Annals of Agricultural Sciences 63 (2018) 163–172

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Annals of Agricultural Sciences

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Formulation of slow release NPK fertilizer (cellulose-graft-poly T

(acrylamide)/nano-hydroxyapatite/soluble fertilizer) composite and
evaluating its N mineralization potential
Kiplangat Ropa, , George N. Karukub, Damaris Mbuia, Immaculate Michiraa, Njagi Njomoa
⁎

a
Department of Chemistry, University of Nairobi. P.O. Box 30197-00100, Nairobi, Kenya
b
Department of Land Resource Management and Agricultural Technology, University of Nairobi. P.O. Box 29053-00625, Kangemi, Nairobi, Kenya

ARTICLE INFO ABSTRACT

Keywords: Polymer nano-composite fertilizer formulation has the potential to enhance nutrient use efficiency. Slow release
Nano-composite fertilizer (SRF) composite was formulated by incorporating nano-hydroxyapatite (nano-HA) and water soluble
Polymer hydrogel fertilizers (urea, (NH4)2HPO4 and K2SO4) into water hyacinth cellulose-graft-poly(acrylamide) polymer hy-
Chemical interaction drogel. Fourier Transform Infra-red spectra revealed existence of chemical interaction between the monomer,
Incubation experiment
cellulose, urea and nano-HA. The release of nutrients was assessed using laboratory incubation experiment.
Kinetics model
Significantly higher content of mineral nitrogen (MN) was observed in the first 4 weeks in conventional fertilizer
(CF) compared to SRF treatments and the control. MN content in SRF treatments increased considerably between
the 8th and 12th week, and declined in the 16th week. The values of potentially mineralizable N estimated using
first order kinetics model related well to the observed cumulative MN at 16th week. No significant difference was
observed between CF and SRF treatments for available P content in the 2nd week. Significantly higher P content
was observed in CF compared to SRF treatment in the 4th week, whereas in the 8th week, some SRFs released
significantly higher content than CF. Available P peaked in the 8th week in all the treatments and remained
constant at 12th and 16th week. Availability of P in SRFs increased with increased content of soluble P and
decreased content of nano-HA. Exchangeable K showed less variation during the incubation period, suggesting
short release time. The data revealed reduced chances of leaching losses and toxic effect to the plant roots, as
well as synchronized nutrient release and requirement by crops.

1. Introduction performance. On the other hand, exclusive use of organic manure is

limited by bulkiness, low nutrient quality, low nutrient mineralization
Over the years, inherent soil fertility has declined in Kenyan farm- (Makokha et al., 2001) and extra labour. The major constraint for usage
lands (Kimetu et al., 2007; Mucheru-Muna et al., 2013) due to con- of fertilizers and profitability to farming in Kenya has been high pro-
tinuous cultivation and inadequate replenishment of nutrients, among ducer prices (Druilhe and Barreiro-Hurlé, 2012) and thus, alternative
other limiting factors. To increase and sustain crop yields, farmers sources of cheaper fertilizers are being sought.
apply conventional fertilizers such as diammonium phosphate (DAP), Of the amount of fertilizers applied to the farms, only a small per-
triple superphosphate (TSP), nitrogen, phosphorous and potassium centage is utilized by plants, while the rest is eventually washed into
(NPK), mono-ammonium phosphate (MAP), single super-phosphate water bodies (Tolescu et al., 2009) through leaching and surface run-
(SSP), calcium ammonium nitrate (CAN), urea (Mathenge, 2009) and to off, or lost by volatilization under reduced conditions. About 40–70% N
some extent, organic manure, to supply the most limiting nutrients and 80–90% P of fertilizers’ applied in the farms is lost to the en-
(NPK). However, use of considerable amounts of fertilizers in sub- vironment resulting not only in economic and resource losses, but also
humid zones result in low nutrient use efficiency (NUE) due to leaching. environmental pollution (Guo et al., 2005; Naderi and Danesh, 2013).
Split application by top-dressing is known to improve NUE, but small- Efforts have been made to minimize these challenges by developing
scale poor-resource farmers consider it a luxury, or apply below re- new generation fertilizers, the so-called “smart” fertilizers. Among
commended rates (Mucheru-Muna et al., 2013) leading to poor crop them, are slow or controlled release fertilizers (SRF) which contain at

Peer review under responsibility of Faculty of Agriculture, Ain-Shams University.

⁎
Corresponding author.
E-mail address: kiplangatrop@uonbi.ac.ke (K. Rop).

https://doi.org/10.1016/j.aoas.2018.11.001
Received 26 July 2018; Received in revised form 27 August 2018; Accepted 2 November 2018
Available online 15 November 2018
0570-1783/ 2018 Production and hosting by Elsevier B.V. on behalf of Faculty of Agriculture, Ain Shams University. This is an open access article under the CC
BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
K. Rop et al. Annals of Agricultural Sciences 63 (2018) 163–172

