Controlled root targeted delivery of fertilizer using an ionically crosslinked carboxymethyl

RESEARCH Open Access
Controlled root targeted delivery of fertilizer
using an ionically crosslinked carboxymethyl
cellulose hydrogel matrix
Drew W Davidson, Mohit S Verma and Frank X Gu*
Abstract
Aims: The recent increases in food prices caused by the corresponding increases in fertilizer costs have highlighted
the demand for reducing the overuse of fertilizers in industrial agriculture. There has been increasing interest in
developing plant root-targeted delivery (RTD) of fertilizers in order to address the problem of inefficient fertilizer
use. The aim of this study is to develop a low cost controlled release device to deliver fertilizers to plant roots and
thereby increase fertilizer use efficiency.
Methods: The Root Targeted Delivery Vehicle (RTDV) is formed by dissolving Carboxymethyl Cellulose (CMC) chains
in water, mixing it with liquid fertilizer and crosslinking using iron and calcium salts. Basic measurements
quantifying nutrient release and green house growth trials were carried out to evaluate fertilizer use efficiency on
wheat growing in nutrient depleted soil media.
Results: Growing wheat in nutrient depleted media showed that the RTDV permits a 78% reduction in the amount
of fertilizer needed to achieve similar levels of plant yield in these conditions. Quantifying the losses associated with
the RTDV synthesis showed that optimizing manufacturing could possibly increase this value as high as 94%.
Furthermore, the delivery device showed a similar lifetime in soil to the plant’s growth cycle, delivering fertilizer
over the course of the plant’s growth before removal from soil by degradation.
Conclusions: These results illustrate the importance of fertilizer delivery in facilitating absorption and may have
potential to vastly increase the use efficiency of fertilizers in soil, resulting in a significant reduction of costs and
environmental damage. With more in depth study to quantify the fertilizer release and refine the device, there is
great potential for the use of the RTDV as an effective means to increase fertilizer use efficiency in agriculture.
Keywords: Carboxymethyl cellulose; Controlled release; Fertilizer; Agrochemicals; Wheat
Background
Fertilizer usage has a critical impact on all aspects of
agriculture, either directly or indirectly, by altering the
cost of growing and the relative profitability of crops.
Use of Controlled Release Fertilizers (CRF) allows the
release of nutrients to be better matched with the life
cycle of the plant (Malhi et al. 2010, Shaviv and
Mikkelsen 1993) in order to increase the efficiency of
nutrient uptake by plants. Furthermore, the nutrient de-
mands of the plant can be met more closely by designing
an appropriate controlled release system to increase effi-
ciency and reducing the risk of overdosing the plant
(Akiyama et al. 2010). CRF also prevent fertilizers from
being leached from the soil and decrease costs for agri-
culture by reducing the amount of fertilizer needed and
the labour and fuel costs associated with repeated applica-
tions of fertilizers (Shaviv and Mikkelsen 1993, Akiyama
et al. 2010, Simonne and Hutchinson 2005). There is also
evidence that different nutrients and micronutrients can
influence the ability of plants to utilize other nutrients
effectively (Shaviv and Mikkelsen 1993, Garcia-Mina et al.
2004).
Polymer coating of large fertilizer granules is the most
commonly applied controlled release mechanism and it
* Correspondence: frank.gu@uwaterloo.ca
Department of Chemical Engineering and Waterloo Institute for
Nanotechnology, University of Waterloo, 200 University Avenue West,
Waterloo, Ontario N2L 3G1, Canada
a SpringerOpen Journal
© 2013 Davidson et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction
in any medium, provided the original work is properly cited.
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relies on the biodegradation of the polymer coating to
release fertilizers over the course of a growing season.
Coating the fertilizers with a polymer serves to help
immobilize the fertilizer pellets and hence make them
resistant to runoff and leaching (Morita et al. 2002);
varying degrees of leaching resistance have been ob-
served depending on the type of CRF used. The encap-
sulation also protects fertilizers from environmental
degradation from microorganisms or chemical reactions
and lengthens the amount of time plants have to absorb
the fertilizer (Tian and Saigusa 2005). However, the
commercially available CRF are vulnerable to changes in
the soil types, moisture contents and other factors which
can affect the release rate (Chen et al. 2008). These com-
plications can lead to fertilizer release not being syn-
chronized with plant demand and may create situations
where the plants are starved of nutrients or unable to
use the fertilizer released (Gumbo et al. 2008).
