Advances in Controlled Release Fertilizers Cost-Effective Coating Techniques and Smart Stimuli-Responsive Hydrogels

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Advances in Controlled Release Fertilizers: Cost-Effective
Coating Techniques and Smart Stimuli-Responsive
Hydrogels
Houssameddine Mansouri, Hamid Ait Said, Hassan Noukrati, Abdallah Oukarroum,
Hicham Ben youcef, and François Perreault*
To meet the needs of a rapidly expanding global population, farmers will need
more fertilizers than ever before to maintain a steady supply of affordable,
nutritious food. The formulation of controlled release fertilizers (CRF) to
synchronize nutrient release according to the demand of plants has emerged
as a viable solution to the current problems associated with the poor nutrient
usage efficiency of fertilizers. Yet, the greatest obstacle that still stands in the
way of broad use of CRF in agriculture is their expensive manufacturing costs.
The first section of this analysis focuses on broad topics related to CRF.
Afterward, the differences between several cost-effective raw materials and
some of the production techniques used to make CRF are examined.
Furthermore, the emerging field of “smart” coating materials, such as
stimuli-responsive coatings, which can accurately tailor nutrients delivery to
the demands of the vegetation, is discussed, and the most important research
work that could lead to their extensive use in agriculture is pointed out. The
purpose of this review is to provide a strong assessment of CRF’s
development over the past several years by highlighting innovations and
providing in-depth analysis of prevailing patterns to better understand the
future of agriculture.
1. Introduction
Nutrients are required for plants to grow and complete their life
cycle. Those nutrients are present in soil naturally, but their quan-
tity will fade with time. Synthetic fertilizers are crucial in this
H. Mansouri, H. Ait Said, A. Oukarroum, H. Ben youcef
High Throughput Multidisciplinary Research Laboratory (HTMR)
Mohammed VI Polytechnic University (UM6P)
Benguerir 43150, Morocco
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adsu.202300149
© 2023 The Authors. Advanced Sustainable Systems published by
Wiley-VCH GmbH. This is an open access article under the terms of the
Creative Commons Attribution License, which permits use, distribution
and reproduction in any medium, provided the original work is properly
cited.
DOI: 10.1002/adsu.202300149
situation since they operate as a direct
source of nutrients for the soil.[1] Chem-
ical fertilizers are now extensively ap-
plied by farmers all over the world since
they can supply all the vital elements that
plants require for development.[1] These
nutrients are categorized into macronu-
trients and micronutrients, based on how
much they are needed for plant growth.
Because they are required in larger quan-
tities, macronutrients, namely nitrogen
(N), potassium (K), and phosphorus (P),
have been the focus of manufacturing ef-
forts by the fertilizer industry.[2]
Despite the increase in agricultural
productivity enabled by intensive agricul-
tural practices, chemical fertilizers still
have a nutrient use efficacy (NUE) of
less than 30%, with phosphorus fertiliz-
ers having an even lower NUE of less
than 20%.[3] The excess fertilizer that
is not quickly absorbed by plants will
become unavailable for plant use due
to microbiological, chemical, or physical
reactions (Figure 1).[4] This poor NUE
can lead to major environmental consequences. For example,
leaching of nitrogen as nitrous oxide and nitrates, caused by the
excess use of nitrogen fertilizers, leads to the eutrophication of
aquatic ecosystems and the emission of greenhouse gases that
can accelerate global climate changes.[3] Those climate changes
H. Mansouri, F. Perreault
School of Sustainable Engineering and the Built Environment
Arizona State University
Tempe, AZ 85287-3005, USA
E-mail: perreault.francois@uqam.ca
H. Noukrati
High Institute of Biological and Paramedical Sciences (ISSB-P)
Mohammed VI Polytechnic University (UM6P)
Benguerir 43150, Morocco
A. Oukarroum
AgroBioSciences Plant Stress Physiology Laboratory (AgBS)
Mohammed VI Polytechnic University (UM6P)
Benguerir 43150, Morocco
F. Perreault
Department of Chemistry
University of Quebec in Montreal
CP 8888, Succ. Centre-Ville, Montreal, QC H3C 3P8, Canada
Adv. Sustainable Syst. 2023, 7, 2300149 2300149 (1 of 19) © 2023 The Authors. Advanced Sustainable Systems published by Wiley-VCH GmbH
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Figure 1. A,B) Loss of excess conventional fertilizers in soil. In (A) excess N fertilizer is lost in the environment through run-off, ammonia volatilization,
leaching of nitrates, and nitrous oxide emissions. In (B) excess P fertilizer is either lost by run-off and leaching, immobilized into organic form, adsorbed
into inorganic form, or precipitated into mineral form.
in turn affect agriculture through a decrease in crops yield, a
reduction in their nutritional quality, or an increased frequency
of extreme weather events detrimental to plants’ growth.[5] The
development of new techniques to help increase fertilizers’ effi-
ciency, both in terms of food production and environmental pro-
tection, is thus critical to achieve more sustainable agricultural
practices.
The low NUE observed for plants in agricultural soils is in part
due to the nature of conventional fertilizers which, once applied,
will release a large amount of nutrients in the early stage of plant
growth, leaving too little at later stages.[4] To overcome this issue,
controlled release fertilizers (CRF) have been explored as an ap-
proach to deliver fertilizers in a way that match the rate required
by the plant, which would avoid losses caused by leaching, evap-
oration, and other factors related to the weather.[6] However, to
accurately control the release of such fertilizers, a number of chal-
lenges and factors that determine their applicability and perfor-
mance still need additional research and investigation. For exam-
ple, there is a pressing need for more research into the processes
that can trigger and govern the release rate of nutrients by CRF, as
well as the primary environmental factors (such as temperature,
moisture, microbes, acidity, and soil type) that influence them.[7]
In this review, we present the most recent research on the de-
velopment of CRF in an effort to highlight the most promising
approaches and materials that may be exploited to coat fertilizers
and achieve controlled release. We will highlight the techniques
and practices that can provide greater performance with less coat-
ing material, as well as any potential difficulties or issues related
with their application. We will also explore the promising field of
stimuli-responsive hydrogels, which can better match nutrient
release to the needs of the crops and identify the key areas that
still require research before they can be successfully applied in
the agricultural sector. This paper will give recommendations for
future research on next generation fertilizers that, by reducing
fertilizer use and increasing agricultural yield, can play an im-
portant role in achieving several of the sustainable development
goals defined by the European Union, such as reducing hunger
and malnutrition, promoting responsible production and con-
sumption in the agricultural field, and decrease the greenhouse
gas emissions associated with fertilizer production and applica-
tion.
2. Controlled Release Fertilizers: Classification,
Synthesis, and Mechanisms of Release
The CRFs are specially designed products that release active fer-
tilizing nutrients in a controlled, slow fashion in synchronization
with the progressive demands of plants for nutrients.[8] The ini-
tial efforts in the developmentA
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onditions (https://onlinelibrary.w
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Ta
bl
e
4.
Su
m
m
ar
y
of
di
ffe
re
nt
st
im
ul
ir
es
po
ns
iv
e
fe
rt
ili
ze
r
or
ag
ro
ch
em
ic
al
de
liv
er
y
sy
st
em
s.
Po
ly
m
er
Sy
nt
he
si
s
m
et
ho
d
Ty
pe
of
st
im
ul
i
A
ct
iv
e
in
gr
ed
ie
nt
R
es
ea
rc
h
fin
di
ng
R
ef
.
W
he
at
st
ra
w
am
in
at
ed
-c
el
lu
lo
se
na
no
fib
er
s
In
si
tu
ra
di
ca
l
co
po
ly
m
er
iz
at
io
n
pH
-r
es
po
ns
iv
e
N
fe
rt
ili
ze
r
•
Lo
w
N
re
le
as
e
in
ne
ut
ra
lp
H
.
•
H
ig
h
N
re
le
as
e
in
lo
w
pH
(r
el
ea
se
m
ai
nt
ai
ne
d
fo
r
90
da
ys
).
[9
6]
Ce
llu
lo
se
na
no
fib
er
s
an
d
so
di
um
al
gi
na
te
Io
ni
c
cr
os
sl
in
ki
ng
pH
-r
es
po
ns
iv
e
N
PK
•
In
cr
ea
se
d
sw
el
lin
g
ra
tio
at
pH
fr
om
3
to
6.