least one nutrient that either delays its availability and utilization 2.2. Research methodology
processes, or is available to the plant for a longer period than the
standard fertilizers which are considered “quickly available” (Zeroual 2.2.1. Synthesis of hydroxyapatite (HA) nano-particles
and Kossir, 2012; Chen et al., 2013). The availability of nutrients is The methodology used in the synthesis was adopted from Kottegoda
prolonged by either slowing the release or altering reactions leading to et al. (2011) with some modifications. 7.716 g Ca(OH)2 was weighed
losses (Olson-Rutz et al., 2011). The enhancement of nutrient use effi- into the beaker and 0.22 mM TX-100 (non-ionic surfactant) solution
ciency (NUE) implies more efficient food production and reduced cost added to make a total volume of 100 mL and the mixture stirred for
for environmental protection (Naderi and Danesh, 2013). Further, SRF 30 min with a motorized stirrer. A 100 mL of 0.6 M H3PO4 was added
can be applied as a pre-plant application and the need for split appli- into the suspension of Ca(OH)2, drop-wise (15 mLmin−1) from the
cation is eliminated, reducing production costs (Chen et al., 2013). burette while stirring vigorously at 1000 rpm. After the reaction, the
In the face of resource scarcity and ever increasing population, dispersion was stirred for 10 min and then allowed to age for 2 h. It was
development in agriculture can be achieved exclusively through effec- then oven-dried at 105 °C to constant weight and then pulverized into
tive use of modern technologies. To date, intensive research is directed fine powder. The surfactant was removed by washing the powder with
towards integrating Nano-technologies into fertilizer development or methanol.
formulation. Due to high surface area to volume ratio, Nano-fertilizers
are expected to be more effective than polymer-coated conventional 2.2.2. Synthesis of cellulose-g-poly(acrylamide)/nano-HA/soluble fertilizer
SRF as they enhance NUE, reduce toxicity and minimize the potential composite
negative effects associated with excess application such as ground Thirty mL (predetermined volume containing 0.8 g dry weight) of
water pollution (DeRosa et al., 2010). Hydroxyapatite (HA) nano-par- cellulose fibers, 1.0 g nano-HA and varied amounts of soluble NPK
ticles are rated as one of the prominent candidates for potential agri- fertilizer blend (NH4)2PO4, urea and K2SO4 weight ratio 3:5:2, respec-
cultural nutrient sources (Kottegoda et al., 2011). However, much of tively, were transferred into a 3-necked flask. The flask was fitted with
the available data on HA is mainly focused on biomedical application reflux condenser and nitrogen line, and then placed in a thermostated
(Mateus et al., 2007; Pang et al., 2010; Pataquiva-Mateus et al., 2013), water bath equipped with a magnetic stirrer. Nitrogen gas was bubbled
while agricultural application is lacking. through the mixture for 10 min, as the temperature was gradually
More recently, there has been an increasing interest in the use of raised to 70 °C. 0.1 g of ammonium persulphate (APS) was added into
polymer hydrogels (PHG) in agricultural production. PHGs are macro- the mixture and stirred for 30 min to generate radicals. 2.7 mL of acrylic
molecular networks with the ability to swell or shrink in the presence or acid (AA), partially neutralized with NH3 to 70% degree of neu-
absence of water, due to hydrophilic groups and slightly cross-linked tralization and 0.25 g of N,N-methylene–bis-acrylamide (MBA) were
structure which resists dissolution (Sannino et al., 2009; Qiu and Hu, mixed, stirred to dissolve and then introduced into the reaction mix-
2013). Polyacrylamide (PAM) is used as a chemical intermediate in the ture. The total volume of the reaction mixture was controlled at 40 mL.
production of PHGs with high absorption capacity (super-absorbents) The mixture was stirred for an additional 1 min after which the reaction
such as disposable diapers, medical and agricultural products, among was allowed to proceed for 2 h. The reaction product was then cooled to
others (Laftah and Hashim, 2014). High molecular weight PAM is room temperature, removed from the flask and then cut into regular
added into the soil through irrigation water as anti-erosion additive pieces. 1:1 NH3 solution was added drop-wise to adjust the pH to 8.
(Charoenpanich, 2013) and it has been reported to be degraded by Fertilizer composite was then oven dried at 60 °C to constant weight
native soil bacterial species such as Bacillus, Pseudomonas and Rhodo- and then pulverized to pass through a 1 mm sieve.
coccus among others, and also fungi (Aspergillus) which are capable of
accessing N through amidase activity (Guezennec et al., 2015; Yu et al., 2.2.3. Chemical characterization of nano-HA and the fertilizer composite
2015). Extracellular amidase enzyme catalyzes the hydrolysis of C-N Fourier Transform Infra-red spectrophotometer, Shimadzu
bond of the amides, resulting in the generation of NH3, NH4+ and IRAffinity-1S, was used to characterize nano-HA and cellulose-g-poly
carboxylic acid group (eCOOH). The production of NH3 under moisture (acrylamide)/nano-HA/soluble fertilizer composite. The sample holder
conditions contributes to mineral N in the soil, whereas carboxylic acid and the probe were cleaned and scanned in absence of the sample to
is further degraded by micro-organisms as source of carbon (energy) to collect the background spectrum. The finely ground sample was placed
CO2 and H2O, thus being environmental friendly. PAM-treated agri- on the sample holder, pressed against the diamond using the probe and
cultural soil has been experimentally demonstrated by Kay-Shoemake scanned between 4000 and 400 cm−1. Field emission transmission
et al. (1998) to exhibit higher bacterial counts, high inorganic N con- electron microscope (Technai F20) was used to study the morphology of
centration and amidase activity, hence considered healthier soil than HA nano-particles and the fertilizer composite.
the untreated ones. In this study, cellulose grafted PAM polymer hy-
drogel was utilized in the formulation of slow release fertilizer com- 2.2.4. Soil sampling for incubation experiment
posite and release of nutrients was assessed using laboratory incubation A field was identified at the College of Agriculture and Veterinary
experiment. Science farm, University of Nairobi located in Kiambu County, Central
Kenya, coordinates 1°15′S and 36°44′E, and an altitude of 1940 m above
sea level. The soils are very deep (> 180 m), well-drained, dark red to
2. Materials and methods dark reddish brown, friable clays (Kimetu et al., 2007; Karuku et al.,
2012) with moderate to high inherent fertility (Mucheru-Muna et al.,
2.1. Materials 2013) and are classified as Humic Nitisols (WRB, 2014). The site ex-
periences a bi-modal rainfall distribution with long rains in mid March-
Swollen cellulose fibers were extracted from water hyacinth May and short rains in October-December. The mean annual rainfall is
(Eichhornia crassipes) and Acrylic acid and N,N-methylene–bis-acryla- about 1000 mm, and the average monthly maximum and minimum
mide, were obtained from ACROS Organics, Germany. Triton X-100 was temperatures are 23.8 and 12.6 °C, respectively. Crops grown in the
obtained from Sigma Aldrich, while Ammonium persulphate, Calcium area include; kales (Brassica olesarea), tomatoes (Lycopaersicon escu-
hydroxide and Phosphoric acid were from Loba Chemie, Mumbai, India. lentum), cabbage (Brassica olesarea), carrots (Daucus carota), onions
All other chemicals such as methanol, ammonia, ammonium acetate, (Allum fistulosum), beans (Phaseolus vulgaris), maize (Zea mays) and
potassium chloride, calcium chloride, hydrochloric acid, sulphuric acid, coffee (Coffea Arabica).
were analytical grade. The surface litter that included leaves, sticks, stumps and other
materials were removed gently to expose the surface soil. Soil samples

