Reuse of coir, peat, and wood fiber in strawberry production

Abstract

Introduction
Production of strawberries in greenhouses and polytunnels is gaining popularity worldwide. This study investigated the effect of reuse of coir and peat, two substrates commonly adapted to soilless strawberry production, as well as stand-alone wood fiber from Norway spruce, a promising substrate candidate.

Methods
The experiment was performed in a polytunnel at NIBIO Apelsvoll, Norway, and evaluated both virgin substrates, as well as spent materials that were used in one or two years. Yield, berry quality and plant architecture of the strawberry cultivar ‘Malling Centenary’ were registered. In addition, chemical and physical properties of virgin and reused substrates were investigated.

Results
While plants grown in peat and wood fiber had highest yield in the first year of production, the berry yield was slightly reduced when these substrates were utilized for the second and third time. However, yield was comparable to the yield level attained in new and reused coir. Interestingly, berries grown in wood fiber had a tendency to a higher sugar accumulation. This substrate also produced the highest plants. Stand-alone wood fiber was the substrate with the highest accumulation of nitrogen during the three consecutive production cycles. All three investigated materials revealed a trend for decreased potassium accumulation. Wood fiber is characterized by the highest percentage of cellulose, however after three years of production the cellulose content was reducedto the same levels as for coir and peat.

Discussion
Implementation of wood fiber as a growing medium, as well as general practice of substrate reuse can be therefore an achievable strategy for more sustainable berry production.

Full text

Reuse of coir, peat, and wood
fiber in strawberry production
Tomasz Woznicki
1
*, Krzysztof Kusnierek
2
, Bart Vandecasteele
3
and Anita Sønsteby
1
1
Department of Horticulture, Norwegian Institute of Bioeconomy Research (NIBIO), Kapp, Norway,
2
Department of Agricultural Technology and System Analysis, Center for Precision Agriculture,
Norwegian Institute of Bioeconomy Research (NIBIO), Kapp, Norway,
3
Plant Sciences Unit, Flanders
Research Institute for Agriculture, Fisheries and Food (ILVO), Melle, Belgium
Introduction: Production of strawberries in greenhouses and polytunnels is
gaining popularity worldwide. This study investigated the effect of reuse of coir
and peat, two substrates commonly adapted to soilless strawberry production, as
well as stand-alone wood fiber from Norway spruce, a promising
substrate candidate.
Methods: The experiment was performed in a polytunnel at NIBIO Apelsvoll,
Norway, and evaluated both virgin substrates, as well as spent materials that were
used in one or two years. Yield, berry quality and plant architecture of the
strawberry cultivar ‘Malling Centenary’were registered. In addition, chemical and
physical properties of virgin and reused substrates were investigated.
Results: While plants grown in peat and wood fiber had highest yield in the first
year of production, the berry yield was slightly reduced when these substrates
were utilized for the second and third time. However, yield was comparable to
the yield level attained in new and reused coir. Interestingly, berries grown in
wood fiber had a tendency to a higher sugar accumulation. This substrate also
produced the highest plants. Stand-alone wood fiber was the substrate with the
highest accumulation of nitrogen during the three consecutive production
cycles. All three investigated materials revealed a trend for decreased
potassium accumulation. Wood fiber is characterized by the highest
percentage of cellulose, however after three years of production the cellulose
content was reducedto the same levels as for coir and peat.
Discussion: Implementation of wood fiber as a growing medium, as well as
general practice of substrate reuse can be therefore an achievable strategy for
more sustainable berry production.
KEYWORDS
strawberries (Fragaria xananassa), growing media, sustainability, circular economy,
soilless culture system, substrate properties, nutrient recycling
Frontiers in Plant Science frontiersin.org01
OPEN ACCESS
EDITED BY
Md Asaduzzaman,
Bangladesh Agricultural Research Institute,
Bangladesh
REVIEWED BY
Rui Manuel Almeida Machado,
University of Evora, Portugal
Himanshu Pandey,
Khalsa College, India
*CORRESPONDENCE
Tomasz Woznicki
tomasz.woznicki@nibio.no
RECEIVED 04 October 2023
ACCEPTED 27 December 2023
PUBLISHED 12 January 2024
CITATION
Woznicki T, Kusnierek K, Vandecasteele B and
Sønsteby A (2024) Reuse of coir, peat, and
wood fiber in strawberry production.
Front. Plant Sci. 14:1307240.
doi: 10.3389/fpls.2023.1307240
COPYRIGHT
© 2024 Woznicki, Kusnierek, Vandecasteele
and Sønsteby. This is an open-access article
distributed under the terms of the Creative
Commons Attribution License (CC BY). The
use, distribution or reproduction in other
forums is permitted, provided the original
author(s) and the copyright owner(s) are
credited and that the original publication in
this journal is cited, in accordance with
accepted academic practice. No use,
distribution or reproduction is permitted
which does not comply with these terms.
TYPE Original Research
PUBLISHED 12 January 2024
DOI 10.3389/fpls.2023.1307240

1 Introduction
The world population is projected to reach 8.5 billion in 2030,
and to increase further to 9.7 billion in 2050 and to 10.4 billion by
2100 (UN, Department of Economic and Social Affairs, Population
Division, 2019). To be able to feed the global population in the
future and maintain sustainability of the food production systems,
substantial improvements in agricultural and horticultural practices
are needed. A broader implementation of soilless culture systems
(SCS) will enhance productivity and reduce negative environmental
impact of plant production by improving water and nutrient use
efficiency (Gruda, 2019). In this type of production, ‘growing media’
or ‘horticultural substrates’provide a root environment that ensures
optimal water and nutrient supply and adequate aeration. Since SCS
popularity is increasing worldwide, the global growing media
demand is predicted to be four times higher in the next 30 years
(Blok et al., 2021).