•
St
ab
le
sw
el
lin
g
ra
tio
at
pH
fr
om
6
to
8
•
D
ec
re
as
ed
sw
el
lin
g
ra
tio
at
pH
fr
om
8
to
11
.
[7
9]
So
di
um
al
gi
na
te
In
si
tu
fr
ee
-r
ad
ic
al
gr
af
t
co
po
ly
m
er
iz
at
io
n
pH
-r
es
po
ns
iv
e
N
PK
•
In
cr
ea
se
d
sw
el
lin
g
ra
tio
at
pH
fr
om
2
to
6.
•
St
ab
le
sw
el
lin
g
ra
tio
at
pH
fr
om
5
to
7.
•
D
ec
re
as
ed
sw
el
lin
g
ra
tio
at
pH
ab
ov
e
7.
[7
1]
C
hi
to
sa
n
In
si
tu
hy
dr
og
el
at
io
n
pH
-r
es
po
ns
iv
e
U
re
a
•
A
tp
H
ab
ov
e
6,
th
e
hy
dr
og
el
m
ai
nt
ai
ne
d
its
st
ru
ct
ur
al
in
te
gr
ity
fo
r
up
to
15
da
ys
.
•
In
cr
ea
se
d
st
ab
ili
ty
w
ith
in
cr
ea
si
ng
pH
.
[7
3]
D
im
et
hy
la
cr
yl
am
id
e/
m
al
ei
c
ac
id
/s
ta
rc
h
R
ed
ox
po
ly
m
er
iz
at
io
n
pH
-r
es
po
ns
iv
e
U
re
a
•
Po
si
tiv
e
su
rf
ac
e
m
od
ifi
ca
tio
n
of
D
M
St
1
ha
d
th
e
gr
ea
te
st
sw
el
lin
g
ra
tio
(2
78
38
%
at
pH
8)
.
[8
2]
Su
lfo
na
te
d
ca
rb
ox
ym
et
hy
lc
el
lu
lo
se
G
ra
ft
co
po
ly
m
er
iz
at
io
n
pH
-r
es
po
ns
iv
e
N
PK
•
H
yd
ro
ge
ls
sw
el
la
tp
H
=
8
w
hi
le
sh
rin
ki
ng
at
pH
=
2
•
H
yd
ro
ge
ls
sw
el
le
d
in
de
na
tu
re
d
w
at
er
an
d
sh
ra
nk
in
0.
1
m
N
aC
l.
[7
5]
Po
ly
(a
cr
yl
am
id
e-
co
-d
ia
lly
ld
im
et
hy
la
m
m
on
iu
m
ch
lo
ri
de
Fr
ee
ra
di
ca
l
co
po
ly
m
er
iz
at
io
n
Th
er
m
o-
re
sp
on
si
ve
(U
C
ST
)
U
re
a,
KH
2
PO
4
,N
H
4
N
O
3
,
M
gS
O
4
•
Fr
om
0.
00
5
to
0.
05
m
ol
L−
1
,t
he
U
C
ST
de
cr
ea
se
d
fr
om
20
to
15
°
C
in
sa
lt
m
ed
ia
.
[8
5]
Po
ly
(N
,N
-d
im
et
hy
la
m
in
oe
th
yl
m
et
ha
cr
yl
at
e)
A
to
m
-t
ra
ns
fe
r
ra
di
ca
l
po
ly
m
er
iz
at
io
n
Th
er
m
o-
re
sp
on
si
ve
(L
C
ST
)
Po
ly
do
pa
m
in
e-
co
at
ed
am
m
on
iu
m
zi
nc
ph
os
ph
at
e
•
A
t2
5
°
C
,p
H
4.
0,
7.
0,
an
d
10
.0
so
lu
tio
ns
re
le
as
e
P
at
59
.2
2%
,
47
.3
6%
,a
nd
40
.8
9%
,c
om
pa
re
d
to
68
.9
8%
,5
6.
26
%
,a
nd
34
.2
1%
at
40
°
C
.
•
A
t2
5
°
C
,p
H
4.
0,
7.
0,
an
d
10
.0
so
lu
tio
ns
re
le
as
e
Z
n
at
57
.0
7%
,
46
.8
4%
,a
nd
38
.5
4%
,c
om
pa
re
d
to
70
.3
6%
,5
5.
13
%
,a
nd
31
.6
7%
at
40
°
C
.
•
A
t2
5
°
C
,p
H
4.
0,
7.
0,
an
d
10
.0
so
lu
tio
ns
re
le
as
e
N
H
4
+
at
63
.8
5%
,5
4.
13
%
,a
nd
48
.7
9%
,c
om
pa
re
d
to
75
.7
1%
,6
4.
34
%
,
an
d
42
.3
6%
at
40
°
C
.
[8
6]
po
ly
et
he
r
po
ly
ol
/p
ol
yc
ap
ro
la
ct
on
e
po
ly
ur
et
ha
ne
O
ne
-s
te
p
co
po
ly
m
er
iz
at
io
n
Th
er
m
o-
re
sp
on
si
ve
N
fe
rt
ili
ze
r
•
N
re
le
as
e
du
ra
tio
n
>
30
da
ys
.
•
Th
e
N
re
le
as
e
ra
te
in
cr
ea
se
d
by
22
.3
tim
es
pe
r
ho
ur
w
he
n
te
m
pe
ra
tu
re
in
cr
ea
se
d
fr
om
32
to
33
°
C
[9
0]
Ca
rb
ox
ym
et
hy
lc
el
lu
lo
se
–
En
zy
m
e-
re
sp
on
si
ve
Ep
ic
hl
or
oh
yd
ri
n
•
Ce
llu
la
se
st
im
ul
i-r
es
po
ns
iv
e.
•
In
se
ct
ic
id
al
ac
tiv
ity
ag
ai
ns
tM
yz
us
pe
rc
ae
.
[9
2]
Ca
rb
ox
ym
et
hy
lc
el
lu
lo
se
–
R
ed
ox
-r
es
po
ns
iv
e
N
ap
ht
hy
la
ce
tic
ac
id
,
6-
be
nz
yl
ad
en
in
e
•
R
ed
ox
-r
el
ea
se
of
ag
ro
ch
em
ic
al
tr
ig
ge
re
d
by
re
du
ce
r.
•
H
ea
vy
m
et
al
io
ns
(C
u2+
an
d
H
g2+
)
ca
pt
ur
e
[9
4]
Ca
rb
ox
ym
et
hy
lc
el
lu
lo
se
G
ra
ft
co
po
ly
m
er
iz
at
io
n
En
zy
m
e-
an
d
re
do
x-
re
sp
on
si
ve
Sa
lic
yl
ic
ac
id
•
R
ed
ox
-r
es
po
ns
iv
e
re
le
as
e
ra
te
of
85
.2
%
tr
ig
ge
re
d
by
hy
dr
og
en
pe
ro
xi
de
.
•
En
zy
m
e-
re
sp
on
si
ve
re
le
as
e
ra
te
of
80
.4
%
tr
ig
ge
re
d
by
ce
llu
la
se
[9
5]
Adv. Sustainable Syst. 2023, 7, 2300149 2300149 (15 of 19) © 2023 The Authors. Advanced Sustainable Systems published by Wiley-VCH GmbH
 23667486, 2023, 9, D
ow
nloaded from
 https://onlinelibrary.w
iley.com
/doi/10.1002/adsu.202300149 by C
A
PE
S, W
iley O
nline L
ibrary on [05/07/2024]. See the T
erm
s and C
onditions (https://onlinelibrary.w
iley.com
/term
s-and-conditions) on W
iley O
nline L
ibrary for rules of use; O
A
 articles are governed by the applicable C
reative C
om
m
ons L
icense
http://www.advancedsciencenews.com
http://www.advsustainsys.com
www.advancedsciencenews.com www.advsustainsys.com
Supporting Information
Supporting Information is available from the Wiley Online Library or from
the author.
Acknowledgements
This work was supported by OCP Group and the University of Mohamed
VI Polytechnique (UM6P) through Specific Agreement number 2.
Conflict of Interest
The authors declare no conflict of interest.