We hypothesize that a Root Targeted Delivery Vehicle
(RTDV) fabricated from Carboxymethyl Cellulose (CMC)
hydrogel is able to decrease the amount of fertilizer ne-
cessary for maintaining plant yields. The efficiency of
fertilizer use by the plant would be increased by deliv-
ering fertilizer to the plant at a rate that neither allows
the fertilizer to be lost nor causes the plant to be
undernourished. CMC material is able to fill the role of
a CRF due to its low cost, due to its common use as
filler in food products, and its ability to directly encap-
sulate fertilizers. Unlike other hydrogel materials being
developed for fertilizer release, the CMC hydrogels de-
tailed in this report do not coat granules of fertilizer
with a layer of hydrogel like commercial fertilizers and
some other hydrogel based devices (Ni et al. 2011) but
instead the hydrogel matrix is mixed with liquid
fertilizer. Intercalation of the fertilizer greatly simplifies
the synthesis of the CRF by removing the need for steps
such as milling and filtering and coating processes such
as fluidized bed used in the manufacture of other emer-
ging hydrogel CRF products (Ni et al. 2011, Zhao et al.
2010). Furthermore, since the fertilizer is contained
within the hydrogel material rather than encapsulated,
the hydrogel is not vulnerable to physical damage or
any other process which can rupture a coating and
cause device failure. The hydrogels can be dried in
order to allow the hydrogel material to be stored and
mixed into soil, which gives the material the potential
to be used for deployment on a large scale.
In addition, this hydrogel device would also be able to
serve as a platform for the release of multiple com-
pounds for root targeted delivery. This control over
fertilizer release is possible due to CMC hydrogels being
able to encapsulate a wide variety of bioactive molecules,
both hydrophobic and hydrophilic, such as glucose
oxidase, aldicarb (a carbamate insecticide), and bovine
serum albumin (Wu and Choi 2004, Kok et al. 1999,
Zhang et al. 2001, Kulterer et al. 2012). Even microor-
ganisms such as Pannonibacter phragmitetus may be en-
capsulated by CMC materials (Xu et al. 2011).
The RTDV attempts to utilize the principles of controlled
drug release to maximize the absorption and utilization of
the released fertilizer by the wheat plants. This strategy in-
volves optimizing the rate of release of the RTDVs while
simultaneously targeting the delivery to plant roots in order
to minimize fertilizer losses to runoff and chemical degrad-
ation. Using this design of CRF, it was demonstrated that
plant growth can be increased and fertilizer use can be sig-
nificantly decreased without a significant decrease in plant
yield. The potential for the RTDV for controlled release of
hydrophilic and hydrophobic materials for applications
such as pest and disease control will also be demonstrated.
The results of the study also demonstrate the potential for
the RTDV to be simultaneously used to increase drought
resistance of crops by improving soil water retention.
The CRF system currently being designed has the ability
to deliver both major nutrients such as nitrogen, phosphor-
ous and potassium compounds as well as small molecules
such as micronutrients. With further research the release of
these nutrients will be targeted to the root zone and
lifecycle of the plant to maximize the usage efficiency of the
plant in order to reduce fertilizer related costs. In addition
to the reduction of costs by increasing the efficiency of
fertilizer use, CRF serve to reduce the environmental pollu-
tion caused by fertilizer application (Gabriels et al. 2001,
Gumbo et al. 2008, Anderson 2009).
Materials and methods
Materials
CMC powder (MW 250000), iron (II) chloride penta-
hydrate, iron (III) chloride hexahydrate were used as re-
ceived from Sigma Aldrich. Anhydrous calcium chloride
and regenerated cellulose membrane (nominal MWCO
12000–14000, width 45 mm) were used as received from
Fischer Scientific. 20-20-20 fertilizer was obtained from
Plant Products Co. Turface used as a nutrient depleted
growth media was obtained from Profile Products. Organic
wheat seeds were obtained from Eating Well Organically.
RTDV synthesis
The RTDVs are manufactured through dissolution of 7 g
CMC powder in 100 ml deionized (DI) water. The gel is
then loaded with the desired release formulation, in this
case the 20-20-20 fertilizer, by dissolving the desired
amount of fertilizer in 10 ml deionized water then mixing
the dissolved fertilizer into the hydrogel. After loading, the
gel is placed into a 30 ml syringe and injected into sections
of dialysis membrane. The membrane is then clamped at
both ends and left in an ionic crosslinking solution com-
posed of 40 g calcium chloride, 4 g iron (II) chloride and 4
Davidson et al. SpringerPlus 2013, 2:318 Page 2 of 9

g iron (III) chloride dissolved in 400 ml DI water for 48
hours in order to obtain the resulting ionically crosslinked
hydrogel. Afterward, the crosslinked hydrogels are taken
out of solution and the membrane is removed. Drying of
the RTDVs involves placing the RTDVs into an oven set at
80°C for 24 to 48 hours depending on the desired level of
crystallization for the hydrogel.
Release study
Bench-top release studies were performed by immersing
the fertilizer loaded RTDVs into 150 ml DI water. Three
300 μl samples were taken on a regular basis and
analysed using UV–vis spectrometry, described below.