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Table 1
The composition of the formulated fertilizer composite and the amounts of N, P and K, in the treatments.
Code Composition of the Fertilizer Composite (% w/w) Soil treatment (mg kg−1)

N-P2O5-K2O PHG SF Nano-HA N P2O5 K2O

T1 14: 0: 0 100 0 0 50 0 0
T2 13.8: 18.8: 4.6 68.8 16.0 16.0 50 68.1 16.6
T3 15.4: 20: 4.8 62.7 22.3 15.0 50 65.0 15.6
T4 16.8: 21.5: 6.2 57.7 28.5 14.3 50 64.0 18.4
T5 20: 24: 9.7 44.5 44.4 11.1 50 60.0 24.2
T6 21.3: 25.2: 11.6 25.0 66.0 9.0 50 58.5 27.2
T7 22: 24: 11 0 100 0 50 54.5 24.9

Legend: PHG = polymer hydrogel, SF = soluble fertilizer, nano-HA = nano-hydroxyapatite.

at a depth of 0–20 cm were randomly collected at selected points using the fractional quantity N susceptible to mineralization, was estimated
a soil auger and bulked to make a composite sample. The composite using single first order kinetics employed by Stanford et al. (1974) and
sample was air-dried in the laboratory, crushed to pass through a 4 mm later adopted by Karuku (1989), and Karuku and Mochoge (2018), Eq.
sieve to remove large pieces of surface materials. A portion of the (1).
composite sample was set aside for chemical characterization and the
dN
other portion for incubation experiments. = kN
(1)
dt
2.2.5. Soil characterization before onset of incubation experiment Integration gives Eq. (2),
Total nitrogen and mineral N were determined by micro-Kjedahl Log (No Nt ) = logNo kt /2.303 (2)
method (Bremner, 1996), available P with Mehlich 1 method (Mehlich,
1953) and exchangeable cations (Ca, K and Mg) by flame photometry Where, Nt is the cumulative N mineralized at time t (days), No is the
after extraction with 1 M NH4OAc. Soil pH was determined with glass amount of potentially mineralizable N and k is the first order rate
electrode, 1: 2.5 soil to water (salt) ratio. constant (day−1). Stanford et al. (1974); Karuku and Mochoge (2018)
found the rate constant k, to be reasonably equal for large number of
2.2.6. Fertilizer composite samples and laboratory incubation experiment soils and a period of 2 weeks incubation following a short term pre-
Table 1 shows the composition of the fertilizer composite and the incubation was sufficient to estimate mineralization potential (N0 )
amount of NPK added to the soil for incubation experiments. T1 is using simplified Eq. (3).
cellulose-g-poly(acrylamide) polymer hydrogel (PHG) and T2-T6 was N0 = 9.77Nt (3)
formulated to contain cellulose-g-poly(acrylamide)/nano-HA/soluble
fertilizer. The amount of soluble fertilizer (SF) in the composites in- Where, N0 is nitrogen mineralization potential and Nt is nitrogen mi-
creases from T2 to T6 with decrease in the content of PHG and nano- neralized in 2 weeks.
HA, whereas T7 represent conventional fertilizer.
The formulated fertilizer composite (< 1 mm) was added to the soil 2.2.8. Statistical analysis
at the rate of 50 mg N kg−1, thoroughly mixed and then put into plastic The data from the incubation experiment was subjected to ANOVA,
incubation bags. This corresponded to 100 kg N ha−1, recommended for using IBM SPSS Statistics Version 20. Tukey honest significant differ-
N application for maize in Kiambu County (planting, 250 kg/ha NPK ence (HSD) post hoc test was used to compare and assess the sig-
23:23:0; top dressing, 125 kg/ha CAN) (Ministry of Agriculture, 2014). nificance of the mean values. The main effects; time (within-subject
The amount of the fertilizer added to 1 kg of soil include; T1 – 397 mg, factor) and treatment (between-subject factor) were considered sig-
T2 – 362 mg, T3 – 325 mg, T4 – 298 mg, T5 – 250 mg, T6 – 235 mg and nificant at a probability level, p ≤ 0.05.
T7 – 227 mg. The treatments were replicated three times, with un-
treated soil serving as the control. Distilled water was added to field 3. Results and discussion
capacity (30% w/w), bags were sealed and incubated in the dark at
20 °C for 16 weeks. The amount of mineral N (NH4-N and NO3-N), P and 3.1. Fourier Transform Infra-red spectroscopic analysis of SRF and Nano-
K were determined bi-weekly from the onset of incubation. Soil was HA
kept moist at field capacity throughout the incubation period by adding
distilled water where the feel method was used to establish the ne- The Fourier Transform Infra-red (FTIR) spectrum of HA nano-par-
cessity. Aerobic conditions were maintained by opening plastic bags ticles is shown in Fig. 1. The absorption bands at 1419 and 875 cm−1
periodically to allow aeration. Each of the samples was divided into two correspond to CO32– ions, attributed to the physical interaction of HA
portions at the time of sampling. For one portion, available N (NH4-N with CO2 during the synthesis at ambient conditions (Iyyappan and
and NO3-N) was extracted and quantified. The other portion was air Wilson, 2013). The spectrum observed in the study is similar to that of
dried, before analyzing for total N, available P and K. Costescu et al. (2010), who reported decreased intensity of the peaks
related to CO32– at high calcination temperatures of 600 and 1000 °C.
2.2.7. Nitrogen mineralization potential, No The broad and weak band at 3600–3000 cm−1 and 1635 cm−1 corre-
Several models have been proposed to simulate N-mineralization spond to HeOeH of lattice water, which also diminish on heating. The
dynamics during long-term aerobic incubation. Simulation models characteristic bands for PO43− group appear at 1022 and 964 cm−1 due
widely employed include; single first-order kinetics model, double first- to stretching vibrations and, 601 and 563 cm−1 corresponding to the
order kinetics model, and mixed first-order and zero-order kinetics bending vibrations. The bands characteristic of CeH stretch at 2928
model (Zhang et al., 2017). Potentially mineralizable-N (No ), which is and 2856 cm−1 due to eCH3 and eCH2 respectively, were found to be

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K. Rop et al. Annals of Agricultural Sciences 63 (2018) 163–172

Fig. 1. FTIR spectrum of hydroxyapatite nano-particles.