Keywords
agriculture, controlled release fertilizers, cost-effective coatings, stimuli-
responsive hydrogels
Received: April 11, 2023
Revised: June 3, 2023
Published online: July 9, 2023
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Houssameddine Mansouri is currently pursuing his Ph.D. at Mohammed VI Polytechnic University,
Morocco, alongside his visiting scholar role at the School of Sustainable Engineering and the Built
Environment, Arizona State University. His work aims to improve nutrient use efficiency by crops and
advance sustainable agriculture through the development of new P-based nanofertilizers and the
study of the complex dynamics between roots and nanoparticles in soil. Houssam holds an engineer-
ing degree from Hassan II Agronomy and Veterinary Medicine Institute, graduating in 2019.
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Hamid Ait Said obtained his Ph.D. from the Faculty of Sciences Semlalia at Cadi Ayyad University in
2021. He currently holds a position as a postdoctoral researcher in the Department of High Through-
put Multidisciplinary Research at Mohammed VI Polytechnic University. His research encompasses
a wide range of interests, with a particular focus on the development of calcium phosphate for bone
regeneration. Moreover, Dr. Ait Said is actively engaged in the development of advanced inorganic and
composite materials for agricultural purposes, specifically in the context of slow-release fertilizers.
Hassan Noukrati is currently an assistant professor at the Faculty of Medical Sciences at Mohammed
VI Polytechnic University. He holds a Ph.D. in Materials Science and Engineering from the National
Polytechnic Institute of Toulouse in France and Cadi Ayyad University in Morocco. His research inter-
ests focus on the development of innovative biomaterials based on calcium phosphates, bioactive
glasses, and biomolecules (biopolymers and drugs) for application in bone regeneration, cartilage
repair, and drug delivery.
Abdallah Oukarroum is a plant physiologist with extensive experience in environmental plant biol-
ogy. He realized his Ph.D. at the University of Geneva, Switzerland, studying the alteration of the pho-
tosynthetic apparatus of plants during environmental stress caused by drought, salt, and heat. As a
post-doc and researcher/lecturer at the University of Quebec in Montreal, Canada, Dr. Oukarroum
studied the cellular level inhibitory effects of metals and nanomaterials in aquatic plants. Currently, Dr.
Oukarroum is a full professor at the University Mohammed VI Polytechnic in Morocco. His research
mainly focuses on the physiological and biochemical responses of plant to abiotic stresses.
Hicham Ben youcef is currently an associate professor at Mohammed VI Polytechnic University.
He holds a Ph.D. in chemistry and materials science from the Swiss Federal Institute of Technology
Zurich, Switzerland. His main research interests focus on the development of smart and nano mate-
rials for different applications, such as agriculture, membrane technology, sensors, coatings, energy
storage, and conversion toward competitive targets (performance, durability, and cost). He is cur-
rently leading the High Throughput Multidisciplinary Research Laboratory.
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François Perreault is a professor in the Department of Chemistry at the University of Quebec in Mon-
treal and an adjunct professor in the School of Sustainable Engineering and the Built Environment
at Arizona State University. He completed his Ph.D. in environmental sciences at the University of
Quebec in Montreal and was a NSERC postdoctoral fellow in the Department of Chemical and Envi-
ronmental Engineering at Yale University before starting as an assistant and then associate professor
at Arizona State University. His research explores the interface between materials and biological sys-
tems, with a focus on environmental nanotechnology, toxicology, and water quality.
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http://www.advancedsciencenews.com
http://www.advsustainsys.comof CRFs started in the 1960s and
were primarily focused on sulfur and polyethylene as coating ma-
terials. Since then, many polymer materials and natural coating
agents were explored, along with a wide range of advanced mate-
rials such as multifunctional super-absorbents or nanocomposite
structures.[9] In this section we will discuss the distinct categories
of CRF, present their synthesis methods, and highlight the differ-
ent mechanisms for the release of nutrients.
2.1. Classification of CRF
The different categories of CRF, as described by Shaviv et al.,[8]
are presented in Figure 2. These categories are: 1) low solubility
organic-N compounds, which can be further classified as either
biologically decomposing products like urea-formaldehyde (UF)
or chemically (mostly) degrading compounds like isobutyledene-
diurea, 2) low-solubility inorganic compounds, which are com-
monly found in forms such as metal ammonium phosphates
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Figure 2. Classification of CRF into low solubility organic and inorganic
compounds, and compounds with a physical barrier to control the release
of the core nutrient (coatings and matrices). Text in bold to highlight the
only categories covered in this review.
(MgNH4PO4) or partially acidulated phosphates rock, and 3) CRF
with a physical barrier that controls the release, which can be in
the form of a continuous matrix where the soluble active ingredi-
ent is spread, or nutrient granules surrounded by a hydrophobic
polymer.
Compared to low-solubility macronutrient formulations,
which simply slows down the release kinetic to increase their
retention in soils, using an external barrier to control the release
also provides an opportunity to develop functional coatings
that control the release within the specific bounds that match
the plant’s nutritional needs.[4] Therefore, this review will be
focusing on CRF materials using a physical barrier, a matrix, or
an external coating to control the release. Coated CRF can be
divided into two subgroups: inorganic, mineral-based coatings
such as sulfur and organic coatings using polymers, thermoplas-
tics, or resins.[4] These coatings can be either hydrophobic, such
as polyolefin or rubber, or gel-forming polymers, also known as
hydrogels.[9] The nature of the physical barrier determines how
nutrients will be released from the CRF and therefore dictates
its potential applications in the field.
2.2. Mechanisms of Release of CRF
Each CRF granule has a distinct three-stage sigmoidal and non-
linear release profile (Figure 3); a lag phase where there is no
evidence of release, a stage of continuous release, and a decay
phase in which the release rate declines considerably. In the first
step, soil water, primarily as vapor, wets the coating, resulting
in the formation of fissures as the water penetrates to the core
and allows the fertilizer to dissolve. No fertilizer is discharged at
this stage due to the vapor pressure differential. The lag may be
caused by the time needed to fill the internal voids with the nec-
essary volume of water or by a steady state between water intake
and solute exiting. In the following step, as water keeps entering,
more material fertilizer is dissolved and the osmotic pressure in
the core increases, allowing fertilizers to be gradually released
through gaps in the polymer coating (Figure 4). Diffusion of nu-
trients to the soil is maintained if the granule’s solution remains
saturated. Once the osmotic pressure surpasses a set level, the
coating layer collapses and the fertilizer bursts out. In the decay
point, most nutrients have been dissolved and released, which
reduces the osmotic equilibrium and thus the release rate.[10]
More specifically, in the case of coated CRF, a multi-stage dif-
fusion model was suggested.[11] After spreading coated fertilizers
in the soil, irrigation water penetrates the coating and condenses
on the solid fertilizer core, causing the dissolution of nutrients.
As osmotic pressure rises, the granule swells and exhibits two
reactions. When osmotic pressure exceeds the resistance of vul-
nerable coatings (e.g., sulfur or modified sulfur), the coating will
burst and the core nutrient immediately liberated. On the other
hand, if the layer resists the increasing pressure, the core fertil-
izer is gradually released, driven by a concentration or pressure
gradient, or a combination of both.[11] The solubility and availabil-
ity of the nutrient in the soil determine the speed at which fertil-
izers will diffuse through the polymer layer and into the soil. The
rate of dissolution of the nutrient is controlled by the concentra-
tion difference between the soil and the CRF’s core.[12] Microor-
ganisms can also play an important role in the fertilizer release
process for several CRFs. Microorganisms in the soil can produce
enzymes or acids that degrade the polymeric coating layer, gradu-
ally releasing the nutrients into the soil.[13] Therefore, the coating
layer must be strong enough to prevent the core from expanding
because of absorbing water and be able to resist microbial degra-
dation in order to maximize the longevity of the fertilizer granule
inside the soil.