Strawberry is one of the most important berry crops globally,
and its production is increasing (FAOSTAT, 2020;Hernandez-
Martı

nez et al., 2023). Peat and coir are commonly used substrates
in tunnel and greenhouse strawberry production; however, use of
these materials has a negative environmental impact (Gruda, 2019).
Peatlands are important carbon storage and water retention
reservoirs which are only partially renewable. Therefore, policy
regulations and customer preferences are becoming more restrictive
toward the use of peat and peat-based substrates. Similarly, one of
the most popular alternatives for peat, coconut coir, has several
drawbacks, for example: environmental degradation due to land use
change during coir production and water pollution during the coir
processing. In addition, the material is produced in Asia, and the
high CO
2
footprint due to the long transportation is an issue from a
European or American perspective. Many regions where coconuts
are grown, including previously forested areas, have experienced
deforestation due to coconut cultivation. This is particularly evident
in places like the Pacific Islands, where coconuts are a major
contributor to deforestation (Kumar and Kunhamu, 2022). SCS
possess a plasticity potential to utilize organic by-products from
other industries, for example forestry (Barrett et al., 2016).
Therefore, conifer wood fiber became recently a promising, more
sustainable alternative, mainly due to the fact that the natural forest
habitats are not changed because of straightforward reforestation.
According to the Quantis (2012), production, use and disposal of
wood fiber releases only 75 kg CO
2
equivalents per 1 m3, whereas
for peat, 300 kg CO
2
equivalents per 1 m3 are released. The material
has been tested as a growing media ingredient and was recently
applied also as a stand-alone substrate for strawberry cultivation
(Woznicki et al., 2021;Aurdal et al., 2023;Woznicki et al., 2023).
The disposal of spent substrates at the end of cultivation is a
known threat to the environment (Incrocci et al., 2012). Even though
some types of spent growing media can be directly recycled as a soil
improver, either incorporated into composting process as a bulking
agent (Viaene et al., 2017), or as other materials, like Miscanthus, can
be used as a solid fuel or biochar precursors (Kraska et al., 2018),
there is a substantial risk that growing media may contain harmful
chemicals and/or nutrients which can directly affect the ecosystems.
In addition, growing media disposed to landfill at the end of the
growing cycle generate additional costs and recently, landfill
availability is becoming more and more reduced (Ahire et al., 2022).
Since sustainability has recently become a major concern,
minimizing the environmental impact of horticultural production
is an important topic among researchers and policy makers.
Therefore, it is suggested that the circular economy concept of
‘3R’(Reduce, Reuse and Recycle) should be applied to the entire life
cycle of growing media used for both hobby and professional
horticulture (Fiorello et al., 2023). In accordance with the
Directive EU2018/851 of the European Parliament and of The
Council, “waste management in the European Union should be
improved and transformed into sustainable material management,
with a view to protecting, preserving, and improving the quality of
the environment, protecting human health, ensuring prudent,
efficient, and rational utilization of natural resources, promoting
the principles of the circular economy”(EU, 2018).
In recent years, the reuse of growing substrates has gained
increasing attention due to its potential economic and
environmental benefits. Recchia et al. (2013) concluded that reuse
of the spent growing medium has less environmental impact when
compared to landfilling. However, reuse of substrates may reduce
their quality. Accumulation of nutrients may occur in organic
substrates during a growing season and their subsequent reuse
may lead to plant stress and production losses. The applicability of
reused substrates depends on the physio-chemical properties of the
material as well as on the crop specific tolerance for unfavorable
conditions in the root zone (Incrocci et al., 2012).
Strawberries grown in SCS have relatively high nutritional
requirements. Under optimal growth conditions, the strawberry
plant has the highest demand for potassium, followed by nitrogen,
calcium, magnesium, and phosphorus, making the fruits especially
rich in N, P and K (Tagliavini et al., 2005). Moreover, it was observed
that in multi-seasonal strawberry production, approximately 40% of
the N stored in the plants was remobilized during the plant regrowth
in the spring (Tagliavini et al., 2005). Therefore, it is important to
provide adequate mineral nutrition not only during the production
phase but also in the period of plant establishment. The nutritional
composition of growing medium in thisperiodmayplayanimportant
role for further biomass production and realizing of the yield potential.
Use of a growing media in a given season may lead to changes of the
physical and chemical properties and the reuse in subsequent growing
seasons may stimulate or suppress plant establishment and crop
production. However, such effects are not fully understood.
To address all these issues, the objective of this study was to
evaluate the effects of reuse of coir, peat and wood fiber in three
production cycles, identify changes in their physical and chemical
composition and record growth, yield and quality parameters of
tunnel-grown strawberries.
2 Materials and methods
2.1 Plant cultivation
Tray plants of the June bearing strawberry cultivar ‘Malling
Centenary’(Fragaria xananassa) were grown in three different
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substrates in a high poly-tunnel (Haygrove Gothic, oriented South-
North) on a table-top system attheNIBIOresearchstation
Apelsvoll, Kapp, Norway (60°40′N), during three consecutive
years and under the same agrochemical conditions.