Volumes of release media removed for sampling were
replaced with DI water. Release media was replaced daily
with fresh DI water to avoid saturation of the media.
The study continued until all fertilizer was released from
the gels as noted by steady readings for the UV–vis mea-
surements. The combination of conditions used for syn-
thesis of RTDVs is summarized in Table 1. Here, full
dose represents the total mass of fertilizer (2.15 g) used
for the positive control experiments given daily applica-
tions of liquid fertilizer. The fertilizer loading (wt%) is
calculated by dividing the mass of fertilizer by the mass
of polymer used.
UV–vis measurements
UV–vis measurements were done to measure the ab-
sorbance of fertilizer in solution in order to determine
the concentration. Readings were taken using a Bio-Tek
Epoch microplate spectrophotometer on 300 μl samples.
Absorbance readings were performed at 630 nm, which
corresponds to an absorbance peak for the 20-20-20
fertilizer compound. Measurements were done in tripli-
cates to account for experimental error. This method
was employed because it was simple and rapid. The lin-
ear dependence of absorbance on the concentration of
the fertilizer was verified by obtaining a calibration
curve. Although this method does not measure all the
nutrients that are being released, it provides an indica-
tion of controlled release of fertilizer which is further
verified by the greenhouse experiments. The UV–vis
measurements are able to indicate the overall rate of re-
lease from the device but are not able to quantify the
concentration of individual fertilizer components. Al-
though it would be preferable to obtain this information,
it was deemed to be beyond the scope of this initial
study. More in- depth quantification of the release of
fertilizer from the RTDV is planned in the future.
Growth testing
Growth testing using plant models was done in a
temperature and humidity controlled greenhouse. The
testing involved placing the CMC hydrogels prepared
using the above method into a small planter filled with
Turface. The gel is covered with Turface and a wheat
seed is placed into the pot and again covered with
Turface. The pots are then watered to hydrate the soil.
The growth experiments involving wheat continue for
50 days with 50 ml deionized water applied daily to
allow the wheat to reach the heading stage, which occurs
at approximately 45 to 50 days for the wheat strain used.
The heights of the plants are recorded throughout the
experiment in order to gauge the hydrogel’s performance
in increasing plant growth. At the end of the experiment
the plants are removed from the soil and are dried com-
pletely in ambient conditions in order to obtain dry mass
measurements. Each growth experiment is performed
alongside a negative and positive control to help minimize
the effect of variation from the experiments occurring at
different times. Positive controls received 50 ml deionized
water for the first 7 days and 1 g/L 20-20-20 fertilizer
dissolved in 50 ml deionized water per day for the
remaining 43 days instead of using a hydrogel device for
fertilizer delivery. Negative controls received only 50 ml
deionized water and did not use a hydrogel device either.
All experiments are done in replicates of three to ac-
count for variation between plants. Plants that do not
germinate within the first 20 days are considered to be
outliers and their measurements were not considered in
the final average.
Results and discussion
RTDV synthesis and release testing
The controlled release device is made through ionic gel-
ation of CMC using iron and calcium salts. Raw fertilizer
containing nitrogen, phosphorous and potassium was
Table 1 Synthesis conditions of root targeted delivery
vehicles (RTDV)
Sample Fertilizer loading
(% of full dose)
Fertilizer loading
(wt%)
Drying
(hours)
RTDV 1 a
22 20 0
RTDV 2 a
33 30 0
RTDV 3 a
44 40 0
RTDV 4 a
22 20 24
RTDV 5 a
22 20 48
RTDV 6 a
100 92 0
RTDV 7 a
100 92 24
RTDV 8 a
100 92 48
RTDV 9 a
44 40 24
RTDV 10 b
44 40 0
RTDV 11 c
44 40 0
a
refers to devices synthesized with the method detailed in RTDV synthesis
above, b
refers to devices synthesized while heating the crosslinking solution
to 40°C as detailed in alternative RTDV synthesis methods below, c
refers to
devices synthesized using elevated levels of iron salts as detailed in alternative
RTDV synthesis methods.

added to the CMC immediately prior to gelation. Free
fertilizer and salts were removed from the hydrogel by
washing with deionized water. The hydrogels were syn-
thesized in 5 cm × 3 cm × 1 cm pieces in order to
maximize the compatibility of the gels with the growth
experiment set-up and for ease of manufacture using the
ionic bath. This size of hydrogel was also necessary for
allowing the hydrogels to be able to hold enough
fertilizer for the duration of the growth experiment. The
hydrogels were tested at three different loading levels,
22%, 44% and 100% dosage, as compared to the total
amount given to plants that were administered daily ap-
plications of fertilizer.