3. Coated CRF: Definition, Coating Techniques, and
Cost-Effective Coating Practices
Coated CRFs are conventional soluble fertilizer products that, af-
ter granulation, prilling, or crystallization are provided a water-
insoluble covering layer to regulate moisture absorption and
hence solubilization rate and nutrient release.[7] Coating mate-
rials may be broken down into two groups: inorganic materials
like sulfur, bentonite, or phosphogypsum; and organic polymers,
which can be either synthetic polymers such as polyurethane,
polyethylene, and alkyd resin or natural polymers, such as starch,
chitosan, or cellulose.[10]
However, sulfur coated CRF can cause soil acidity if sulfur is
converted into sulfuric acid after application.[14] Moreover, while
CRF created from natural materials decomposes rapidly in the
soil, CRF made from synthetic materials such as polyurethane
and polyethylene generate volatile organic compounds and have
a detrimental effect on the soil as a result.[15] This kind of poly-
mer coatings can also break down into microplastics over its
lifetime, generating another contaminant of emerging concern
due to their persistence. Therefore, although the use of CRF has
many positive effects in agriculture, such as lower fertilizer use
and decreased nutrient pollution, the choice of the material must
be made carefully as to not impact negatively the soil quality over
time.[16]
3.1. Coating Techniques
Polymer-coated CRF can be produced using several types of com-
mercially available coating processes. Examples of such tech-
niques are the rotating drum, pan coater, and fluidized bed coater
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www.advancedsciencenews.com www.advsustainsys.comFigure 3. Schematic representation of the temporal development of soil nutrient availability, as supplied by CRF and conventional fertilizers. As a result
of their burst release, the conventional fertilizers release most of the nutrients during the first growth stages of the plant. The CRF release follows a
sigmoidal curve involving three phases: an initial lag period, followed by a linear release, and last, a decay release phase.
(Table S1, Supporting Information). Rotary pan or drum coat-
ing is one of the most common techniques used to make com-
mercial CRF due to its simplicity: the rotary pan technique in-
volves rotating a pan or a drum containing fertilizer granules
and spraying them with a polymer solution using nozzles, which
will form a coating upon drying. Warm air can be blown over
the wet coatings to reduce the drying time. This simple process
has been in use for producing CRF commercially; however, it is
known to have a low coating efficiency and, as a result, a lot of
coating material is wasted to obtain a good film on the surface
of the granules.[17] To reduce solution losses during the coating
process, CRF can be produced using a fluidized bed technique.
Since fluidization allows for precise regulation and adaptability,
it improves the efficiency of the operation. Spraying the coating
material into a supporting air stream causes the coating material
to create a fluidized bed. Once the fertilizer is added, the coat-
ing substance covers the granule, resulting in a new compound
with gradual nutrient release.[18] A schematic representation of
the main coating processes used for CRF production is shown in
Figure 5.
3.2. Cost-Effective Coating Materials and Practices: Making
Innovation Affordable Again
Industry is increasingly focusing on new ways to augment the
productivity of agricultural systems while decreasing production
costs, which has led to the adoption of novel alternative materi-
als in agriculture.[19] Agricultural fertilizers are being used on a
massive scale, which provides a strong financial reason for inno-
vative, cost-effective formulations to be developed. For large scale
applicability, it is important to use coating materials that are easy
to apply, inexpensive, and environmentally benign to ensure the
broad use of coated CRF.
3.2.1. Economic Benefits of Coated CRF
Coated CRF are designed to ensure that the late nutrient release
is matching in time with the nutrient demands of crops,[18] mak-
ing a single application able to meet the nutrient requirements
for the entire season. That way, the use of CRF can reduce spread-
ing costs by allowing for an earlier application that avoids the “an-
nual spring rush,” when access to the field is challenging. Like-
wise, single basal application of CRF can lower the demand for
additional short-season top dressing applications during critical
periods, such as for rice paddies, which reduces labor expenses.[8]
Since financial benefit is the main driver for farmers when se-
lecting a management practice, Zhang et al. demonstrated that
a single application of blending urea (BU) which is composed of
both coated controlled-release urea and uncoated common urea
saved half of the labor expenses, which balanced the higher fertil-
izer N prices. Thus, the farmer’s financial earnings were not af-
fected by the implementation of BU and could potentially been
in fact improved.[20] Through a 2 years field trials using aromatic
brown rice, Shivay et al., showed that 0.5% boron-coated urea,
5.0% sulfur-coated urea, and 2.5% zinc-coated urea provide bet-
ter yields than non-coated prilled urea.[21] These treatments max-
imized returns and benefit–cost ratios, as shown in the Figure 6.
However, despite these proven benefits of CRF fertilizers, they
are not commonly used in Europe outside of gardening and for
the fertilization of lawns and ornamental plants.[18]
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Figure 4. Schematic representation of the different processes through which CRF releases nutrients. A) The coating layer failed, owing to the osmotic
pressure induced by water penetration exceeding its pressure resistance threshold, resulting in a burst release. B) Gradual release of nutrients from the
core through the expanding pores caused by the water penetrating the coating, resulting in increased internal osmotic pressure. C) Water absorption by
the hydrogel coating, which swells to slowly release the dissolved nutrients under osmotic pressure (deswelling).
Figure 5. Schematic illustration of the two most common coating techniques. A) The rotary pan/drum; B) The fluidized bed process used to form
polymeric coatings on granular fertilizers. The movement of the air current causes the grains to swirl and hover around. A nozzle is used to coat fertilizer
granules by spraying coating material over them, producing coated CRFs.
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Figure 6. Costs of coating prilled urea with borax, sulfur, and zinc, as well as an economic assessment of net return and benefit:cost ratio per fertilizer
treatment for a 1 hectare aromatic rice crop (Shivay et al. 2019) *At the rate of 7% of fertilizer prices (USD ha−1). Prevailing prices of fertilizer materials
during 2013–14: i) Borax (12% boron) at the rate of 2.0 USD kg−1, ii) sulfur dust (90% S) at the rate of 0.50 USD kg−1, iii) ZnSO4·7H2O (20% Zn) at
the rate of 0.50 USD kg−1; iv) ZnO (80% Zn) at the rate of 1.49 USD kg−1, v) prilled urea at the rate of 0.087 USD kg−1 (Note: Prilled urea at the rate of
883 USD tonne−1; 24.97 USD for an application of 130 kg ha−1).
3.2.2. CRF Production Cost Factors
The higher production costs of CRF compared to conventional
fertilizers is one of the most important factors that limit their
widespread adoption. Because of the prices of coating materi-
als and the inefficiency in the coating practices, the production
costs of fertilizers may be increased by 10 to 30 times when or-
ganic polymer coatings are added.[7] However, improvements in
the CRF coating processes could help mitigate this higher pro-
duction cost. Production methods for CRF can be divided into
chemical and physical techniques with the latter being the most
used today because they are simpler and less expensive to op-
erate. However, physical coating methods use higher amount of
raw coating material in order to obtain homogeneous coating lay-
ers on the fertilizer granules, which raises the overall production
cost.[17] An important aspect of the cost efficiency of CRF coating
is therefore maximizing the use of coating material. Techniques
such as fluidized bed coating can help minimize solution loss
and make the physical coating method more affordable, but this
approach usually has a high capital cost and is not ideal for gran-
ules of increasing sizes.[10] Production size also matters for the
CRF costs. Forthe moment, CRF production is often realized in
small batches (1000 to 5000 kg) with select raw granular mate-
rial sizes to ensure optimal coating quality, which increases the
production costs.[7] This cost is likely to be mitigated as CRF be-
comes more abundant in the global market and production vol-
umes increased.
This cost reduction with scale-up can also be expected for
chemical coating techniques, which for CRF is mostly used to
make hydrogel-based coatings for controlled release or stimuli-
responsive fertilizers. For chemical coatings, the use of organic
solvents during hydrogel synthesis can be an important aspect
of the synthesis’ costs. Reusability of the solvent should there-
fore be an important selection criterion for hydrogel-based CRF.
The polymerization step is a critical component for this, since for
some hydrogel synthesis, like solution polymerization synthesis,
the solvent cannot be recovered but, for other approaches such
as inverse suspension polymerization, which relies on perform-
ing the polymerization reaction in monomer droplets dispersed
in a non-miscible medium such as water or oil, the solvent can be
recovered and purified for reuse.[10] The use of inexpensive, bio-
based raw materials (discussed in the next section) as well as in-
novative green chemistry techniques to avoid organic solvents[22]
are potential avenues of development to improve the affordability
of hydrogel-based CRF coatings.
3.2.3. Cost-Effective Coating Materials and Production Methods
Fertilizers can be coated with a variety of materials to obtain con-
trolled release properties. The choice of coating material, along
with the production method, will be determining factors to CRF
costs and eventual marketability. Moreover, using bio-based and
biodegradable coatings is becoming an increasing area of interest
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Table 1. Overview of cost-effective coating materials and techniques.
Coating Material Cost-effective technique Research findings Ref.
Castor oil-based
polyurethane
Water polishing • Water polishing DAP grains lowered the angle of repose by 2.48–10.57% and the
specific surface area by 5.70–48.76%.
• The release rate of water polished coated DAP granules is 5.36 times slower.