The following growing media were used in the experiment:
-100% coir (Botanicoir Precision Plus Ultra, UK), (C)
-80% peat (H2-H4, limed, Tjerbo, Norway) and 20% perlite
[Agra-perlite, Pull Rhenen, NL (Grade 3 –0-6.5mm)], (V/V), (P)
-100% Norway spruce wood fiber, coarse (Fibergrow, Hunton
Fiber AS, Norway), (WF)
Each year, the plants were acquired from NORGRO AS,
Norway, as cold-stored tray (plug) plants and planted in 8 L
plastic trays (length = 50 cm, width = 18 cm, height of substrate
= 14 cm) with four plants in each tray and with two trays
representing one replicate (8 plants per replicate). After the first
and second year of production the above-ground plant biomass was
removed, and the substrates were stored under the roof of unheated
plastic tunnel, to be reused in the following year. The substrates
dried during winter and were rewetted before use the next growing
season. To simulate real farming conditions and minimize cost of
operation, the old plug/root from the tray plants was kept in the
trays, and the new plants were planted in-between the old root
plugs. The only exception was after the second year of production,
when the remaining root plug of one of the old plants was removed
and replaced by a new plant. Fertigation (EC 1.6 mS/cm, pH 6.0,
Calcinit and Kristalon Scarlet, Yara, Norway,50%/50%, containing
macronutrients:10.6; 1.0; 4.7; 0.7; 0.8; 3.2 mmol/l of N, P, K, Mg, S,
Ca and micronutrients: 34; 21; 0.75; 5; 10; 0.5 mmol/L of Fe, Mn, Cu,
Zn, B, Mo, respectively), was applied continuously during the entire
study by one drip (1.2 L/h) per plant and timing was adjusted to the
environmental conditions in the tunnel using a Priva-system
sensors (Priva, ON, Canada). A detailed fertigation schedule is
presented in Table 1. Fixed watering duration times (4 min per
watering event) were applied throughout the experiment. A
standard commercial plant protection strategy, including
application of predatory mites and pulsed water mists against
powdery mildew (Asalf et al., 2021), was successfully used to
prevent pests and diseases. Runners were removed throughout the
season. In the third and final year of production, the experiment
included a 2x3 split-plot design involving two factors (substrate
type and year of cultivation), each with three levels: three substrates
(coir –C, peat –P, and wood fiber - WF), which were 1) not used
before, 2) were used for one or 3) two seasons before. The results
obtained in the final season are presented here, and the numbers 1,
2 and 3 included in the figures, indicate the age of the substrate at
the end of the experiment as explained above.
Berries were harvested three times a week throughout the
season and the weight of all berries collected per replicate was
recorded. Here, only the yield of marketable berries is included
because the proportion of the other yield fractions (rotten and
exceptionally small berries) was negligible. Marketable berries
representing each week of production were frozen (-20°C) and
further analyzed for their chemical composition. The obtained
results were further averaged and are presented as mean values
representing the whole growing season. At termination of the
experiment, biomass production (g FW) and plant height (cm) as
well as number of crowns and leaves were recorded for all the plants
in each treatment. The results are presented on a per plant basis.
2.2 Chemical composition of strawberries
For soluble solids and titratable acids analysis, 200 g of
representative and uniform frozen fruits from each treatment and
each harvest week was thawed overnight at 20°C and homogenized
using a blender (Braun MR400, Karlsruhe, Germany). The samples
were then filtered (Whatman 125 mm, Schleicher & Schuell, Dassel,
Germany) and centrifuged at 400 rpm for 15 min (Eppendorf 5810
R, Hamburg, Germany) to obtain juice. Soluble solid concentration
was determined from the juice by a digital refractometer (Atago
refractometer model PR-1 CO, LTD, Tokyo, Japan), measured as
Brix0 and expressed as % soluble solids. Titratable acids were
determined by a radiometer endpoint titrator (Metrohm 716
DMS Titrino and 730 Sample Changer, Herisau, Switzerland) that
calculated citric acid expressed as a percentage. For determination
of dry matter %, berry homogenate (10 g) was dried at 100°C for 24
h in a drying oven (Termaks, Bergen, Norway) and stabilized in a
desiccator before weighing.
2.3 Physical and chemical properties of
the substrates
Properties of the substrates were analyzed after termination of
the experiment. Both unused virgin materials and reused substrates
(after removal of all plugs remains) were tested for its physical and
chemical characteristics at ILVO, Belgium.
Methods for substrate analysis are based on European
Standards developed by the European Committee for
standardization (CEN) and are assigned to the European
Standard EN numbers. Sample preparation of growing media for
analyses were conducted according to EN13040 (CEN, 2007). To
assess the dry bulk density, water on fresh and dry weight (at −10
cm, −50 cm and −100 cm, as an indicator of water holding
capacity), total pore volume (at −10 cm), air and water volume %
(at −10 cm, −50 cm and −100 cm), easy obtainable water, water
buffering capacity, shrinkage, moisture content, dry matter content,
organic matter content and ash content, the EN13039 and EN13041
(CEN, 2012) procedures were employed.
TABLE 1 Implemented watering strategy.
Time of
the day
Watering criterion
9.00-10.00 When temp. > 20°C and solar radiation > 500W/m
2
10.00-13.00 Fixed watering at 10.00 and 12.00
13.00-17.00 When daily radiation sum > 500J/m
2
(min. 1.5 h
between watering)
17.00-21.00 When temp. > 23°C, (min. 1.5 h between watering)
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Prior to chemical analysis, each growing media sample (tray)
was dried at 70°C, ground, and considered as individual biological
replicates. Total nitrogen (N) content was determined using the
Dumas method, EN13654–2(CEN, 2001), and organic carbon (OC)
was measured using a Skalar Primacs SNC 100 analyzer (Skalar, The
Netherlands). For the assessment of total contents of phosphorus
(P), potassium (K), magnesium (Mg), calcium (Ca) and sodium
(Na), 0.5 g of dried and ground material was digested using a
DigiPREP MS 200 Block Digestion System (SCP SCIENCE, Quebec,
Canada) with 4 mL HNO3 (p.a. 65%) and 12 mL HCl (p.a. 37%) for
120 minutes at 105°C. An Agilent 5110 VDV ICP-OES (Agilent,
Santa Clara, CA, USA) was used to analyze the extract. Neutral
detergent fibre (NDF), acid detergent fibre (ADF) and acid
detergent lignin (ADL, lignin percentage) content was determined
using an Ankom220 Fiber Analyzer extraction unit according to
Van Soest et al. (1991). Based on NDF, ADF and ADL percentage,
the following calculations were performed to obtain percentage of
hemicellulose and cellulose: %hemicellulose = %NDF −%ADF, and
%cellulose = %ADF −%ADL. The lignin, cellulose, hemicellulose,
and residual fraction are expressed on OM base (%/OM) by
conversion of the fraction expressed on DM base (%/DM) to
100% OM:100 x (fraction (%/DM))/(%OM/DM). Residual
fraction (%/DM) = %OM/DM - lignin (%/DM) - cellulose
(%/DM) - hemicellulose (%/DM).