In order to determine the optimal loading levels for
the ionically crosslinked RTDV, measurements of the re-
lease rates of several different hydrogels were taken in
order to quantify the effects of increasing the fertilizer
loading level in the RTDV. The results showed that as
the fertilizer concentration increased, the initial burst re-
lease of fertilizer increased and the release was less
sustained than with lower fertilizer loading levels. The
RTDVs released 80 to 90% of the loaded fertilizer within
the first 80 hours. However, after the initial release, there
was a steady release of fertilizer until after 500 hours,
after which the release ceased. The efficiency of release,
the total amount of fertilizer released from the RTDVs
compared to the amount loaded, is on average 96%, indi-
cating that the RTDVs are able to release the majority of
their payload with minimal irreversible binding. On the
basis of the release profiles of the RTDVs at different
loadings (Figure 1), 22% dose and 44% dose loadings
were chosen for testing since 22% dose shows the
strongest sustained release of fertilizer over the long
term, while the 44% dose device had a similar release
profile to the 33% dose device but with a higher loading
level.
The ionic crosslinking process of the RTDV relies on
the interaction of the carboxyl group of the CMC with
polyvalent salts (Rathna et al. 1996) in order to create a
more stable hydrogel than that which is obtained natur-
ally by the dissolution of CMC in water. The objective of
the study is to create a controlled release device capable
of increasing the fertilizer use efficiency of plants. How-
ever, the 20-20-20 fertilizer used in the experiment has a
number of ionic compounds which weaken the overall
integrity of the hydrogel. At high concentrations, the
presence of chelators in the fertilizer weakens the bond
strength of the CMC crosslinks (Rathna et al. 1996).
Thus, the RTDVs loaded with lower fertilizer levels are
expected to sustain release for a longer period of time.
Natural soils were avoided in this experiment to prevent
confounding of results. A nutrient depleted medium as-
sures that the growth was due to the implanted RTDV in-
stead of nutrients from the soil itself. The release
study indicated a burst release by the hydrogel in the
first 24 hours in advance of the RTDV’s sustained release
period. Turface was chosen as the soil media for the
growth experiments in order to compensate for any pos-
sible burst release by the RTDV by preventing the build-
up of harmful levels of nutrients in the soil. Typically,
plants gain little benefit from soil nutrients before seed
germination so this burst release was expected to have lit-
tle impact on the study, as the coarse grain of the soil
media would not retain the fertilizer effectively, preventing
excessive nutrient concentrations that may harm the
plant. Thus, the growth experiments give a better repre-
sentation of the effects of the sustained release fertilizer
due to the nutrient depleted soil matrix. In addition, the
presence of the burst release coupled with the low reten-
tion of the soil indicates that plants are likely receiving
only a fraction of the fertilizer loaded into the RTDV. In
order to prevent this excess fertilizer from being wasted
an extra processing step where the RTDV was washed in
deionized water before implanting in soil, removing the
loosely bound fertilizer from the device and making them
available for recycling, is planned to be added to the de-
vice synthesis.
The sustained release from the RTDV in water in the
release study is still somewhat rapid, however, other
water and soil based release studies done using CMC
(Kumar et al. 2010) have shown that release of hydro-
phobic compounds in soil using CMC based controlled
release media can be nearly twice as slow as in water
(Kumar et al. 2010). Considering that the fertilizer being
released from the RTDVs is hydrophilic and observa-
tions from growth testing showing fertilizer remaining in
Figure 1 Fertilizer release profiles of RTDVs in deionized water.
The hydrogels were loaded with 20-20-20 fertilizer at 22% (“blue
triangle symbol” RTDV 1), 33% (“red circle symbol” RTDV 2) and 44%
(“black square symbol” RTDV 3) dose levels. The amount of fertilizer
released (mean ± S.D., n = 3) was quantified using UV-Visible
spectrophotometry.

the RTDVs even after 50 days, it is likely that the release
of fertilizer in soil will be much more prolonged com-
pared to the timing given by the water based release
experiments.
An alternate setup using fertilizer pellets coated with a
different form of CMC hydrogel showed that their for-
mulation reached a release of 80% after approximately 5
days in soil (Ni et al. 2011). Due to the differences in the
release study procedure between the two methods, it is
difficult to determine which formulation has a longer
sustained release period in soil, however, observations of
the performance of the CMC hydrogel matrix used in
this study during growth testing suggest that the
sustained release period in soil is much longer than was
observed in deionized water during the release study.