[29]
Castor oil-based
polyurethane
Wax-based surface modification • Wax surface modification lowered DAP particle surface area and repose angle.
• Prior research used 9 wt% coating material to release P for 25 days, while
wax-surface–modified coated DAP granules released P for 93.4 days using 4 wt%
coating material.
[31]
Cassava starch modified
with di-sodium
tetraborate
Heated pre-coat solution • N release time from uncoated urea granules in water was 63.33 s, rising with
pre-coat solution temperature from 50 to 80 °C, reaching 209 s at 80 °C.
[32]
Recycled polystyrene
foam
Large tablets size • Same release rate as the commercial polymer coated urea (PCU) with 70–80%
less coating material.
• Coating large urea tablets with recycled polystyrene foam is 7–8 times cheaper
than commercial PCU.
[30]
Recovered lignin from
industrial wastewater
• Lignin retrieval from Kraft and
Sulfite black liquors
• Acetylation of lignin using acetic
acid and sodium metabisulfite
• Acetylated lignin-coated urea granules released 36.3% for Kraft lignin and 45.3%
for Sulfite lignin after 7 days, while commercial sulfur-coated urea released 59%.
[27]
Biochar Microwave irradiation • Biochar-hydrogel–coated urea released 20.03% N after 30 days.
• 40% cost reduction at lab-scale compared to conventional polymerization.
[24]
Vegetable Oil polymer
coating
Oil-based polyurethane films • After 70 days, 80% of the fertilizer content is released when using an Oil–PCU
film with an isocyanate index of 1.58
• After 180 days, the degradation percentage of five polyurethane films with
different isocyanate indices ranged from 10.23% to 29.63%
[26]
for fertilizer production since, in addition to regulating nutrient
release, there is a need to cut down on the emission of potentially
hazardous chemicals during CRF production and use.[23] Table 1
provides an overview of novel potentially cost-effective materials
used for the synthesis of CRF coatings.
Residue materials from industries like agriculture are a
promising source of low-cost materials to explore given the scale
of agricultural production. For example, biochar, as a bio-waste,
is an interesting candidate due to its low cost and widespread
availability. Chemical synthesis of biochar using microwave irra-
diation was recently introduced as a rapid, high-efficiency, low-
energy, and low-pollution bulk heating option to create biochar–
hydrogel composites.[24,25] Microwave irradiation, unlike tradi-
tional polymerization synthesis, does not involve the use of or-
ganic solvents or any other byproducts. Using urea coated with a
biochar–hydrogel composite, only 20.03% of N was released af-
ter 30 days. Another type of agricultural materials usable for CRF
synthesis are oils, as demonstrated by Feng et al., who prepared
a fully vegetable oil–based polyols using epoxidized soybean oil
and oleic acid as raw materials. These polyols were then used to
form polyurethane films for CRF coatings.[26] Based on the iso-
cyanate index of the polyurethane, coating degradation varied be-
tween 10% and 30% after 180 days in soil. When used to coat urea
fertilizers, the resulting CRF took up to 70 days to release 80%
of its cumulative capacity, showing that these oil–polyurethane
coating have good prospects for controlled nutrient release. In
addition to these good performances of CRFs derived from
agricultural waste, the opportunity to valorize the waste back
into the agricultural cycle is interesting for a circular economy
perspective.
The pulp and paper Industry are another large source of use-
ful waste in the form of lignin. Behin and Sadeghi experimented
with modified lignin as a coating for urea fertilizer particles. Kraft
and sulfite black liquors, as sources of sulfate and sulfite lignin,
were modified using an acetylation reaction to improve their hy-
drophobicity. Then, by coating urea granules with the acetylated
lignin, a release of 36.3% and 45.3% after 7 days for the Kraft
and sulfite lignin, respectively, compared to a release of 59% for
sulfur coated urea.[27] These improved rates suggest that recov-
ered lignin from industrial wastewater can be used to achieve
good performance in CRF. Waste polymers can also be recycled
into new coating materials for fertilizers. For example, Yang et al.
used recycled polystyrene foam, either as is or blended with wax
or polyurethane, as a coating material for the coating of urea gran-
ules. This approach was estimated to not only reduce the cost of
urea coating compared to commercial polymer coated urea but
also provide a novel solution to the recycling of plastic products.
Beyond the nature of the coating material, coating costs can
be reduced by reducing the amount of coating needed for con-
trolled release. A study by Tian et al. showed a surface modi-
fication technique based on abrasive particles that reduced the
micro/nanoscale surface morphology of urea particles and im-
proved the coating adherence and uniformity. By reducing the
surface roughness of the urea particles, the amount of coating
material needed to produce coated urea CRF was reduced by up
to 28.6% for an equal duration of release.[28] Water polishingis
another way to improve to reduce the roughness of fertilizer parti-
cles and therefore reduce the amount of coating material needed.
Lu et al. used water polishing to treat the surface of diammo-
nium phosphate (DAP) granules, which decreased their specific
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surface area by 5.70–48.76% (based on the water % used) and
therefore reduced the amount of surface to cover with the coat-
ing material. When applied to maize crops, the fertilizer granules
coated with a castor oil–based polyurethane after water polishing
had a release rate 5.36 times slower than non-polished coated
granules.[29] In a similar approach, using large fertilizer tablets
also reduces the specific surface area of the fertilizer material,
resulting in less coatings needed. Large urea tablet (12 mm in
diameter and 8.5 mm thick) had the same release rate as a com-
mercial polymer coated urea particles (diameter of 0.5–2 mm)
but used 70% to 80% less coating material.[30] Substantial sav-
ings can therefore be gained simply by better understanding and
optimizing the amount of coating needed for CRF fabrication.
4. Matrices CRF: Definition, and Loading
Techniques
Water-soluble fertilizers with physical barriers that regulate nu-
trient release take the form of a matrix of active fertilizer nutrients
spread throughout a coating material that impedes fertilizer dis-
solution. Both hydrophobic matrices, like polyolefin and rubber,
and gel-forming hydrophilic matrices, commonly referred to as
hydrogels, can be used as a controlled release matrix.[9]
4.1. Hydrophilic Matrices: Hydrogels CRF
Hydrogels are a form of cross-linked polymer with hydrophilic
groups attached to the polymeric backbone that can absorb sig-
nificant amounts of water without dissolving.[33] By itself, this
high-water adsorption capacity is attractive for agriculture appli-
cations, knowing that farmers in arid and desert places across
the world have limited access to water during the dry season but,
at the same time, considerable precipitation is lost during this
period.[34] Therefore, beyond their use to control nutrient release,
hydrogels may also decrease water losses due to evaporation and
reduce irrigation periodicity.[35] In the design of CRF, hydrogels
are used for their high water absorption capacity, high crosslink-
ing gel fraction, low toxicity, affordability, outstanding swelling
and storage stability, and the ability to rewater after drying.[36]
Moreover, the hydrogel is resistant to physical damage or other
stress that would damage most coatings and result in the unde-
sirable burst release of nutrients.[37] Research on using hydrogels
as CRFs is summarized in the Table 2 below.
4.1.1. Cross-Linking Methods of Hydrogels
Because hydrogels are usually derived from water soluble com-
pounds, cross-linking the hydrogel matrix is essential to preserve
its structure as a coating when used in the field. The addition
of cross-links between polymer chains influences the material’s
physical characteristics, the nature of which is determined by the
degree of cross-linking as well as the crystallinity of the poly-
mer. Several cross-linking techniques are available to be used for
hydrogel formation, such as electrostatic, thermal, or chemical
cross-linking.