The cation exchange capacity (CEC) was determined by
ammonium acetate (p.a. > 99%, Chem-Lab NV) at pH 7.0 and
KCl (p.a. > 99.5%, Chem-Lab NV), modified from the method by
Rajkovich et al. (2012). Electrical conductivity (EC) (EN 13038) and
pH-H
2
O (EN 13037) were measured in a 1:5 solid to water (v/v)
suspension. As an indicator of substrate stability, oxygen uptake
rate (OUR) was calculated from the oxygen consumption due to
microbial activity. 20 g of substrate was mixed in 200 mL buffered
nutrient solution during 5 days of shaking in a closed Oxitop
respirometer based on the method reported by Grigatti et al. (2011).
The materials were tested for immobilization of mineral N
(Vandecasteele et al., 2018a) by adding 350 mg N/L material
followed by incubation at 37°C for 7 days. CO
2
emission was
measured 13 times during 30 days using a LI-8100 Automated
CO
2
Flux System equipped with a soil flux chamber and a non-
dispersive infrared gas analyzer (LI-COR Biosciences, Lincoln, NE,
USA). Two liters of material was mixed with 4 g/L Haifa Multi-Mix
Potting Soil 14 + 16 + 18(+micronutrients) fertilizer (MF),
moistened based on the squeeze test, and put in PVC rings
(height: 12 cm, diameter: 25 cm) at 20°C. The rings were closed
at the bottom with a plastic cover. The mixtures were rewetted twice
a week based on the recorded weight loss. The cumulative CO
2
release after 30 days was expressed as mol CO
2
/kg OM
(Vandecasteele et al., 2020). Approximation of the amount of
residual macronutrients in spent growing media after each
production cycle (in kg/ha) was based on assumption that one
hectare of table-top strawberry production utilizes ca. 13000 half
meter trays, and that each of them contains 8L of growing medium.
Based on the dry bulk density of the substrates (73, 90 and 37 kg/m3
for C, P and WF, respectively) their dry matter was calculated per
tray basis (0.6 kg for C, 0.7 kg for P and 0.3 kg for WF).
2.4 Statistical analysis
Statistical analysis for substrate properties and the plant growth
trial followed the data presentation paradigm suggested by
Weissgerber et al. (2015);Amrhein et al. (2019) and Muff et al.
(2022). Due to a relatively small dataset, all available data are
presented whenever possible and narrative language of evidence is
applied. Before the analysis, data were tested for normality and
homogeneity of variances using Bartlett’s test. Since all data satisfied
the assumptions for analysis of variance, a General Linear Model was
employed to analyze the relationship between the factors (substrate
type and year of cultivation as fixed factors) and the responses
variables, and to show the presence of the possible interactions
between the factors. Further, Fisher LSD post-hoc tests were applied.
The analyses were conducted using MiniTab®Statistical Software
program package (Release 17.2.1 Minitab Inc., State College, PA, USA).
3 Results
3.1 Physical and chemical characteristics of
virgin materials
Physical and chemical properties of unused substrates are
presented in Tables 2,3, respectively. Virgin defibrated wood
fiber is characterized by more than two-fold lower dry bulk
density than the traditionally used substrates (Table 2). Water on
fresh and dry weight, water volume %, easy obtainable water and
water buffering capacity are much lower in wood fiber compared to
coir and peat (Table 2). On the other hand, wood fiber is
characterized by the highest air volume % (83.5 at 10 cm and
90.7 at 100 cm, respectively) among the studied substrates (Table 2).
Organic carbon content was identical in wood fiber and coir,
and slightly lower in peat (Table 3). While hemicellulose levels are
comparable in all three virgin materials (varying from 11.2% in
wood fiber to 13.8% in peat), the cellulose fraction is much higher in
wood fiber (52.4%) than in coir (36.7%) and peat (25.9%). Wood
fiber seems to be the most inert material, with the lowest
concentration of plant nutrients. Consequently, it has the lowest
CEC and EC. This substrate had also the lowest pH (4.41 vs 6.49
and 6.59 for coir and peat, respectively). Typically for lignocellulosic
materials, wood fiber is characterized by a high C/N ratio and a
much higher CO
2
release rate and oxygen uptake rate compared to
peat and coir. Interestingly, the nitrogen immobilization essay
revealed the highest values for coir (30.9%), followed by wood
fiber (22%) and peat (-4,4%). Negative values observed in peat
indicate nitrogen mineralization in this material (Table 3).
3.2 Physical and chemical characteristics of
reused substrates
After the first year of production the highest cellulose
percentage was detected in wood fiber, followed by coir and peat
(Figure 1A). There was only weak evidence for decrease in cellulose
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content during consecutive cycles of coir and peat reuse. However,
data revealed very strong evidence for decrease in cellulose
percentage in wood fiber. After 3 growing cycles the cellulose
content in this material was comparable to coir and peat. Overall,
hemicellulose was highest in peat (Figure 1B). In both coir and peat,
the percentage of hemicellulose was relatively stable across the age
of substrates (Supplementary Table 1). The opposite situation was
observed in wood fiber, where a rather low initial hemicellulose rate
increased throughout all years (Figure 1B). For the lignin
percentage in the substrate, the lowest levels were observed in
peat, while the highest were in coir and wood fiber. In general,
during the reuse cycles, the total percentage of lignin increased in
peat and wood fiber and remained stable in coir (Figure 1C).