Growth testing
Growth testing was used to determine if the RTDVs
were effective in increasing the fertilizer use efficiency of
the wheat plants compared to simply applying fertilizer
dissolved in water. The experimental runs for each for-
mulation of hydrogel were compared to each other by
analysing the relative difference in the growth of the
plants as compared to the controls rather than directly
comparing the runs to each other in order to reduce the
impact of extraneous factors such as day to day variabil-
ity in humidity, light levels and temperature. Growths of
the plants were compared using measurements of plant
height and dry mass (at the end of the experiment). The
focus of the study was primarily to reduce the amount
of fertilizer used to promote plant growth, quantified
using plant height, while maintaining plant yield, quanti-
fied using dry mass. The dry mass data gives a more
complete measure of the plant growth. However, the fact
that the plant must be uprooted in order to acquire a
mass measurement makes the height measurement the
best tool for determining the performance of the RTDVs
over time as the plants grow. The plant height data gives
a more complete picture of the effect of the hydrogels
on the growth of the plants during the entire lifecycle,
compared to the dry mass data, which can in turn be
used to improve the release characteristics of the
hydrogels to enhance performance.
Fertilizing the plants daily over 50 days gives an effect-
ive dose of 2.15 g of fertilizer. RTDVs loaded with the
same amount of 20-20-20 fertilizer as the total fertilizer
given to the plants administered daily fertilizer doses.
The results, show that the plants initially lagged behind
those given daily applied fertilizer but from day 20 to
day 43 the plants are approximately equal in height and
after this point the plants given the RTDV outperform
the plants fertilized daily in terms of growth (Figure 2,
RTDV 6). All of the wheat plants reached the heading
stage before the end of the experiment except for some
of the plants given only deionized water, likely due to a
lack of nutrients. After the experiment, the plants were
uprooted and we observed that the hydrogels had a blue
colouration from the residual fertilizer. These observa-
tions confirm that the release of fertilizer in soil is more
sustained than the release observed in deionized water
(Figure 1).
In addition to height data, dry mass measurements
taken at the end of the growth experiment also show
that the RTDVs were effective at increasing the growth
and yield of the wheat plants. The data, shown in Table 2,
indicates that the 100% dose hydrogels were able to in-
crease the seed mass of the wheat plants almost double
the amount produced by the plants receiving daily doses
of fertilizer.
Figure 2 The effect of drying RTDVs on the growth of wheat
plants. The plants were supplied with deionized water daily as a
negative control (“black square symbol”), 20-20-20 fertilizer daily as a
positive control (“red circle symbol”) or RTDVs at the start of the
experiments. The RTDVs were loaded at 100% dose of 20-20-20
fertilizer and dried in the oven at 80°C for 0 h (“blue triangle symbol”
RTDV 6), 24 h (“pink triangle symbol” RTDV 7), 48 h ("green triangle
symbol" RTDV 8). Plant height (mean ± S.D., n = 3) is reported.
Table 2 Mass measurements of wheat plants grown with
root targeted delivery vehicles (RTDVs)
Sample Dry mass ± S.D. (%) Dry seed mass ± S.D. (%)
Fertilizer applied
daily
100.0 ± 20.8 100.0 ± 16.2
Deionized water
daily
9.9 ± 2.4 11.6 ± 11.6
RTDV 4 20.6 ± 2.7 81.0 ± 6.8
RTDV 5 16.9 ± 0.0 66.5 ± 4.8
RTDV 6 78.9 ± 15.9 197.3 ± 63.7
RTDV 7 76.0 ± 2.8 198.9 ± 20.5
RTDV 8 69.5 ± 7.0 271.6 ± 32.4

RTDV drying
In order to reduce burst release at the start of the growth
period and increase release duration, we dried the
hydrogels at 80°C for varying amounts of time. Drying the
hydrogels before implantation resulted in significant im-
provement in the growth of the wheat plants. The plant
heights for experiments performed with 100% dose RTDV
subjected to 24 and 48 hour drying (Figure 2 RTDV 7, 8)
show an increase in final plant height in the range of 3 to 5
cm for the plants with the hydrogels subjected to drying.
Contrasted with the performance of the hydrogels without
drying, the plants are consistently outpacing the plants
given daily fertilizer in terms of height, rather than just in
the last 10 days. In addition to increased plant height there
was also a corresponding increase in the dry seed mass for
the 48 hour drying of the hydrogels, as shown in Table 2.
The RTDV with 48 hour drying had a 2.72 times increase
in seed yield as compared to the daily fertilized plants,
whereas the non-dried and 24 hour dried RTDV had a 1.97
and 1.99 times increase, over the daily fertilized plants.
There was significant improvement in seed yield using
the RTDV compared to daily applied fertilizer. However,
due to research indicating that the strength of the ionic
bonds in the hydrogel is weakened by having a large
concentration of ionic species (Rathna et al. 1996),
additional methods of increasing the integrity of the
hydrogels were investigated. Drying the hydrogel was in-
troduced in order to attempt to minimize the amount of
fertilizer released during the early growth period of the
plants where the plants are using the nutrients contained
in the seeds and instead provide nutrients more evenly
over the course of the plant’s lifecycle to achieve higher
growth yields (Sarkar et al. 2007, Yinbo et al. 1997).