Hydrogels can be cross-linked electrostatically through ionic
interactions, complex coacervation, or hydrogen bonding.[45]
Cross-linking reactions using ionic interactions or complex coac-
ervation are based on molecules or polyelectrolytes with oppos-
ing charges. Ionic cross-linking is achieved by the introduction of
counterions to initiate an in situ gelation reaction, which will be
influenced by factors such as ionic strength, pH, type of coun-
terion, and functional charge density of the solution. Alginate
is a common example of an ionic cross-linked hydrogel when
it is mixed with divalent cations.[46] On the other hand, com-
plex coacervation is the rapid aggregation of two polyelectrolytes
with opposing charges, resulting in a liquid–liquid phase sepa-
ration in an aqueous medium.[47] Some examples of this type
of physical hydrogels are those made by coacervating chitosan
and xanthan.[21] Polymers of the same charge can also be cross-
linked through hydrogen bonding; for example, poly(acrylic acid)
and poly(methacrylic acid) can make stable hydrogels in the pres-
ence of poly(ethylene glycol) by creating hydrogen bonds between
the carboxyl group (─COOH) of acrylic acids and the oxygenated
groups of poly(ethylene glycol).[48] Hydrogen bonding for cross-
linking is of particular interest for hydrogels that possess self-
healing properties due to the reversible nature of the bonds as
the microenvironment of the polymer changes.[49]
Thermally crosslinked hydrogels form after cooling a heated
biopolymer solution. During cooling, an interchain connection
is made when two or more molecules align (whether in parallel
or perpendicular to the axis of the chain length), which cross-
links the biopolymer into a hydrogel. The polysaccharides agar
and carrageenan are two common examples of thermally cross-
linked hydrogels.[50] A similar process occurs during the crys-
tallization process generated by freeze–thaw cycles. During the
freeze–thaw process, the crystallization maximizes the density of
polymer by decreasing the chain gap in the polymer, which al-
lows the chains to align and crosslink with one another to create
a network structure. After going through a series of freeze–thaw
cycles, the hydrogel formed ends up with a porous morphology
due to the voids left by the melting particles. Variations in poly-
mer solution, frequency of freeze–thaw rounds, freezing interval,
and freezing degree all affect the freeze–thawed hydrogel’s me-
chanical characteristics.[51] The cryogelation of xanthan is an ex-
ample of an hydrogel generated by freeze–thaw cycling.[52] These
thermal cross-linking benefits from not using any added chem-
icals; however, the energy required for the temperature change
may be prohibitive unless heating is already required to make
the hydrogel solution.
Chemical cross-linking relies on covalent interaction between
polymer chains to create permanent chemical hydrogels.[53] Dif-
ferent chemical cross-linking techniques have been used in the
literature, including functional groups crosslinking, free radi-
cal polymerization, irradiation induced crosslinking, and enzyme
mediated crosslinking.[48] For CRF synthesis, the primary meth-
ods used have been based on free radical polymerization synthe-
sis and functional group cross-linking. Free radical polymeriza-
tion is based on the formation of a free radical site by an initia-
tor, which results in the accumulation of monomers in a chain-
like pattern.[54] Free radical polymerization can be triggered by
a variety of initiators, such as chemical redox reagents, ther-
mal decomposition, electro- or sono-chemical activation, ioniza-
tion radiation, or photopolymerization with visible or ultravio-
let light. The type of initiator to be used depends on the types
of monomers to be cross-linked as well as the properties of the
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Table 2. Overview of hydrogel-based CRF studies.
Polymer Active
ingredient
Maximum
swelling
capacity
Release time Plant Agronomic tests findings Ref.
Gelatin Ammonium
nitrate
(AN)
218% 50 h in water Cucumber • Hydrogels CRF increased seed
germination from 70% to 100%, and
enhanced stem elongation by 42%.
[38]
Tomato • Hydrogels CRF increased seed
germination from 76% to 94%, and
enhanced stem elongation by 41%.
Wheat straw Urea 862 g g−1 16.3%, 60.2%, and 78.5% within
1, 5, and 10 days in soil
– – [39]
corncob-g-poly(acrylic
acid)/bentonite
Urea 1156 g g−1 56.6% was released after 30 days
in soil
Cotton • Hydrogels CRF increased cotton seedlings
plant height, root length, fresh weight, and
dry weight by 37.76, 33.48, 44.07, and
64.29%, respectively, compared to
commercial urea.
• Hydrogels CRF increased cotton
germination rate to 86.67% compared to
commercial urea (66.67%).
[40]
Hydroxy propyl methyl
cellulose, PVA, glycerol
and blended paper
Urea 15.6 g g−1 87.01% release in soil in 44 days – – [41]
Double hydrogel network
of sodium alginate
and polymerization of
𝛽-cyclodextrin, acrylic
acid, and acrylamide
in the presence of
halloysite
Urea 107.9 g g−1 45.6%, 73%, and 87.8% within
0.5, 2, and 4 h in water.
39.8%, 68.7%, and 79.5% within
0.5, 2, and 4 days in soil.
– – [42]
NaCMC and
carrageenan (CG)
Zinc 635% for
CG and
110% for
NaCMC
50.4% and 20.5% on the third
day, 71.4% and 38.7% on the
fifth day, and 96.1% and 66.7%
on the eight day in soil for CG
and CMC hydrogels,
respectively
Wheatgrass • Zinc-loaded NaCMC and CG hydrogels
increase wheatgrass plant height,
germination rate, fresh weight, and dry
weight.
[43]
Acrylic acid Urea 909 g g−1 0.18, 0.37, 0.58, 0.81, 1.06, 1.59,
2.67, and 3.71 wt % within 1, 3,
5, 7, 10, 20, 30, and 40 days in
water, respectively.
Maize • Low application rate of the hydrogel
(hydrogels.
stimuli-responsive nanomaterials with the goal of using them as
drug delivery or diagnostic carriers. These synthetic systems can
enable localized drug delivery by inducing a wide range of reac-
tions via a series of intrinsic or extrinsic triggers, which allows
for a reduced amount of drug needed and a higher treatment
efficiency.[60] The same objectives are shared by the fertilizer in-
dustry, where the ability to control the delivery of nutrients can
reduce phytotoxicity, leaching losses, volatilization, drift, and soil
degradation, as well as enable greater safety during application.
Regulating the release of active substances using passive coatings
is challenging because of their reliance on diffusion, capsule ero-
sion, or osmotic pressure. Coatings that actively respond to minor
signals or variations in the surrounding environment by altering
their physicochemical characteristics to promote the release of
loaded chemicals are, however, one way to achieve this controlled
release.[61] Therefore, inspired by the advance and breakthroughs
in the stimuli-responsive administration of therapeutic and diag-
nostic ingredients, more functional fertilizer coatings have been
explored for the targeted delivery of agrochemicals to plant roots.
Hydrogels are a common polymeric system for the design of
stimuli-responsive materials. They can be engineered to have
triggered reactions, such as compression or expansion, in re-
sponse to changes in their surrounding environment (Figure 8).
Variations in temperature, electric or magnetic fields, light, pres-
sure, and sound are all examples of physical stimuli, whereas
chemical stimuli include changes in pH, solvent composition,
ionic strength, and specific molecular species. The hydrogel’s
response to changes in its external environment may cause
swelling or de-swelling of such a magnitude that the event is
known as volume collapse or phase transition.[33]
The release of nutrients from hydrogel CRF occurs rapidly
upon moisture exposure, with particle swelling and diffusion
mechanisms instantly occurring.[62] Therefore, the design objec-
tive of stimuli-responsive systems is to match that release to a
trigger associated with the need of the plant. Stimuli-responsive
materials have been explored for agrochemical release in reac-
tion to pH, temperature, redox conditions, enzymes, and light.[63]
A variation in these parameters can be found in soils as part of
environmental perturbations or as a result of the plant to these
perturbations.[64] For example, pH variations in the rhizosphere
can be caused by H+ or OH− release by roots to adjust for imbal-
anced cation–anion absorption at the soil–root interface. Roots
exudation and respiration increase CO2 concentration in the rhi-
zosphere, forming carbonic acid that dissociates in neutral to al-
kaline soils and decreases the pH near the plant, which alters nu-
trient and toxic element bioavailability. Redox-coupled reactions
by plant roots and microorganisms can also modify the pH of
the rhizosphere. Table 3 compiles some examples of the pH dif-
ferential between the bulk soil and the plant’s rhizosphere. While
these changes are very species- and soil type-dependent, the pres-
ence of a significant pH difference between the root system and
the soil can provide a target pH window to engineer the release
of nutrients in the roots’ vicinity, which would maximize nutri-
ent uptake by the plant. The following section provides informa-
tion on the efforts to tailor the nutrient release to specific stimuli,
which can inform on potential strategies to target specific physi-
ological triggers associated with the plant’s nutritional needs.
4.2.1. pH Responsive Hydrogels
When a polymer possesses acidic or basic groups that can give or
accept protons in reaction to variations in pH, it is considered pH
responsive. The ionizable functional groups of pH-sensitive poly-
mers are utilized to produce pH-responsive hydrogels that either
receive or release protons in response to environmental changes.