Residual organic matter percentage was the highest in peat. Both
coir and peat revealed relatively stable levels of residual organic
matter, while a significant rise was noted in wood fiber (Figure 1D).
The lowest percentage of organic carbon was observed in peat and
was comparable to unused virgin material and after three years of
production. However, a slight increase was observed after the second
and third year of production (Table 3,Figure 2A). On the other hand,
both unused and reused wood fiber and coir had stable percentages of
organic carbon (Table 2,Figure 2A). Relatively high content of
sodium (Na), which is a potentially harmful element, was observed
especially in virgin coir and peat (Table 3). However, decrease of Na in
those substrates was noted after the reuse (Figure 2B). Accumulation
of N was observed in all substrates when compared to N contents in
virgin materials (Table 3). Across the three analyzed growth cycles,
the most rapid N accumulation was noted for wood fiber, resulting in
concentrations comparable to or higher than those observed in peat
and coir (Figure 2C). Consequently, the C/N ratio of wood fiber,
which has the highest values across the substrates after one year of
production, after two and three years of use drops to the levels
observed in substrates traditionally utilized in SCS strawberry
cultivation (Figure 2D).
Mineral composition of spent growing media is highlighting the
trend in nutrient accumulation during the repetitive production
years in various substrates (Figure 3). While unused substrates are
highly different in mineral composition (Table 2), such contrasts
decreased after the first production cycle (Figure 3). Compared to
the virgin materials, the spent growing media were clearly higher in
N, P, K, and Mg while Ca contents were slightly lower in the spent
peat. P content was similar and stable in all spent materials and was
not affected by age (Figure 3A,Supplementary Table 1). On the
other hand, K accumulated to comparable levels in all materials
after the first year of production and then decreased when the
substrates were further reused. The highest decrease in K content
TABLE 2 Physical properties of virgin materials.
C* P WF
Dry bulk density kg/m
3
72.9 90.7 36.9
Water on fresh weight g H
2
O/100 g 10 cm 85.3 87.4 79.3
50 cm 77.7 79.1 65.5
100 cm 76.6 76.2 64.2
Water on dry weight g H
2
O/100 g 10 cm 582.2 692.8 383.8
50 cm 349.4 379.4 189.6
100 cm 327.3 319.5 179.7
Total pore volume ml/100 ml (humid 10 cm) TPV 95.4 95.0 97.6
Air volume % (ml air/100 ml fresh sub.) 10 cm 52.9 32.2 83.5
50 cm 68.5 61.2 90.1
100 cm 71.7 67.9 90.7
Water volume % (ml H
2
O/100 ml fresh sub.) 10 cm 42.5 62.8 14.1
50 cm 26.7 33.9 7.2
100 cm 23.7 27.4 6.9
Easy obtainable water 15.8 28.9 6.9
Water buffering capacity 3.0 6.5 0.4
Shrink % 12.6 12.1 <5.0
Moisture content % (g/100 g fresh weight) 73.1 66.2 46.7
Dry matter content % (g/100 g fresh weight) 26.9 33.8 53.3
Organic matter % (g/100 g dry weight) 95.8 64.8 99.7
*C, coir; P, peat; WF, wood fiber.
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A
B
CD
FIGURE 1
(A) Cellulose (%/OM), (B) hemicellulose (%/OM), (C) lignin (%/OM) and (D) residual fraction (%/OM) content in coir (C), peat (P) and wood fiber (WF)
after one (1) two (2) and three (3) growing seasons. For all treatments n = 3, where n is one randomly selected tray representing each substrate type.
Means that do not share a letter are significantly different based on the Fisher LSD Method (95% CI).
TABLE 3 Chemical properties of virgin materials.
C* P WF
ORG. CARBON (OC) %/DM 47.9 34.2 47.9
LIGNIN %/OM 40.0 23.8 25.7
HEMICELLULOSE %/OM 13.4 21.3 11.2
CELLULOSE %/OM 38.3 40.0 52.6
Residual %/OM 8.4 15.0 10.5
N g/kg DM 4.43 7.55 0.64
C/N - 108 45 749
P g/kg DM 0.163 0.283 <0.05
K g/kg DM 1.514 0.749 0.33
Mg g/kg DM 0.621 0.996 <0.6
Ca g/kg DM 5.67 15.67 <3.0
Na g/kg DM 0.81 0.69 0.041
CEC cmolc/kg DM 59.8 109.0 3.4
pH-H
2
O - 6.49 6.59 4.41
EC µS/cm 120 57 55
OUR (O
2
Uptake) mmol/kg OM/hr 1.5 1.5 2.6
CO
2
Release mol CO
2
/kg OM 0.32 0.30 2.19
N Immobilization % 30.9 -4.4 22
*C, coir; P, peat; WF, wood fiber; NDF, Neutral detergent fibre; ADF, acid detergent fibre; CEC, cation exchange capacity; OUR, oxygen uptake rate.
Woznicki et al. 10.3389/fpls.2023.1307240
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AB
CD
FIGURE 3
(A) Phosphorus (g/kg DM), (B) Potassium (g/kg DM), (C) Magnesium (g/kg DM) and (D) Calcium (g/kg DM) content in coir (C), peat (P) and wood fiber
(WF) after one (1), two (2) and three (3) growing seasons. Horizontal lines represent nutrient levels observed in unused materials. For all treatmentsn
= 3, where n is one randomly selected tray representing each substrate type. Means that do not share a letter are significantly different based on the
Fisher LSD Method (95% CI).