An increase in plant height should correlate with an
increase in mass, however, the mass data (Table 2) and
height data (Figures 2 and 3) shows that although the
plants grown with the device are taller than the fertilized
plants, the mass of the fertilized plants is higher than
the ones grown with the device. This discrepancy arose
later in the plant growth cycle during the heading stage
of plant development. In this stage the tillers (the grain
bearing stalks) of the wheat grown with the RTDV grew
much taller than those of the plants given daily fertilizer.
This growth resulted in the height of the RTDV plants
exceeding those for the fertilized plants despite a lower
total mass for the RTDV wheat. Higher plant mass but
lower seed yield can occur in cases of excessive nitrogen
concentration (Oscarson, 2000). Based on these observa-
tions, it appears that the RTDV creates favourable condi-
tions for development of grains on the plant, which
corresponds to their higher seed yield, compared to the
plants given daily fertilizer.
In order to reduce the reliance of the device on ionic
bonding, drying of the devices was investigated as a
method of increasing the strength of the RTDV. Research
on CMC/polyvinylamine films showed that dried CMC
films are able to form additional hydrogen bonds after
already being bonded using ionic crosslinking (Feng and
Pelton 2007). Drying the CMC hydrogel was intended to
increase the bonding strength of the material through
both the formation of these additional hydrogen bonds as
well as increasing the time for water to fully hydrate the
device.
RTDV fertilizer loading variation
We performed growth experiments at varying degrees of
fertilizer loading to test the limits of reduced fertilizer
application before yield was adversely impacted. This is
expected to yield an estimate of the improvement of nu-
trient absorption by the wheat. Growth experiments
using RTDV 4 resulted in similar levels of growth to
wheat plants given full daily doses of fertilizer (Figure 3).
The lower level of fertilizer loading was able to maintain
plant height and mostly maintain the seed mass of the
plants but did not have the drastic improvement in
growth seen with the 100% Dose RTDV. Table 2 shows
that despite an approximate 78% reduction in the total
amount of fertilizer delivered to the plants; yield only
suffered by 19% for the 22% Dose RTDV 4 compared
with an 89% reduction in the plants that did not receive
fertilizer. Furthermore, assuming that the fertilizer re-
leased during the burst release phase could be recycled
through the addition of a pre-washing step, it is possible
Figure 3 The effect of loading dose and RTDV synthesis
condition on the growth of wheat plants. The plants were either
supplied with 20-20-20 fertilizer daily as a positive control (“red
horizontal line”) or RTDVs at the start of the experiments. The RTDVs
were loaded with 20-20-20 fertilizer: 22% dose, 24 h drying (“blue
triangle symbol” RTDV 4), 44% dose, 24 h drying (“pink triangle
symbol” RTDV 9), 44% dose, heating during cross-linking (“green
triangle sysmbol” RTDV 10). Plant height (mean ± S.D., n = 3)
is reported.

that the actual dose delivered to the plant could be re-
duced by as much as 94% compared to the daily applied
fertilizer by incorporating the reductions from both re-
moving the burst release fertilizer and the initial loading
of only a 22% dosage. The effectiveness of measures to
remove the effect of the burst release will be evaluated
in subsequent studies.
Growth experiments performed for RTDV 5, showed
that increased drying time resulted in higher levels of
plant height being achieved, however, seed yield and
plant mass both decreased compared to the hydrogels
dried for only 24 hours. Observations of the hydrogels
that were removed from soil at the end of the experi-
ment indicated that there was still residual fertilizer
present in the RTDV, suggesting that the extended dry-
ing period of the RTDVs, coupled with the low loading
of fertilizer, resulted in a release rate that was too slow
to adequately supply the plants with nutrients.
RTDV 9 using a 44% fertilizer dose was tested using
growth experiments in order to further quantify the
effect of fertilizer loading on plant growth. The growth
data (Figure 3) indicates that the 44% dose was effective
at increasing growth in the wheat plants, according to
plant height. The increase in fertilizer loading had
diminishing returns according to the plant height data.
Increasing the fertilizer loading from 22 to 44 to 100%
dosage increased the growth each time, but the amount
of growth gained decreased as the fertilizer level in-
creased. This is likely due to the issue of the hydrogels
being less able to effectively achieve sustained release at
higher loading levels, as suggested by the release experi-
ments and other studies (Rathna et al. 1996).
Based on the results of the previous release study and
research showing the weakening effect of the high con-
centration of ionic compounds (Rathna et al. 1996), such
as those present in the fertilizer, it was hypothesized that
reducing the fertilizer loading in the hydrogel would in-
crease the proportion of fertilizer released during the
sustained release phase. The tests were performed at 22
and 44% dosage levels in the hydrogel in order to enable
comparison with the previous release experiments and
confirm the hypothesis that the lower fertilizer loading
levels exhibited a more sustained release. 33% dose
RTDVs were not tested due to the relative similarity be-
tween the release curves for the 33 and 44% release pro-
file. These tests were also performed with varying levels
of drying before implantation, as with the 100% dose
RTDV.