Fig. 2. FTIR spectrum of cellulose-g-poly(acrylamide)/nano-HA/soluble fertilizer composite.

absent in the FTIR spectrum. This confirms complete removal of T-X 3182 and 1543 cm−1 are assigned to NeH stretching vibration for
100 upon washing HA nano-particles with methanol. primary amide (Bundela and Bajpai, 2008).
FTIR spectrum of the fertilizer composite is shown in Fig. 2. The Eritsyan et al. (2006) and Fernandes et al. (2015) proposed radical
broad and strong band at 2500–3500 cm−1 can be assigned to OeH polymerization reaction mechanism between acrylic acid and urea via
stretch due to carboxylic acid (acrylic acid) and alcoholic group (cel- the carbonyl carbon reaction scheme (a). According to these authors,
lulose), and also, NeH for amide group (acrylamide). The bands at the moderately strong band at 1635 cm−1 (Fig. 2) is assigned to

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K. Rop et al. Annals of Agricultural Sciences 63 (2018) 163–172

Fig. 3. TEM images of rod-shaped agglomerates of HA nano-particles at 50 and 20 nm scales.

adsorption of NH3+ and COO– groups as a result of intra- and inter- (Fig. 2) corresponds to OeH bending vibration for carboxylic acid, re-
molecular interactions between eCOOH and eNH2 which lead to the vealing incomplete neutralization of acrylic acid. Spectral bands at
formation of a salt.Since acrylic acid was partially neutralized with 1153 to 1049 cm−1 are assigned to CeOeC bridging resulting from the

or

Inter-molecular interaction

Intra-molecular interaction

Reaction Scheme (a): Radical polymerization between urea and acrylic acid

NH3, it therefore implies that ammonium acrylate could react the same reaction between ammonium acrylate (monomer) and the eOH group
way according to reaction scheme (b). Alongside radical polymeriza- of cellulose. The band at 898 cm−1 is assigned to CeOeC stretch of
tion, condensation reaction between urea and acrylic acid may also glucosidic bonds for amorphous cellulose (Synytsya and Novak, 2014).
occur (Fernandes et al., 2015), yielding a branched co-polymer ac- The FTIR peaks from 1049 to 920 cm−1 are assigned to PeOeC (Fig. 2),
cording to reaction scheme (c).The strong band at 1438–1400 cm−1 suggesting an overlap between bands attributed to CeOeC and PeOeC

70 °C
- H2O

Reaction Scheme (b): Radical polymerization between urea and ammonium acrylate

Reaction Scheme (c): Condensation reaction between urea and acrylic acid

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K. Rop et al. Annals of Agricultural Sciences 63 (2018) 163–172

Fig. 4. TEM images of cellulose-g-poly(acrylamide)/nano-hydroxyapatite/soluble fertilizer composite at 50 and 20 nm scales.