AB
CD
FIGURE 2
(A) Organic carbon (OC, %), (B) Total sodium content (Na, g/kg DM), (C) Total nitrogen content (N, g/kg DM) and (D) C/N ratio in coir (C), peat (P)
and wood fiber (WF) after one (1), two (2) and three (3) growing seasons. Horizontal lines represent nutrient levels observed in unused materials. For
all treatments n = 3, where n is one randomly selected tray representing each substrate type. Means that do not share a letter are significantly
different based on the Fisher LSD Method (95% CI).
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was observed for wood fiber (Figure 3B). The greatest levels of Mg
were detected in peat, while lower in coir and wood fiber
(Figure 3C). Peat had the highest ability to accumulate excess Ca,
and the levels were relatively stable across production years
(Figure 3D). On the contrary, wood fiber was characterized by
the lowest ability to accumulate Ca, however, during the
consecutive production cycles the increase in Ca contents was
significant and finally similar to values observed in coir (Figure 3D).
Approximation of the amount of macronutrients incorporated
into a production system through the reuse of spent growing media
show that the supply is affected by both substrate type and number
of reuse cycles (Table 4). Under the applied fertigation strategy, coir,
peat, and wood fiber after the first production cycle provide to the
second production cycle ca. 90, 117 and 38 kg of N/ha, respectively,
and those values increase by ca. 10kg after each of reuse (Table 4).
The amount of P, K and Mg accumulated in the spent substrate per
hectare of production is ca. 7-10 times lower than N, being the
highest in peat and the lowest in wood fiber. While the content of P
and Mg was relatively stable in each substrate across the reuse
cycles, K revealed slight depletion (Table 4,Supplementary Table 2).
3.3 Plant performance, berry yield
and quality
All plants were visually assessed, and no visible symptoms of
any stressful conditions were recorded. However, variability in the
plant material was very high (each dot in Figure 4 represents
parameters of an individual plant). In general, the highest
strawberry plants were recorded in wood fiber independently of
the age of substrates. Comparable performance was observed in
plants grown in new peat; however, reused peat tended to produce
shorter plants. On the other hand, the lowest strawberry plants were
observed in coir, Figure 4A). Number of leaves, crowns and plant
weight are plant architectural parameters which are usually
correlated with plant height, and a similar trend was observed
here (Figures 4B-D). Number of leaves and crowns are comparable
across the treatments; nonetheless, there is a visible tendency that
strawberry plants were more vegetative when grown in substrates
reused three times (Figure 4B). Plant biomass production varied
greatly but was generally maintained, even though it was slightly
reduced in the substrates used for two years (C2, P2, WF2;
Figure 4D,Supplementary Table 3).
In general, strawberry yield was satisfactory and comparable
across all investigated substrates (Figure 5A). A slightly higher yield
wasobservedinplantsfrompeatandwoodfiber in the first year of
production compared to coir. Yield from plants in coir, however,
remained relatively stable across reused substrates while for plants in
peat and wood fiber weak evidence for reduction of their yielding
potential in the second (p<0.008 for peat) and third year of
production in the recycled substrates is present. However, the
reduction was not large, and the plants performed similarly to
those grown in coir with no statistical differences observed
(Figure 5A). Dry matter of the berries was also relatively stable
across the substrates (Figure 5E), increasing only slightly in berries
grown in peat used for the third time (Figure 5E). Berry dry yield,
obtained by the recalculation based on dry matter content (Figure 5E)
revealed a pattern parallel to the fresh weight yield (Figures 5A,F).
Comparison of the means suggests a tendency for higher sugar
accumulation in strawberries grown in wood fiber (Figure 5B,
Supplementary Table 3) and reduced berry acidity in this
substrate (especially in the variant reused twice, Figure 5C)
consequently resulted in strawberries with a slightly better taste
(sugar/acid ratio ca. 0.5 to 1 point higher in WF than in C and P,
Figure 5D). However, a high variability in the sugar/acid ratio
(strawberry taste) in a given plant didn’t allow to detect very strong
statistical evidence for differences between the growing media age
(Figure 5D,Supplementary Table 3).
4 Discussion
4.1 Sustainability
Worldwide, large quantities of spent, nutrient-rich growing
media residues are disposed, not being recycled. It is suggested
TABLE 4 Approximation of the amount of macronutrients provided with spent growing media (in kg/ha of table-top strawberry production).
N (kg/ha) P (kg/ha) K (kg/ha) Mg (kg/ha)
C1* 93 c 11 a 37 abc 13 c
C2 103 bc 10 ab 36 abc 12 c
C3 114 abc 11 a 34 c 14 bc
P1 117 ab 14 a 43 a 17 a
P2 130 a 13 a 41 ab 17 a
P3 130 a 13 a 35 bc 16 ab
WF1 38 e 6 c 17 d 5 d
WF2 48 de 6 c 15 d 5 d
WF3 63 d 7 bc 13 d 6 d
*C, coir; P, peat; WF, wood fiber; 1, 2, 3 –years of reuse.
Means that do not share a letter are significantly different based on the Fisher LSD Method (95% CI).
Woznicki et al. 10.3389/fpls.2023.1307240
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that circular economy (CE) strategies for materials used in
agricultural production include reuse within the same or between
different value chains. This would minimize waste generation and
decrease the need to incorporate new materials (Sayadi-Gmada
et al., 2019;Circle Economy, 2021). Despite of the observed changes
in chemical properties of the substrate after a production cycle, the
results here show that all the tested growing media (both traditional,
like peat and coir, and a novel substrate candidate, stand-alone
wood fiber) can be successfully reused in SCS strawberry
production. A similar observation was made by Baevre and
Guttormsen (1984) when tomato and cucumber produced in
reused peat bags was analyzed. Shuttleworth et al. (2021) proved
the potential of SCS strawberry production in reused coir. Beyond
that, the present study indicates that the stand-alone wood fiber,
previously investigated as a potential raw material which can
replace peat and coir (Woznicki et al., 2023) can also be utilized
in consecutive growing cycles without significant deterioration or
adverse effect on yield. This provides possibilities to fulfill the
potential of this material for economic and environmental
benefits in a CE approach as well as being an important step
toward broader implementation of “lean production”paradigm
(Gruda, 2019;Dahmani et al., 2021;Velasco-Muñoz et al., 2022).