Comparing the performance of the RTDV at different
loading levels and drying times indicates that the drying
process does improve the sustained release behaviour of
the RTDV at higher loading levels. Drying the hydrogels
for 48 hours resulted in a decrease in plant mass at 22%
dosage but resulted in a large increase at 100%. Drying
may be effective at mitigating the effect of higher
fertilizer loads on the sustained release from the hydro-
gel, but the long drying time, cost of energy and risk of
undersupplying nutrients to the plants make it undesir-
able for use in large scale production.
Alternative RTDV synthesis methods
Each of these trials used RTDVs with 44% fertilizer dos-
age. The growth data of the hydrogels subjected to
heated crosslinking (Figure 3) indicates that each of the
different methods for increasing crosslinking density of
the hydrogels were effective at increasing the growth of
the wheat plants. Each method resulted in similar in-
creases to plant height as 44% dose RTDVs that were
subjected to the 24 hour drying time. The growth of the
wheat plants in the trials for both alternative synthesis
methods had more dramatic increases in growth during
the latter half of the experiment compared to simple
drying of the hydrogels. This fact suggests that these
methods were more effective than drying at sustaining
the release of the fertilizers from the hydrogels.
Alternatives to drying were investigated in order to at-
tempt to avoid the large amount of synthesis time re-
quired for dried hydrogels and make the synthesis more
economical. These alternatives centred on altering the
synthesis process for the ionically crosslinked RTDVs in
order to provide a more robust hydrogel that would be
able to resist the weakening of the ionic bonding from
the high fertilizer loads. The two alterations that were
investigated were increasing the proportion of iron to
calcium salts by increasing iron (II) and iron (III) chlor-
ide by 2 g and decreasing calcium chloride by 4 g per
400 ml crosslinking solution. Previous experiments sug-
gested that increasing iron salt concentration in
crosslinking CMC hydrogels resulted in a more heavily
crosslinked structure. This is expected because the triva-
lent iron ions would increase ionic bonding strength be-
tween the CMC chains compared to divalent calcium
ions. The previous experiments on CMC hydrogels were
done without loading the hydrogel with any payload and
resulted in hydrogels that were brittle and possessed a
hard shell. The brittle morphology was not observed
after the introduction of fertilizer as a payload which
made this approach more viable. The second approach to
increasing the hydrogel robustness involved subjecting the
hydrogels to heating during the ionic crosslinking process.
In this experiment, the hydrogels were heated to 40°C
while crosslinking. The increase in temperature during
crosslinking should increase the diffusion of salts into the
hydrogel and result in a more densely and uniformly
crosslinked hydrogel and a corresponding increase in
sustained fertilizer release.
Overall, heating the hydrogels during crosslinking is
the best alternative method due to differences in cost

and time required compared to the altered crosslinking
solution recipe or drying the hydrogels. Heating the
hydrogels during crosslinking does not require the extra
synthesis time that drying requires for improving the
sustained fertilizer release of the hydrogels. In addition,
the heated crosslinking hydrogels did not retain the
colour of the fertilizer after the duration of the growth
experiment, indicating a more complete release of
fertilizer over the growth period of the plants compared
to the dried hydrogels. This may indicate that the
heating of the hydrogel serves to reduce both the
amount of fertilizer that is loosely bound as well as the
binding of the hydrogel shell phase, while increasing the
proportion of fertilizer contained in the moderately
crosslinked core phase, leading to a better sustained re-
lease profile. In comparison to the hydrogels synthesized
with increased levels of iron salts, the heated cross-
linking gels were more repeatable. In addition, it is un-
known what effect changing the proportion of residual
iron and calcium salts had on the growth of the wheat
plants, it is possible that increasing the amount of iron,
a potent micronutrient for many plants, delivered by
the hydrogel served to mitigate excess salinity that may
have been introduced by the calcium salts or fertilizer
(Delgado and Sanchez-Raya 2007).
RTDV degradation testing
RTDVs were left in soil and regularly exhumed and
weighed to gauge the amount of mass loss as a result of
degradation of the gel. The results of these trials (Figure 4)
show an initial increase in hydrogel mass in the first 15
days due to water absorption, followed by steady degrad-
ation of the hydrogel. Even with a large amount of water
exposure, the hydrogels are able to avoid complete deg-
radation despite being subjected to 50 watering cycles,
ending with approximately 10% of their original mass. In
addition, the remaining hydrogel was observed after this
time period to have retained a small amount of the encap-
sulated fertilizer due to the presence of the 20-20-20 fertil-
izer’s blue colouration in the hydrogel. This experiment
suggests that the hydrogel would be able to survive and
continue releasing fertilizer for an extended period of time
in soil but would be removed from the soil by degradation
after the end of the growing season once they are no lon-
ger needed. Although separate degradation tests for dried
hydrogels were not performed, it was observed during the
growth experiments that the dried hydrogels were much
more resistant to degradation over the course of these ex-
periments than non-dried hydrogels. Hydrogels watered
on a weekly basis had much less loss of mass compared to
those watered on a daily basis over the same time period.