groups. The peaks at 1049 and 956 cm−1 are assigned to PeOeC 3.4. Nitrogen mineralization
stretching vibrations, indicating the reaction between eOH groups at
the surface of HA nano-particles and the monomer. The bands at 1319 The content of mineral N (NH4-N + NO3-N) during 16 weeks in-
and 516 cm−1 (Fig. 2) are attributed to P]O stretching vibrations for cubation period is shown in Table 3. The results revealed low content of
the PO43− ion, whereas the bands at 613 and 439 cm−1 corresponds to MN in the initial stages of incubation with some decrease in the 4th
SO42− ion for the inorganic salts i.e., (NH4)2HPO4 and K2SO4. week, followed by significant increase through the 8th week and a peak
From FTIR spectrum of the fertilizer composite, there is an indica- at the 12th week then a decline in the 16th week. Low mineralization of
tion of the existence of chemical interactions between (a) the monomer, N in the initial stages of incubation reflects the lag phase (Deenik and
cellulose and nano-HA, and (b) monomer and urea molecules towards Yost, 2008) associated with immobilization of nutrients by micro-or-
the formation of 3-D network structure. Additionally, due to large ganisms to nourish and increase their biomass (Karuku and Mochoge,
surface area of HA nano-particles, Kottegoda et al. (2011, 2017) asso- 2016; Tambone and Adani, 2017). Micro-organisms require sufficient
ciated the formation of urea-HA Nano-hybrid (molar ratio, 6:1) to the water, inorganic nutrients, carbon sources and trace elements for
existence of H-bonds between eOH group on the surface of HA and maintenance and growth. The period between the 4th and 12th week
eNH2 group of urea. relates to microbial exponential growth phase and the microbes have
proliferated hence able to act on the substrate. Although they are well
satiated, more are left for mineralization process and the rate of mi-
3.2. Transmission electron microscopy neralization is much higher than immobilization. The decline in MN
content beyond the 12th week is attributed to depletion of mineraliz-
Figs. 3 and 4 show transmission electron microscopy (TEM) images able substrate. Also, the microbes may have passed the stationary phase
of HA nano-particles and cellulose-g-poly(acrylamide)/nano-hydro- and had entered the endogeneous growth phase, leading to decline in
xyapatite/soluble fertilizer composite. The images of nano-HA dis- immobilization. Low N mineralization observed within the treatments
played rod-shaped nano-particle agglomerates with particle size of less up to 4 weeks might favour annual crops such as maize, because the
than 50 nm (Fig. 3). The TEM images of the fertilizer composite (Fig. 4) uptake of N is slow at establishment, faster at development and re-
showed dispersion of the HA nano-particles and the salt crystals. productive phases, and declines at maturity.
The release of N from the SRF composites T2 to T6 (cellulose-g-poly
(acrylamide)/nano-HA/soluble fertilizer composite) occurred in two
3.3. Chemical characteristics of soil at the onset of the experiment phases: i) diffusion of urea-N and NH4-N and ii) hydrolysis of amide-N
(Liu et al., 2007). The highest content of MN in the first 4 weeks was
Table 2 shows salient characteristics of the soil used in the study.
The soils at the site were acidic with low available P content. The soil
acidity could be attributed to the humid conditions in central highlands Table 3
which lead to the leaching of Ca, Mg and K, and other basic cations. Concentrations of Mineral N (NO3-N + NH4-N) during 16 weeks incubation
Low amounts of available P could be attributed to soil acidity which period.
renders P unavailable through fixation and also, continuous removal by Treatment Incubation period (weeks)
crops.
2 4 8 12 16 Cumulative
MN at 16th
wk
Table 2
Some salient chemical characteristics of soil used in incubation experiment. Cntrl 43.6a 24.2 a 107.2 a 145.8a 85.1 a 405.8 a
T1 50.8ab 33.0 b 152.9 b 176.6ab 117.9 b 531.1 b
Parameter Units Value
T2 59.0abc 44.1 cd 176.4 c 200.1bc 128.4 cd 608.0 cd
T3 55.0abc 41.5 c 189.5 cd 192.2bc 119.3 b 597.4 c
pH (soil: H2O, 1: 2.5) – 5.25
T4 55.3abc 46.0 cd 205.1 de 266.1e 139.1 e 711.7 e
pH (CaCl2: 1: 2.5) – 4.50
T5 63.5bc 42.9 c 190.1 cde 210.6 cd 124.7 bcd 631.8 cd
Electrical conductivity ds/m 0.26
T6 58.3abc 48.8 cd 211.7 e 242.1de 131.2 de 692.1 e
Cation exchange capacity C mol kg−1 15.62
T7 73.2c 51.7 d 184.9 cd 207.5bc 129.6 d 646.9 d
N % 0.29
Available P ppm 8.50
Exchangeable K C mol kg−1 1.10 Notes; different letters in the same column are significantly different (p ≤ 0.05
Ca C mol kg−1 8.51 level). Cntrl = No treatment, T1 = 14: 0: 0, T2 = 13.8: 18.8: 4.6, T3 = 15.4:
Mg C mol kg−1 4.26 20: 4.8, T4 = 16.8: 21.5: 6.2, T5 = 20: 24: 9.7, T6 = 21.3: 25: 11.2, T7 = 24:
22: 11.

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observed in conventional fertilizer T7 which was attributed to the hy- Table 4