This causes the hydrogel to swell in an aqueous solution.[57] If the
solvent pH is adjusted, an ion concentration gradient between
within and outside the gel is created, which allows mobile ions
to enter and exit the hydrogel and increase the osmotic pressure
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Table 3. Root-induced change of pH between bulk soil and rhizosphere.
Plant Soil type pH Root distance
[mm]
Days after sowing Ref.
Bulk soil Rhizosphere
Alpine pennycress Loam 5.6 7.0 – 6–8 weeks [65]
Silt loam 7.5 7.9
Maize Loam 5.6 5.5 8–9 days
Silt loam 7.4 7.5
Loam 6.2 6.5
Ryegrass Silt loam 7.4 7.9 4–5 weeks
Loam 6.2 7.7
Luvisol 6.8 4.4 0 10 days [66]
≈ 5 1
≈ 6 2
≈ 6.5 3
Rice Ultisol 5.9 ≈ 4.2 0 6 weeks [67]
≈ 5 2
≈ 5.5 4
Haplaquoll 7.3 7.1 0 12 days [68]
7.2 5
7.24 10
Durum wheat 4.7 7.4 – 4 weeks
(1 week after
transplantation)
[69]
Millet – 4.4 5.6 – 75 days [70]
Sorghum 5.2 5.5
Cowpea 4.5 5.6
Groundnut 5.2 5.2
on the surface. The osmotic pressure causes the hydrogel’s vol-
ume to change.[45]
A variety of low-cost natural polymers and biomaterials have
been explored to obtain stimuli responsive CRF. For example, us-
ing in situ free-radical graft copolymerization, sodium alginate,
acrylic acid, and acrylamide were combined with rice husk ash
(RHA) and fertilizers to obtain a pH-responsive NPK formula-
tion. The authors showed that as pH rises from 2 to 6, the equi-
librium water absorbency of this composite also increased, reach-
ing its highest value at pH 6. The swelling ratio was also found
to be steady between pH 5 and 7 while it decreased with the pH
increasing above 7.[71] Chitosan is another widely used natural
pH sensitive polymer because of the number of amino groups
among its chain, which makes it readily soluble at acidic pH
but insoluble at alkaline pH levels.[72] Using in situ hydrogela-
tion of chitosan with salicylaldehyde in the presence of urea, If-
time et al. developed a pH responsive hydrogel that maintained
its structural integrity for up to 15 days at a pH >6, with en-
hanced stability with increasing pH.[73] Sodium carboxymethyl
cellulose (CMC), a water-soluble anionic cellulose derivative, is
also frequently used to form hydrogel, particularly in the form of
gel beads.[74] To obtain a pH-responsive hydrogel from CMC for
the release of NPK, Olad et al. grafted sulfonated carboxymethyl
cellulose with acrylic acid in the presence of polyvinypyrrolidone
and silica nanoparticles.[75] This hydrogel material was shown to
swell at pH = 8 due to anion–anion electrostatic repulsion but
shrink at pH = 2 due to the protonation of carboxylate groups.
Additionally, this hydrogel responsive to salinity, with samples
swelling in denatured water and shrinking in a 0.1 m NaCl solu-
tion. A composite CMC material made with monocalcium phos-
phate and zeolites was also developed by Singh et al. for P release
in soils.[76] The composite hydrogel possessed a “burst release” in
neutral pH but a delayed diffusion–driven release in acidic (pH
= 4.2) conditions, emphasizing the pH sensitive release mecha-
nism for this material. While production is constrained by high
fixation rates and low phosphate utilization efficiency, this release
profile is of particular significance for P delivery in the rhizo-
sphere of acidic soils.
Hydrogels can be relatively fragile structures, especially in
their swollen configuration.Therefore, several groups have com-
bined hydrogels with cellulose nanofibers (CNF), which are typi-
cally characterized by high strength and stiffness, to balance the
properties of the hydrogel.[77] The CNF also have moderate ther-
mal expansion, high crystallinity, and hydrophilicity, and an eas-
ily adjustable surface chemistry, all of which can be of interest
for CRF synthesis.[77] Shaghaleh et al. successfully synthesized
a novel N pH-responsive/sustained release fertilizer composite
based on wheat straw aminated-cellulose nanofibers and cationic
poly(acrylamide-co-2-aminoethyl methacrylate hydrochloride) by
direct AN fertilizer encapsulation.[78] While the results revealed
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that N release was lowest in a neutral pH medium, it was high-
est in a lower pH condition, where N supply was maintained for
90 days in a neutral-irrigated soil growing rice. In another study,
CNFs were mixed with SA and PVA, and then crosslinked in the
presence of NPK fertilizer.[79] This SA/CNF/PVA composite re-
tained more water in both deionized water and mixed soil, and
exhibited a pH-sensitive swelling behavior, with a rising trend
in equilibrium swelling ability from 3 to 6, stability from 6 to 8,
and a reduction at greater pH levels from 8 to 11. These studies
emphasized the potential of CNFs as a component for designing
advanced functional hydrogel fertilizers.
Combining the hydrophilic properties of acrylic polymers with
the biodegradable nature of starch-based composites can allow
for CRF materials that will not become persistent in the soil.[80]
However, care must be taken when altering the crosslinking of
starch-based hydrogels if biodegradability is the desirable prop-
erty, as extensive crosslinking and substitutions in starches can
reduce their biodegradability.[81] Dudu et al. synthesized a novel
N,N-dimethylacrylamide—maleic acid (MA)—Starch (St)-based
superabsorbent (DMSt1) and tested its abilities as a controlled
release fertilizer in lettuce growing. Dudu et al. used HCl/NaOH
to make negative and positive surface modifications of DMSt1,
labeled DMSt2 and DMSt3, respectively. The latter had the great-
est maximum swelling rate of 37.38% in deionized water envi-
ronment. The anionic and cationic characteristics of the DMSt1
changed and the hydrogel displayed varied swelling behaviors at
different pH levels after being treated with acid and base. The
DMSt1, DMSt2, and DMSt3-based hydrogels were also shown to
have excellent water uptake potential, with equilibrium swelling
values of 7163.7% at pH 10, 15708% at pH 10, and 27838%, re-
spectively, at pH 8. This water retention ability resulted, during
pot experiments with lettuce plants, in a decrease in membrane
damage index compared to the control sample, showing that the
hydrogels, in addition to nutrient release, can also prevent water
deprivation, in crops.[82]
Some possibilities for customizing nutrient release to the rhi-
zosphere microenvironment can be found by comparing the pH
window of stimulus response hydrogels to the pH gap between
bulk soil and plant’s rhizospheres. Several investigations have re-
ported hydrogels that are stable at near-neutral pH (about 6 to
8), but that expand more rapidly at lower pH levels.[71,73,79] These
pH ranges may coincide with the soil and rhizosphere conditions
seen in some crops (as noted in Table 3), indicating the poten-
tial usefulness of adopting stimuli-responsive techniques. Some
crops, however, have narrow pH windows between the bulk soil
and the roots, which may necessitate additional optimization of
the hydrogel chemistry. In addition, a better understanding of
the changes found in the rhizosphere needs to be provided be-
fore implementing such solutions for a specific crop, as the dif-
ferences between the bulk soil and the pH microenvironments of
the rhizosphere are likely to change with the season, soil chem-
istry, plant growth cycle, or physiological status.
4.2.2. Thermo-Responsive Hydrogels
Thermo-sensitive carrier systems have a lot of potential in agri-
culture since the temperature of the environment is always
changing. The physicochemical characteristics of the hydrogel
polymer can be made to vary in response to temperature and
release active ingredients.[61] Hydrophilic and hydrophobic do-
mains coexist within the structure of thermosensitive polymers.