Some of the mass loss observed is a result of the initial
loading of water in the hydrogel being diffused out of the
hydrogel by the drier soil over time. These hydrogels
slowly degraded over the 50 day period, but retained
much of their initial mass. The weekly watered hydrogels
were able to retain approximately 89% of their original
mass (not including absorbed water).
In order to be effective as a controlled release fertilizer
agent, the RTDVs should remain intact in soil over the
course of the growing season. Optimally the RTDVs will
only require insertion into soil during crop planting with
no need for reapplication until the next growing season,
but will be not remain in the soil once no longer needed.
In order to investigate the RTDVs ability to remain in-
tact for this length of time, tests in which RTDV with
22% fertilizer dosage and no drying were placed into soil
and subjected to conditions similar to those experienced
by the hydrogel during the growth experiments. How-
ever, wheat plants were not grown along with these gels,
instead the hydrogel mass was monitored to give an in-
dication of the amount of physical degradation of the
RTDV.
This indicates that the RTDVs are likely a viable
method for fertilizer release over the course of an entire
growing season in field conditions. The observed oscilla-
tion of the hydrogel mass in time with the watering cycle
also suggests that the RTDVs may be effective at storing
water in soil over periods of drought, as it takes approxi-
mately 27 days of weekly watering before the hydrogel
reaches a minimum mass. Even though the hydrogels
were able to repeatedly absorb and release water, signifi-
cant changes in the volume of the RTDVs were not ob-
served, this is advantageous since changing the height of
soil and disturbing root systems is undesirable during
plant growth. The resistance of the dried RTDVs to deg-
radation could be an issue with regard to a build-up of
Figure 4 Degradation of RTDV in soil. RTDV 1 loaded with 22%
dose of 20-20-20 fertilizer was placed in the soil and supplied with
deionized water daily. The mass (mean ± S.D., n = 3) of RTDV has
been normalized against the initial mass.

litter in fields if the hydrogels do not effectively bio-
degrade after the growing season. However, combined
with their lengthened release time, the dried RTDVs
may be viable for release over multiple seasons if they
are able to withstand being in soil over the winter
months. The ability of the RTDVs to act as a medium
for improving drought resistance of crops will be ex-
plored in subsequent investigations.
Conclusions
Overall, the RTDVs are a promising platform for in-
creasing the fertilizer use efficiency of agricultural crops.
The RTDV maintained seed yield of crops despite reduc-
tions of fertilizer use of up to 78%, with the possibility
that this reduction could be increased to as much as
94% with further optimization. The difference in the
total plant mass and seed mass observed may also indi-
cate that the CRF was altering the nutrient availability in
the soil. In addition, there is potential for improving the
performance of the RTDVs further in order to maximize
the efficient use of fertilizer and allow larger reductions
in fertilizer use in order to help reduce costs and envir-
onmental issues caused by fertilizer pollution. Alterna-
tive uses for the RTDVs in terms of drought mitigation
were also identified as possible avenues of investigation.
However, further investigation will be required in order
to fully explore the ability of the RTDVs in increasing
fertilizer use efficiency in a wider variety of crop plants
and field conditions.
Abbreviations
(CMC): Carboxymethyl cellulose; (CRF): Controlled release fertilizer;
(RTDV): Root targeted delivery vehicle.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
DD executed the synthesis of RTDVs, greenhouse testing of the RTDVs,
release study of fertilizers from hydrogels and prepared the manuscript. MV
participated in the synthesis of CMC hydrogels, release studies, prepared
figures and revised the manuscript. FG designed the greenhouse
experiments, analyzed the data and revised the manuscript. All authors read
and approved the final manuscript.
Acknowledgements
This work was financially supported by Natural Sciences and Engineering
Research Council of Canada (NSERC) and Landscape Ontario. Drew Davidson
is funded by Ontario Graduate Scholarship and the Canadian Agri-Food
Policy Institute and Mohit Verma is funded by NSERC Vanier Graduate
Scholarship.
Received: 14 March 2013 Accepted: 14 June 2013
Published: 15 July 2013
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doi:10.1186/2193-1801-2-318
Cite this article as: Davidson et al.: Controlled root targeted delivery of
fertilizer using an ionically crosslinked carboxymethyl cellulose hydrogel
matrix. SpringerPlus 2013 2:318.

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