drolysis of urea-N and release of NH4-N from DAP. However, no sig- Concentrations of NH4-N and NO3-N during 16 weeks incubation period.
nificant difference was observed between T7 and T2, T3, T4, T5 and T6 Treatment Incubation Period (weeks)
in the 2nd and the 4th week of incubation. This insignificant difference
may be attributed to small particle sizes of SRF composite (< 1 mm) 2 4 8 12 16 Cumulative
which enabled faster diffusion of the soluble fertilizer into the soil, and MN at 16th
wk
thus slow release of N might be enhanced by increasing the particle size
of the fertilizer composite. Significant higher content of N observed in NH4-N
T6 in the 8th and 12th week and T4 in the 12th and 16th week relative Cntrl 29.4 a 17.1 a 72.5 a 87.0 a 53.3 a 259.0 a
to T7, may be attributed to early exposure of NH4-N (in case of T7) to T1 34.0 ab 24.5 b 83.0 ab 99.6 ab 69.3 b 310.3 b
T2 38.8 c 26.8 b 97.4 bc 116.8 bc 78.9 cd 358.7 cd
exchangeable sites in the soil and possibility of immobilization through
T3 34.0 ab 25.0 b 95.4 bc 114.5 bc 73.6 bc 342.5 bc
fixation. NH4+ fixation occurs in soils with clay minerals of the type 2:1 T4 35.1 ab 29.3 b 109.8c 131.7 c 86.4 d 392.3 bc
such as illite and vermiculite, due to the formation of NH·O bond in the T5 35.8 ab 23.5 ab 86.29 ab 103.5 ab 78.9 cd 328.0 bc
hexagonal holes and the balancing of positive charge deficiency which T6 38.8 c 28.3 b 113.7 c 136.4 c 78.9 cd 396.1 d
T7 38.8 c 27.9 b 89.5 ab 107.4 ab 73.6 bc 342.6 c
arises from isomorphous substitution of Si4+ and Al3+ ions (Chen,
NO3-N
1997). Among the factors which have been found to enhance fixation Cntrl 14.2 a 7.2 a 34.6 a 58.8 a 31.7 a 146.5 a
are increased concentration and contact time (Kissel et al., 2008). No T1 16.8 ab 8.7 b 69.9 b 77.0 ab 48.5 b 220.8 b
significant difference was observed in the 12th week between T7 and T2 20.2 abc 17.2 cd 79.1 c 83.3 bc 49.5 cd 249.2 c
T1, T2 & T3, indicating considerable mineralization of N in SRF treat- T3 20.9 abc 16.5 c 94.1 cd 77.7 bc 45.7 bc 254.9 c
T4 20.2 abc 16.7 c 95.4 de 134.4 e 52.7 e 319.3 d
ments. The observed increase in MN contents between the 8th and 16th
T5 27.6 bc 19.5 c 103.9 cde 107.1 cd 45.7 bcd 303.8 d
week in SRF T1 to T6 may be attributed to the release (hydrolysis) of T6 19.4 bc 20.5 cd 98.1 e 105.7 de 52.3 de 295.9 d
amide-N. T7 34.3 c 23.8 d 95.4 cd 100.1 bc 56.0 d 309.7 d
The cumulative MN in SRF treatments generally increased from T1
to T6, reflecting increased amount of soluble fertilizer and decreased Notes; different letters in the same column are significantly different (p ≤ 0.05
content of nano-HA incorporated into the PHG. Since N mineralization level). Cntrl = No treatment, T1 = 14: 0: 0, T2 = 13.8: 18.8: 4.6, T3 = 15.4:
20: 4.8, T4 = 16.8: 21.5: 6.2, T5 = 20: 24: 9.7, T6 = 21.3: 25: 11.2, T7 = 24:
is a biological process, the release of N depends on the chemical con-
22: 11.
stituent of the fertilizer such as N content and C:N ratio (Masunga et al.,
2016) among other factors. The C:N ratio influences mineralization
from DAP and easily hydrolysable N from urea. The microbes were
rate, as microorganisms immobilize N to break carbon bonds/chains in
provided with easy source of N and to acquire energy (carbon), the
the organic material for their energy requirement (Dong et al., 2012;
polymer has to be degraded. Polymer medium supplemented with mi-
Karuku and Mochoge, 2016; Tambone and Adani, 2017). The sig-
neral N, liquid paraffin and sucrose has been shown to contribute to
nificant difference observed in SRF T1 {cellulose-g-poly(acrylamide)}
PAM degradation and microbial biomass compared to the control (Yu
MN content compared to T2, T3, T4, T5 and T6, from the 4th to the
et al., 2015). The significantly (p ≤ 0.05) lower content of MN observed
16th week, could be related to the carbon content. Slow mineralization
in SRF treatment T5 relative to T4 and T6 could be attributed to the
of N may have been contributed by higher C content in T1 (i.e., 14% N,
experimental errors.
the rest being carbonaceous material of acrylic and complex cellulose
Table 4 shows the contents of ammonium-N and nitrate-N during
chains), compared to T2 to T6 of which the C content decreased on
different incubation stages. The NH4-N content in T7 was significantly
incorporating soluble fertilizer and nano-HA.
higher (p ≤ 0.05) than T1, T2, T4 &T5 in the 2nd week, whereas for
Nevertheless, significantly higher MN content in T1 compared with
NO3-N, T7 showed significantly higher value than T1, T3, T4 &T5 in the
the control indicates substantial hydrolysis of the amide-N within the
4th week of incubation. No significant difference in both NH4-N and
incubation period. Cellulose chains being part of the polymer composite
NO3-N content was observed between T7 and SRF T2, T3 and T5 in the
provide easily degradable-C to the microorganisms, enhancing break-
8th, 12th and 16th week, but significantly higher values were observed
down of the copolymer and hence N release. Addition of organic ma-
in T4 and T6 compared to T7. Significantly higher N content in T7 at
terial to PAM-amended soil has been reported to effect degradation
early stages of incubation (2nd & 4th week) reveals availability of N
through increased microbial activity. Higher amounts of soil ag-
which crops may not fully utilize as it may be lost through leaching or
gregating fungi was reported by TonThat et al. (2008) in macro-ag-
fixation by clay minerals, while in SRFs it may be preserved for future
gregates generated from PAM-wheat residue amended soil compared to
use and hence better synchronization. From the 8th to the 16th week,
the control. Award et al. (2012) also reported stimulating effect of
both T7 and SRF showed nearly equal N content, implying that the
synthesized PAM biopolymer (BP) and biochar (BC) on the decom-
plant can utilize N released by SRF more efficiently than T7 which
position of soil organic matter and maize residue. Higher enzymatic
might get depleted sooner due to earlier losses.
activities were observed in both BP and BC amended soil compared to
NH4-N content was higher than NO3-N throughout the incubation.
the control and, fungi contributed highly to plant residue decomposi-
Subsequently, the cumulative NH4-N content at the end of incubation
tion. Watson et al. (2016) observed stimulation of nitrification and C-
period recorded higher values than NO3-N content and generally in-
mineralization in maize straw-amended soil conditioned with PAM, a
creased from T1 to T6. No significant difference was observed in cu-
phenomenon attributed to improved microbial conditions and partial
mulative NH4-N content between T7 and SRF T2, T3, T4 & T5, and also,
utilization of PAM as a substrate.
in the cumulative NO3-N between T7 and SRF T4, T5 & T6. The higher
Addition of MN from inorganic source to organic fertilizer enhances
NH4-N content compared to NO3-N may be attributed to the acidity of
decomposition of organic material (Abbasi and Khaliq 2016). Further,
soil which could have inhibited the growth and activities of nitrifying
cultures of bacteria derived from agricultural soils have been reported
bacteria. The pH of soil during the incubation period showed some
to utilize PAM as N source (Kay-Shoemake et al., 1998). Bacterial strain
increase in the 12th and 16th week, particularly in SRF T2 to T6,
(Pseudomonas putida H147) studied by Yu et al. (2015) showed 31.1%
though not significant among the treatments (Table 5). The lowest pH
degradation efficiency of PAM in 7 days and exceeded 45% under op-
value was 5.15 recorded in the 2nd week, while the highest value was
timum culture conditions. Degraded PAM showed low molecular
5.97 recorded in the 16th week. Nitrification process which is a bio-
weight oligomer derivatives while acrylamide monomers did not ac-
logical oxidation of NH4+ to NO3– has been found to occur in soil pH
cumulate. The observed increase in the content of MN in SRF treat-
values ranging between 5.5 and 10.0 (Sahrawat, 2008) with an
ments T2 to T6, could be attributed to increased amount of soluble N

169
K. Rop et al. Annals of Agricultural Sciences 63 (2018) 163–172

Table 5 Table 7
Soil pH during the incubation period. Content of available P (ppm) at different incubation times (weeks).
Treatment Incubation period (weeks) Treatment Incubation period (weeks)

