMP-Abdeckungen und WC-Mulchen eektiv
sind und auch kosteneziente Methoden zur Verbesserung der
Bodenqualität und Stimulans von Baumwachstum in verdichteten
urbanen Landschasböden leisten können.
Résumé. La croissance des arbres est grandement aectée par
l’enlèvement de la couche de terre arable et la compaction des
horizons du sol associés au développement des sites urbains. Une
parcelle de recherche a été aménagée comportant 60 Acer rubrum
et 60 Betula nigra, reproduisant les perturbations typiques aux-
quelles est exposé un site urbain. Des paillis de bois raméal frag-
menté, du compost, de l’engrais inorganique, du compost de thé,
un produit biologique commercial et une irrigation contrôlée ont
été mis à l'essai an d’évaluer leur impact sur la qualité du sol et la
croissance des arbres après cinq ans. Le paillis de bois raméal frag-
menté a diminué de manière signicative la densité volumétrique
et a augmenté l'humidité du sol, le taux de matière organique et
la respiration microbienne. Pour sa part, le compost a augmenté
l'humidité du sol, le pourcentage de matière organique, la respira-
tion microbienne, le pH, le taux d'azote, de phosphore et de potas-
sium. Le niveau de phosphore a été amélioré avec l'utilisation de
l’engrais inorganique. La croissance des arbres a été améliorée de
façon signicative avec les traitements de paillage avec bois raméal
fragmenté, de compost et d’engrais inorganique. Aucune améliora-
tion signicative des propriétés du sol ou de croissance des arbres
n’a été observée avec le compost de thé ou le produit biologique
commercial par rapport à l'irrigation contrôlée; et par rapport aux
parcelles de sols témoins ou aux autres traitements (par exemple, le
compost ou le paillis de bois raméal fragmenté) le compost de thé et
le produit biologique commercial n’ont fourni que des quantités rel-
ativement minimes de microbes et d’éléments nutritifs. Cette étude
démontre que l'utilisation de compost et de paillis de bois raméal
fragmenté sont des méthodes ecaces et rentables pour améliorer
la qualité des sols et stimuler la croissance des arbres dans les sols
compacts rencontrés sur les sites urbains.
Resumen. El crecimiento de los árboles se ve afectado negativa-
mente por la eliminación de la capa supercial y la compactación
del subsuelo asociada con el desarrollo de sitios en paisajes urbanos.
Se creó una parcela de investigación con 60 Acer rubrum y 60 Betula
nigra, imitando la perturbación típica del paisaje urbano. Se evalu-
aron los tratamientos con mulch de triturado de madera (WC),
composta (COMP), fertilizantes inorgánicos (FERT), té de compost
aireado (ACT), un producto biológico comercial (CBP) y control
con agua (NULL) para conocer sus impactos en la calidad del suelo
y crecimiento de los árboles después de cinco años. El tratamiento
WC disminuyó signicativamente la densidad aparente y aumentó
la humedad del suelo, materia orgánica y respiración microbiana. El
tratamiento COMP aumentó la humedad del suelo, materia orgáni-
ca, respiración microbiana, pH, N, P y K. El P del suelo aumentó
con el tratamiento FERT. El crecimiento de los árboles fue signi-
cativamente superior con WC, COMP y FERT. No se observaron
cambios signicativos en las propiedades del suelo o crecimiento
de los árboles con ACT o CBP en comparación con NULL; y en
comparación con los niveles del suelo u otros tratamientos (por
ejemplo, COMP y WC) ACT y CBP aportaron cantidades relati-
vamente mínimas de microorganismos y minerales. Esta investig-
ación muestra fuertes evidencias de que COMP y WC en cobertura
y mulches son métodos económicamente ecaces para mejorar la
calidad del suelo y estimular el crecimiento de los árboles en suelos
compactados en el paisaje urbano.