As a result of temperature fluctuations in the interaction among
both hydrophilic and hydrophobic portions in the polymer and
water components, a sol–gel phase transition takes place in these
kinds of materials. The phase transitions of some polymers are
characterized by a lower critical solution temperature (LCST),
below which they are in a soluble and hydrated state but be-
comes hydrophobic and precipitates above this temperature (sol–
gel transition), while the phase transitions of certain other poly-
mers are characterized by an upper critical solution tempera-
ture (UCST), where the opposite transformation actually hap-
pens (gel-to-sol).[83] Polymer networks in temperature-sensitive
hydrogels show swelling responses that can be either positive,
where the gel material swells as temperature rises, or negative,
where the gel material contracts as temperature rises.[57] The
LCST or UCST of the hydrogels may be adjusted by tuning the
ratio of hydrophilic to hydrophobic segments.[45]
Hydrogels having an interpenetrating polymer network
(IPN) of polymer chains like polyacrylamide and PAA or
poly(acrylamide-co-butyl methacrylate) crosslinked with N,N′-
methlenebisacrylamide are among the few known instances
of UCST-type of polymers.[84] As an example,[85] examined an
innovative thermo-sensitive semi-IPNs formed by incorporat-
ing cellouronic acid sodium, which is derived from bagasse
pith, into a poly(acrylamide-co-diallyldimethylammonium chlo-
ride) (poly(AM-co-DAC)) network. These semi-IPN gels dis-
played UCST-type temperature sensitivity and excellent water-
absorbency. The UCST could be adjusted to higher temperatures
by increasing the ratio of H-bonds to zwitterionic pairs and the
total interchain connections between the two polymers. However,
when urea and other fertilizer salts (i.e., KH2PO4, NH4NO3, and
MgSO4) were introduced to the solution, the UCST was found
to decrease, emphasizing the need to consider the release envi-
ronment’s chemistry when designing the phase transition tem-
peratures of CRF. The LCST thermosensitive hydrogels were
also explored by Feng et al., who used surface-initiated atom-
transfer radical polymerization to produce a “smart” fertilizer
from poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA)
brushes grafted onto polydopamine-coated ammonium zinc
phosphate.[86] They showed that temperatures below the LCST
can increase the rate of nutrient release whereas temperatures
above the LCST decreased nutrient release. These findings sug-
gested that fertilizer made with PDMAEMA brushes can be de-
signed to possess controlled–release capabilities.
Polyurethane is another polymer that can be engineered to
manifest reversible changes in chemical and physical character-
istics upon exposure to temperature variations.[87–89] One exam-
ple of polyurethane used as a thermo-responsive coating mate-
rial for CRF application is the study by Qiao et al., who em-
ployed polycaprolactone (PCL) with varying molecular weights
(PCL500, PCL1000, PCL2000, and PCL3000) as raw materi-
als to produce six typesof polyurethane-based CRF coatings
by reacting polyether polyol (PPG)/PCL blends with methy-
lene diphenyl diisocyanate (MDI). Results demonstrated that
PPG/PCL2000-based PUCF blends can copolymerize with MDI
to generate a block copolymer with a two-phase structure (crys-
talline/amorphous). The CRFs coated with the block copolymer
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of PPG/PCL2000 at a molar ratio of 8 to 2 showed a sustained N
release time of more than 30 days. The N release rate increased
by 22.3 times per hour as the temperature rose from 32 to 33 °C,
revealing a clear thermo-responsive behavior.[90] These findings
emphasize the potential of using polyurethane to control nutrient
release based on the temperature environment, warranting fur-
ther investigation, alongside rigorous plant growth assessments,
to establish its benefit as a CRF coating material.
For temperature-responsive hydrogels, targeting the seasonal
shifts in soil temperature may be the main interest for the de-
sign of CRFs. Indeed, seasonal shifts in temperature are observ-
able in many parts of the world. The soil temperature shift will
also be depth dependent, being most pronounced in the soil’s up-
per layers. By adding CRF to the soil in the off-season, and wait-
ing for them to become active, as desired, nutrient release can be
timed to coincide with a certain growing season. As a result, this
stimuli-responsive CRF could be employed to cut down on costs
associated with human labor during peak, high-demand periods
of the growing season. For other stimuli-responsive CRFs, such
as those made from pH-responsive materials, it is necessary to
have a firm grasp on how soil properties evolve on a seasonal ba-
sis in a given region.
4.2.3. Other Stimuli-Responsive Hydrogels
Many chemical and biological processes in cells rely on en-
zymes, which are crucial biological catalysts. In terms of sub-
strate specificity and selectivity, enzyme-catalyzed reactions are
unmatched.[91] Making enzyme-responsive polymer complexes
by covalently linking enzymatic substrates to amphiphilic copoly-
mers is a typical approach. Since insects and their larvae have
and release enzymes, most of enzyme-responsive hydrogels
are being applied for the targeted delivery of insecticides. For
example,[92] created a new enzyme-responsive emamectin ben-
zoate microcapsule by cross-linking silica with carboxymethyl
cellulose using epichlorohydrin. The silica–epichlorohydrin–
carboxymethylcellulose microcapsules demonstrated outstand-
ing cellulase stimuli-responsive characteristics, maintained in-
secticidal activity against Myzus percae while protecting the ac-
tive ingredient, epichlorohydrin, against light- and temperature-
induced degradation. However, as of yet, enzyme responsive hy-
drogels have not been investigated for nutrients delivery in soils.
It has been shown that redox-responsive hydrogels can alter
their color, fluorescence reactivity, or chiral structure in response
to external redox reactions or electric field stimulus by shifting
the self-assembly phase of the gel component.[93] To regulate
the release of agrochemicals and capture heavy metal ions at
the same time, Hou et al. developed a redox-responsive hydro-
gel where cystamine was used as a crosslinker in a CMC matrix
to produce a flexible material. A decrease in agrochemical losses
following application to crops was attributed to the 3D network
generated by CMC during the crosslinking process. Toxic metal
leaching was decreased when redox-responsive hydrogels were
applied to soils contaminated with Cu2+ and Hg2+ by complex-
ing the metal ions through their thiol groups. Agrochemicals in-
corporated in hydrogels were delivered when the responsive net-
works were decomposed by the reducer.[94]
Products that can include two or more sensitive groups
(temperature and pH, redox and enzyme, enzyme and pH,
etc.) are called multi-responsive carriers.[93] Using salicylic
acid as a standard agrochemical, Hou et al. developed redox-
and enzyme-responsive macrospheres by self-assembling 𝛽-
cyclodextrin–modified zeolite and ferrocenecarboxylic acid–
grafted carboxymethyl cellulose. Salicylic acid liberation from
macrospheres was increased in the presence of hydrogen per-
oxide (oxidant) and cellulase (enzyme), with comparable release
rates of 85.2% and 80.4%, respectively, after 12 h, compared to
the control, non-responsive sample (12.6% salicylic acid release).
These results suggest that dual-responsive macrospheres may be
a useful carrier for the controlled release of agrochemicals.[95]
Table 4 provides an overview of the different materials
and approaches for the design of stimuli-responsive CRF or
agrochemical-delivery systems. As discussed above, pH- and
temperature-responsive hydrogels are the most actively explored
for fertilizer delivery, due to their clear relationship with soil
chemistry and its alteration by the plant’s metabolic activity. Com-
bining these types of CRFs with other functionalities to make
them responsive to multiple stimuli at the same time may offer
an opportunity to further refine the nutrient delivery and there-
fore avoid competition with the soil components in fertilizer use.
This is likely to result to an increase in production cost for these
more advanced stimuli-responsive CRFs, as their design become
more refined and precise. However, considering that several of
the materials proposed for the fabrication of stimuli-responsive
CRFs are natural materials such as cellulose, chitosan, or alginate
(see Table 4), there is a possibility to advance this next-generation
of CRFs in a way that mitigate the costs and environmental im-
pacts.
5. Conclusion and Future Outlooks
Implementing CRF has a lot of potential as a strategy to improve
NUE in agriculture. However, due to their greater production
costs, their broad application is still limited. The choice of coat-
ing materials and methodology for a cost-effective CRF produc-
tion remains to be defined; however significant progress in the
synthesis of novel CRF materials based on improved coating pro-
tocols or using low-cost, renewable coating materials have high-
lighted several promising avenues to improve the performance of
CRF and reduce their production costs. Furthermore, innovation
in this niche sector of CRF can also be attained by stimuli respon-
sive CRF, which not only enables passive controlled nutrient re-
lease via diffusion but also regulates this process to only occur in
response to certain environmental stimuli. This strategy allows
for greater control of fertilizer release to meet crop uptake. Re-
search in this area has begun to emerge for pH- and temperature-
responsive CRF; however, other stimuli more closely related to
the plant’s activity, such as enzyme-responsive hydrogels, have
not been explored yet. Finally, while CRF are generating a lot of
interest on their ability to reduce fertilizer use, improve nutrient
use by the plants, and reduce environmental impacts, more in-
depth financial and cost-benefit analyses are still needed to really
identify the type of crops or soil environments where the switch
from conventional fertilizers to CRF would be beneficial.
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