2 4 8 12 16 2 4 8 12 16

Cntrl 5.34 a 5.60 a 5.53 a 5.33 a 5.42 a Cntrl 21.0 a 10.4 a 25.3 a 23.9 a 26.1 a
T1 5.31 a 5.54 a 5.43 a 5.66b 5.71b T1 24.1 ab 12.9 abc 25.9 a 26.3 a 28.5 a
T2 5.28 a 5.23 a 5.56 a 5.74b 5.76 bc T2 24.1 ab 12.5 ab 39.3 b 37.8 b 41.1 b
T3 5.21 a 5.69 a 5.30 a 5.63 ab 5.67b T3 22.3 ab 13.6 abc 46.5 bc 46.6 c 46.2 bc
T4 5.33 a 5.46 a 5.36 a 5.68b 5.83 bc T4 26.1 ab 14.4 bc 55.2 cd 54.3 d 51.6 cd
T5 5.15 a 5.47 a 5.44 a 5.59 ab 5.71 bc T5 25.6 ab 16.3 cd 63.5 d 66.2 e 66.2 e
T6 5.28 a 5.80 a 5.34 a 5.47 ab 5.92c T6 27.4 ab 16.3 cd 76.3 e 80.4 f 76.6 f
T7 5.24 a 5.65 a 5.23 a 5.52 ab 5.68b T7 27.8 b 19.6 d 54.2 cd 53.7 d 54.3 d

Notes; different letters in the same column are significantly different (p ≤ 0.05 Notes; different letters in the same column are significantly different (p ≤ 0.05
level). Cntrl = No treatment, T1 = 14: 0: 0, T2 = 13.8: 18.8: 4.6, T3 = 15.4: level).
20: 4.8, T4 = 16.8: 21.5: 6.2, T5 = 20: 24: 9.7, T6 = 21.3: 25: 11.2, T7 = 24: Legend: Cntrl = No treatment, T1 = 14: 0: 0, T2 = 13.8: 18.8: 4.6, T3 = 15.4:
22: 11. 20: 4.8, T4 = 16.8: 21.5: 6.2, T5 = 20: 24: 9.7, T6 = 21.3: 25: 11.2, T7 = 24:
22: 11.

optimum pH value of about 8.5, while the process is inhibited at pH less

than 5 and optimal at pH greater than 6. Higher content of NH4-N than polymer composite enhances mineralization of N. Thus, the sig-
NO3-N was observed by Omar and Ismail (1999) in soil treated with a nificantly higher (p ≤ 0.05) MN values observed in T4 and T6, relative
mixture of urea and Ca2Cl2 or K2SO4. The population of bacteria and to T7 may be attributed to improved mineralization leading to release
fungi decreased in urea treatments except cellulolytic fungi, and the of higher amounts of MN in the later stages of incubation.
same decrease in microbial population was also observed in Ca2Cl2 or The coefficient of determination, R2 ranged from 0.742 to 0.917,
K2SO4 soil amendments. Soil pH increased in urea amendment, but was indicating a good fit of the experimental data to the single order ki-
decreased in inorganic salts amendments to values lower than that of netics model. The mineralization rate constant ranged from 0.051 to
the control. The toxic effect of urea and inorganic salts reduced when 0.056 week−1 which resulted in half-life time (t1/2 ) ranging from 11.6 to
they were applied as a mixture. Giroto et al. (2017) observed higher pH 13.6 weeks, suggesting that mineralization of most of the N would
values (6.3–7.9) after 42 days of aerobic incubation of soil amended occur within the growing period of most annual crops of about
with urea/HA and thermoplastic starch urea/HA amendments com- 20 weeks. The t1/2 values obtained in the experiment were similar to the
pared to untreated soil, whereas the pH of soil amended with HA and average value of 12.8 weeks reported by Stanford and Smith (1972) on
SSP remained close to the pH of the control (≈5). The increase in pH in evaluating No of 39 soil types in the USA. Since there was less variation
Nano-composite amendments was attributed to high hydrolysis of urea in the t1/2 among the treatments, the advantage of SRF over T7 could be
in the soil with low CEC and buffering capacity. The existence of more attributed to the slow initial N mineralization, leading to release of
of MN in the form of NH4-N is beneficial because it is not susceptible to significantly (p ≤ 0.5) higher amounts in the later stages of incubation.
leaching losses. The incubation experiment was however carried out at optimal condi-
Table 6 shows N mineralization potential (No ), mineralization rate tions of moisture, temperature and aeration, for the growth and activity
constant (K ), coefficient of determination (R2 ) and time taken for 50% of soil microbes and hence N-mineralization rate might be lower/higher
of potentially mineralizable N (t1/2), to be mineralized. in the field than in the laboratory due to varying conditions that could
T7 had the highest potentially mineralizable nitrogen (No) compared affect the performance of micro-organisms.
to all other treatments though not significantly different from T2, T3,
T4, T5 and T6. The low N mineralization observed in SRF treatments 3.5. Available phosphorous
may be attributed to slow release of nutrients in the initial stages of
incubation. No related well to the observed cumulative mineral N as at Available P at different incubation times are shown in Table 7. The
16th week of incubation. However, no significant difference was ob- lowest P values were recorded in the 4th week, highest in the 8th week
served for cumulative MN between T7, T2 and T5 at 16th week of in- and remained nearly constant in the 12th and 16th week of incubation.
cubation, implying that incorporation of soluble fertilizer into the The decline in P content between the 2nd and 4th week could be at-
tributed to microbial immobilization and adsorption of soluble P into
Table 6 the soil. The increased P availability after 4 weeks in all SRF treatments
Nitrogen mineralization potential (No ), mineralization rate constant (K ), half- may be attributed to its release through microbial solubilization of
life (t1/2 ) and cumulative MN. nano-HA and degradation of the copolymer. Insoluble phosphates such
as apatite have been shown to be solubilized by native soil micro-or-
Treatment No R2 K (week −1
) t1/2 (wks) Observed cumulative
MN at 16 wk
ganisms. Phosphate solubilizing bacteria (Pseudomonous, Enterobactor,
Anthrobactor) and fungi (Aspergillus, Penicillium) present in the soil and
Cntrl 425 a 0.903 0.052 13.3 405 a the rhizosphere have been reported to hydrolyze insoluble P by se-
T1 495 ab 0.917 0.051 13.6 531 b creting low molecular mass organic acids, to chelate mineral ions or
T2 576 abc 0.829 0.059 11.7 608 cd
T3 536 abc 0.716 0.051 13.6 597 c
lower the pH (Khan et al., 2014; Alori et al., 2017). Besides organic
T4 539 abc 0.865 0.056 12.4 712 e acids, mineral acids such as HCl, HNO3 and H2SO4 produced by che-
T5 619 bc 0.742 0.060 11.6 632 cd moautotrophs and H+ pump, for instance in Penicillium rugulosum has
T6 569 abc 0.910 0.053 13.1 692 e been reported to solubilize P (Khan et al., 2014). Soil fungi such as
T7 714 c 0.831 0.057 12.2 647 d
mycorrhizae have been shown to better solubilize P than bacteria as
Legend: No = Nitrogen Mineralization Potential, K = Mineralization rate con- they traverse longer distances within the soil and also, produce and
stant and t1/2 = Time taken for 50% of potentially mineralizable nitrogen to be secrete more acids such as gluconic, citric, lactic, 2-ketogluconic,
mineralized. Cntrl = No treatment, T1 = 14: 0: 0, T2 = 13.8: 18.8: 4.6, oxalic, tartaric and acetic acid (Alori et al., 2017). Additionally, as-
T3 = 15.4: 20: 4.8, T4 = 16.8: 21.5: 6.2, T5 = 20: 24: 9.7, T6 = 21.3: 25: 11.2, similation of NH4+ within microbial cells release H+ that solubilize P
T7 = 24: 22: 11. without production of organic acids. Acidification of microbial cells and