4.2 Nutrient dynamics
During the reuse of substrates, some nutrients can be
accumulated (sometimes reaching the toxicity levels) in the
material, and further transferred into newly established plants and
utilized as a start fertilizer (Vandecasteele et al., 2018b;Aurdal et al.,
2023). Also, in the present study, this phenomenon was observed,
however, any excess accumulation of salts, including sodium, was not
noted (Figure 3), indicating that the utilized fertigation strategy was
adequate and did not cause any serious salt stress or excessive
nutrient retention in any of the substrates. This was reflected in the
biomass production and overall, well-being of the plants (Figure 4).
The amount (kg/ha) of nutrients transferred into the next production
cycle by the spent growing media (Table 4) was generally parallel to
the range observed by Vandecasteele et al. (2023) in greenhouse
strawberry production. However, in the present study a higher
accumulation of K in comparison to Mg was observed, while
Vandecasteele et al. (2023) noted the opposite trend. This might be
due to fertigation strategy implemented by Vandecasteele et al., 2023,
which had almost identical ratios between N, P and K as those
implemented here, but provided approximately two times higher
levels of Mg (See: Materials and Methods section).
Interestingly, wood fiber showed the highest and most rapid
nitrogen accumulation, and therefore, might be the reason for the
most vigorous growth (plants were the highest among all tested
substrates, Figure 4). Consequently, the observed C/N ratio also
dropped rapidly, and after three years of production, the used
wood fiber was characterized by a C/N ratio comparable to peat.
This also indicates thatmicrobial nitrogen immobilization, which can
be a serious problem for plants grown in wood fiber-based substrates
(Harris et al., 2020) was avoided by a precise adaptation of the
fertigation strategy with slightly higher EC and more dense
A
B
CD
FIGURE 4
(A) Plant height (cm), (B) number of leaves (C) crowns and (D) fresh weight (g) of aboveground plant biomass (g) in strawberries grown in coir (C),
peat (P) and wood fiber (WF) after one (1), two (2) and three (3) growing seasons. For all treatments n = 48. Each dot represents one plant. Means
that do not share a letter are significantly different based on the Fisher LSD Method (95% CI).
Woznicki et al. 10.3389/fpls.2023.1307240
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distribution of drips in comparison to common practice (Aurdal
et al., 2023;Woznicki et al., 2023). A trend with decreasing potassium
(K) accumulation during reuse of substrates (the highest in wood
fiber) agrees with Vandecasteele et al. (2018b);Vandecasteele et al.,
2023), and might be explained by the fact that strawberry has a
relatively high demand for K. It is known that during fruit ripening,
fruits represent the largest sink for K and N (Tagliavini et al., 2005).
The stand-alone wood fiber substrate is characterized by highest
porosity and lowest CEC of the tested substrates (Tables 2,3)and
therefore substantial leakage of nutrients should be compensated.
Thus, it can be hypothesized that an updated fertigation strategy for
wood fiber should include additional source of K to satisfy the needs
of high yielding cultivars. It is worth mentioning that the applied
fertigation strategy was also suitable for the plants grown in peat and
coir, indicating plasticity and adaptation ability of strawberry plants
when grown in an organic substrate.
4.3 Quality attributes
The sugar –acid ratio is a broadly used parameter for indication
of strawberry taste and consumer acceptance (Li et al., 2022) and
may be a proxy for a sensorics panel evaluating strawberry
attractiveness. This parameter showed constantly the highest
values in strawberries grown in wood fiber (Figure 5). It was the
consequence of a relatively high sugar accumulation and
simultaneously, stable acidity of the studied berries, with
exception of berries from plants grown in the oldest wood fiber
substrate, which reduced acidity. This observation agrees with the
previous study, where strawberry plants grown in wood fiber-based
substrate revealed a slightly higher sugar accumulation (Woznicki
et al., 2021). For optimal aerobic respiration of the roots, sufficient
oxygen level in the root zone is crucial. Anaerobic conditions in the
root zone can inhibit uptake of nutrients as well as cell division rate
AB
CD
EF
FIGURE 5
(A) Total yield of marketable berries (>28mm), (B) soluble solids (°Brix), (C) acidity (% of citric acid), (D) ratio between sugars and acids, (E) berry dry
matter (%) and (F) dry yield (g DW/plant) grown in coir (C), peat (P) and wood fiber (WF) in one (1), two (2) and three (3) growing seasons. For all
treatments n = 6, where n is the average of eight plants from the same repetition. Means that do not share a letter are significantly different based
on the Fisher LSD Method (95% CI).
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(Soffer et al., 1991;Rocksch et al., 2008). Due to higher porosity
(Woznicki et al., 2023) and much higher air volume % (Table 3)of
wood fiber, strawberry plants are less likely to experience stressful
anaerobic conditions. This fact might be partially responsible for
the increase in sugar accumulation in berries as observed in the
present study.