170
K. Rop et al. Annals of Agricultural Sciences 63 (2018) 163–172

Table 8 and hence, just like in P, it was impossible to ascertain the optimum
Concentrations of exchangeable K (C mol kg−1) at different incubation times amount to be incorporated into the fertilizer composite.
(weeks).
Treatment Incubation period (weeks) 4. Conclusion and recommendations

2 4 8 12 16 A SRF composite has been formulated and assessed for release of

nutrients using laboratory incubation experiment. Characterization by
Cntrl 1.63 a 1.70 a 1.87 a 1.58 a 1.70 a
T1 1.75 ab 1.85 ab 1.90 ab 1.85 a 1.80 a FTIR spectroscopy revealed synthesis of nano-HA and existence of
T2 1.87 ab 1.83 ab 1.98 bc 2.05 b 2.08 b chemical interaction between the monomer, cellulose, nano-HA and
T3 2.00 ab 1.92 ab 1.90 c 2.06 b 2.18 c urea molecules. The incubation experiment revealed low MN content in
T4 2.05 ab 2.05 bc 1.95 d 2.00 c 1.95 d
the first 4 weeks and a peak at the 12th week corresponding to the most
T5 2.10 ab 2.10 bc 2.05 d 2.10 d 2.37 e
T6 2.17 ab 2.13 bc 2.25 e 2.17 e 2.27 e active and nutrient demand stages of development and reproduction of
T7 2.10 b 2.07 bc 2.30 d 2.37 c 2.28 d most crops, hence proper synchronization of SRF. The highest MN
content was observed in T7 in the first 4 weeks, whereas between the
Notes; different letters in the same column are significantly different (p ≤ 0.05 8th and 16th week, both T7 and SRF showed similar MN content with
level). some SRFs, for instance T4 and T6, releasing significantly higher
Legend: Cntrl = No treatment, T1 = 14: 0: 0, T2 = 13.8: 18.8: 4.6, T3 = 15.4:
amounts. Single order kinetics model predicted well N mineralization
20: 4.8, T4 = 16.8: 21.5: 6.2, T5 = 20: 24: 9.7, T6 = 21.3: 25: 11.2, T7 = 24:
and the half-life time (t1/2 ) showed less variation among the treatments.
22: 11.
Low contents of P were observed in the first 4 weeks, increased to the
maximum in the 8th week and remained constant thereafter.
their surroundings release P through substitution of H+ for Ca2+ ions.
Availability of P increased significantly in SRF with increased content of
The release of Ca2+ ions into the soil could be the reason for the ob-
soluble P and decreased content of nano-HA. The specific objective of
served increase in soil pH towards the end of the incubation period
the study was achieved and SRF T5 & T6 could provide synchronized
(Table 5). Ca2+ ions are bases and have the effect of neutralizing soil
release N & P, although the release of K was almost immediate. The SRF
acidity (Mucheru-Muna et al., 2013).
composite would be more suitable for use in annual crops and should be
No significant difference was observed in the 2nd week between T7
applied during planting, so as to match nutrient release with crop up-
and SRF T1 to T6, while in the 4th week T1 to T4 recorded significantly
take. However, the findings were based on laboratory incubation ex-
lower P content compared to T7. From the 8th to 16th week, highest P
periments and evaluation should be done under field conditions before
value was observed in T6 which was also significantly different from all
they can be recommended with confidence.
the treatments. The observation could be attributed to solubilization of
nano-HA and release of soluble P which was physically protected by
Acknowledgements
composite from adsorption into the soil in the initial stages of incuba-
tion.
The authors acknowledge DAAD for the scholarship award, National
Fertilizer composites were quantified to deliver a specific amount of
Research Fund (NRF) for financial assistance and University of Nairobi
N (50 mg N kg−1 of soil) into the soil regardless of NPK formulae, hence
technical staff, Mr. Kimotho and Mr. Anyika for their assistance.
the amount of P in the amendments varied as: T2 = 68.1 mg kg−1,
T3 = 65 mg kg−1, T4 = 64 mg kg−1, T5 = 60 mg kg−1, T6 = 58.5
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