4.4 Substrate stability
Microbial degradation is a common process that affects all
organic-based growing media. When combined with shrinkage
and swelling, these factors can lead to negative consequences for
the structural stability of the media (Jackson et al., 2009;Carlile
et al., 2015). In this study, the degradation process was most visible
in stand-alone wood fiber (Figure 1) where the percentage of
cellulose dropped to the levels observed in coir and peat. In
contrast, coir and peat revealed relative stability across production
cycles (Figure 1). The degree of stability loss is mainly affected by
the chemical composition. Peat exhibits high resilience against
decomposition, due to the fact that Sphagnum mosses incorporate
lignin-like polymers into the cell walls, which plays a pivotal role in
the resistance to degradation (Prasad and Maher, 2004). Moreover,
the unique pectin-like substances, such as sphagnan, found in the
cell walls of Sphagnum mosses (Stalheim et al., 2009), contribute to
their durability, as suggested by Hajek et al. (2011). The resistance
to degradation seen in coir can be attributed to the intricate
composition of lignocellulose complexes, comprising cellulose,
hemicellulose, and lignin, which are interlinked through both
covalent and non-covalent bonds. These bonds confer significant
resistance against degradation (Carlile et al., 2015). On the other
hand, the biological degradation susceptibility of conifer wood fiber
can be especially attributed to the high decomposing vulnerability
of cellulose and hemicellulose (Fengel and Wegener, 1989). In the
present study, the observed change in hemicellulose (and to lesser
extent lignin) percentage in wood fiber (Figure 1) is likely related to
the proportional decrease in percentage of cellulose which led to an
overall increase of percentage share of other constituents.
Despite high variability in plant architecture within each
treatment, there is an apparent trend that plants grown in
substrates used for a second production cycle had slightly reduced
number of leaves, crowns, and overall biomass production
(Figure 4). This phenomenon might be explained by the influence
of the planting strategy. Plugs planted in substrates used for two
years were planted next to the old plugs’residuals, while in the
substrates used for the third time, half of the old plugs were
removed, and new ones were planted instead. The differences in
local nutrient distribution and space for root development might be
responsible for this unexpected observation. However, it can be
concluded that the placing of new plants in the old substrate had
only marginal effect on strawberry yield.
As shown in the present study, it is possible to grow strawberries
in reused peat, coir, and wood fiber without any pretreatment.
However, during the scenario where rootzone diseases are present,
the reuse of substrate can become a risky strategy which can lead to
great economic losses. Therefore, it can be suggested that steam
sanitation, which eliminates pathogens and weeds, should be applied
whenever the risk of pathogen or weed contamination is serious
(Vandecasteele et al., 2020). Recently, a successful eradication of
powdery mildew from strawberry transplants was obtained using
aerated steam (Stensvand et al., 2023). It can be hypothesized that the
same equipment utilizing elevated temperature of treatment can be a
promising tool for effective substrate sanitization.
The reuse of growing media reduces the costs of purchase of
new blends and the gate fee for waste collection for the grower. The
net return of reusing growing media is strongly dependent on the
price of virgin growing media. Spent growing media can be directly
reused for another strawberry cultivation, without negative impact
on yield or fruit quality. When destined for direct reuse by the
grower, the need for sanitation needs to be assessed by the grower
based on experiences and observations during the previous growing
cycle. Sanitation is necessary whenever spent growing media are
transferred from one grower to another, to avoid any risks related to
weeds or pests/diseases.
5 Conclusion
This work evaluated the feasibility of substrate reuse for SCS
strawberry production. Two commonly used substrates, peat and
coir,aswellasanovelalternative,woodfiber from Norway
spruce, revealed potential for successful implementation even
after two cycles of production. Wood fiber revealed an ability for
nutrient accumulation. Its chemical composition changed,
making it more similar to the commonly used growing media
utilized for SCS production. Therefore, it can be concluded that
not only peat and coir, both commercial standards with
sustainability issues, but also wood fiber, a more sustainable
alternative, can be successfully reused, ensuring satisfactory
yields of high-quality strawberries.
Data availability statement
The original contributions presented in the study are included
in the article/Supplementary Material. Further inquiries can be
directed to the corresponding author.
Author contributions
TW: Conceptualization, Data curation, Formal analysis,
Investigation, Methodology, Validation, Visualization, Writing –
original draft, Writing –review & editing, Funding acquisition,
Resources. KK: Conceptualization, Data curation, Formal analysis,
Funding acquisition, Investigation, Methodology, Resources,
Validation, Writing –original draft, Writing –review & editing.
BV: Conceptualization, Data curation, Formal analysis,
Investigation, Methodology, Resources, Validation, Writing –
review & editing. AS: Conceptualization, Funding acquisition,
Investigation, Methodology, Project administration, Resources,
Supervision, Writing –review & editing.
Woznicki et al. 10.3389/fpls.2023.1307240
Frontiers in Plant Science frontiersin.org11

Funding
The author(s) declare financial support was received for the
research, authorship, and/or publication of this article. TW, KK,
and AS acknowledge financial support from the Norwegian
Agricultural Agreement Research Fund/Foundation for Research
Levy on Agricultural Products, grant number 302129 and
Grofondet, grant number 190024 and POLNOR QualityBerry,
project implemented under the Norwegian Financial Mechanism
for 2014-2021 "We work together for a green, competitive and
favorable social integration in Europe", contract number: NOR/
POLNOR/QualityBerry/0014/2019-00.
Acknowledgments
The authors would like to thank Unni Roos, Sofie Andersen,
and Mirjana Sadojevic for help in performing experiments and
Signe Hansen and Kari Grønnerød for performing
chemical analyses.
Conflict of interest
The authors declare that the research was conducted in the
absence of any commercial or financial relationships that could be
construed as a potential conflict of interest.
Publisher’s note
All claims expressed in this article are solely those of the authors
and do not necessarily represent those of their affiliated organizations,
or those of the publisher, the editors and the reviewers. Any product
that may be evaluated in this article, or claim that may be made by its
manufacturer, is not guaranteed or endorsed by the publisher.
Supplementary material
The Supplementary Material for this article can be found online
at: https://www.frontiersin.org/articles/10.3389/fpls.2023.1307240/
full#supplementary-material
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