A Novel Slow-release Urea Fertiliser Physical and Chemical Analysis of Its Structure and Study of...

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b i o s y s t em s e n g i n e e r i n g 1 1 5 ( 2 0 1 3 ) 2 7 4e2 8 2Available online at wjournal homepage: www.elsevier .com/locate/ issn/15375110Research PaperA novel slow-release urea fertiliser: Physical andchemical analysis of its structure and study of itsrelease mechanismNi Xiaoyu, Wu Yuejin*, Wu Zhengyan, Wu Lin, Qiu Guannan, Yu Lixiang

Key Laboratory of Ion Beam Bio-engineering, Institute of Technical Biology & Agriculture Engineering of Chinese

Academy of Sciences, 350# Shushanhu Road, Hefei 230031, PR Chinaa r t i c l e i n f o

Article history:

Received 12 February 2012

Received in revised form

28 September 2012

Accepted 5 April 2013

Published online 17 May 2013* Corresponding author. Tel.: 86 551 559317E-mail address: [email protected] (W. Yuej

1537-5110/$ e see front matter 2013 IAgrEhttp://dx.doi.org/10.1016/j.biosystemseng.201Reducing the release rate of urea can increase its efficiency of use and reduce nitrogen

pollution. A slow-release urea (S-urea) was produced using a new method; a bentonite and

organic polymer (OP) were used to form a three-dimensional lattice structure bymelting urea

directly. The structure affected the recrystallisation of urea and increased its stacking density.

The specific surface area of S-urea was 0.046 m2 g1, much lower than that of common urea

(1.698 m2 g1). The static release experiment showed that 75% of 12 g sample of S-urea was

released in1 lwater for about 14h,much longer than thatof commonurea (

Nomenclature

Symbols

A Absorbency value

C Constant incorporating the characteristics of the

adsorption of N2Ct Concentration of urea at time t (mg ml

1)k Constant of the carrier-active agent system

k1 Diffusion constant

k2 Dissolving-erosion constant

m Diffusion exponent

n Diffusion exponent

P Partial pressure of N2 (Pa)

P0 Saturated vapour pressure of liquid N2 (Pa)

Qt Fraction of active agent released at time t

R2 Coefficient of determination

t Time (h)

V Total gas volume adsorbed by sample (ml)

Vm Gas volume adsorbed bymonolayer of sample (ml)

Abbreviations

BET Brunauer, Emmett, & Teller equation to calculate

specific surface area B-urea Urea added with

bentonite

IR Infrared spectra

OP Organic polymer

P-urea Urea added with organic polymer.

SEM Scanning electron microscopy

S-urea Slow-release urea added with bentonite and

organic polymer

XRD X-ray diffraction

b i o s y s t em s e ng i n e e r i n g 1 1 5 ( 2 0 1 3 ) 2 7 4e2 8 2 275developing countries consume more and more nitrogen fer-

tiliser and yet have only 20e35% efficiency of nitrogen use

(Fan & Liao, 1998; Jiang, Hu, Sun, & Huang, 2010).

In this study a novel slow-release urea (S-urea) is pre-

sented whose structure and release mechanism is quite

different from that of coated urea. The formulation forms a

three-dimensional lattice structure in the urea solution that

could influence its release process (Cai et al., 2009; Chinese

Patent Specification ZL200610040631.1, 2006). The method of

production is developed by melting urea directly and using

bentonite which is a cheap and safe material as a main

substrate (Chinese Patent Specification CN201110003090.6,

2011). This new type of urea can reduce costs greatly since

it increases cost only by about 30e50U t1 above commonurea and it is much cheaper than coated urea

(200e2000U t1). This improvement should make this newtechnique popular, particularly in developing countries. In

order to investigate the slow-release mechanism of this

new type of urea, its structure was analysed using infrared

spectra (IR), scanning electron microscopy (SEM), X-ray

diffraction (XRD) techniques and a static release experiment

designed mainly according to the model of Higuchi (1963).

The affect of the proportion of additives was tested using

the release kinetics data and the results simulated using the

equation of Peppas (Lenaerts, Dumoulin, & Mateescu, 1991;

Peppas, 1985) and the double-exponent equation (Kaunisto,

Marucci, Borgquist, & Axelsson, 2011; Peppas & Sahlin,

1989).2. Materials and methods

2.1. Materials

Bentonite (Zhejiang Fenghong Bentonite Co. Ltd., China)

sieved through a 200 mesh screen was washed with distilled

water, and then dried at 105 C for 8 h before use. Organicpolymer (OP) (chemically pure, Shanghai Chemical Regent

Factory, Shanghai, China) and urea (Shanghai Chemical

Regent Factory, Shanghai, China) were dried at 80 C for 8 hbefore use.2.2. Preparation of bentonite-urea (B-urea), organicpolymer-urea (P-urea) and slow-release urea (S-urea)

Aquantityofbentonite (5%)waspreparedandmixedequally in

the melting urea according to the method in Chinese Patent

Specification (CN201110003090.6). The admixture was taken

into a mould and recrystallised at room temperature; the final

product (B-urea) was dried at 80 C for 8 h before use. Using aquantity of OP (0.15%) to replace the bentonite and the final

product (P-urea) was also dried at 80 C for 8 h before use.OPwithaproportionof bentonite (from1%to5%)wasadded

to the urea andmixed according to the samemethod. The final

product of S-urea was dried at 80 C for 8 h before use.

2.3. Physical and chemical analysis of structure

The common urea and S-urea samples were tested respectively

using IR, SEMandXRD.The IRspectrawereobtained in thewave

number range of 400e4000 cm1 using a Fourier transform IRspectrophotometer (Alpha-T, Bruker Company, Germany). The

SEM images were recorded using scanning electronmicroscope

(Sirion200, FEI Company, USA). The common urea and S-urea

samples were scanned in the angle range of 10e60 on the in-strumentofX-raydiffraction (Xpert, PhilipsCompany,Holland).

The specific surface area of the two samples was also

measured (Ommishop 100CX, Coulter Company, USA) and in-

formationonspecific surfaceareaandpore sizedistributionwas

obtained using Brunauer, Emmett, and Teller equation (1938):

PVP0 P

1VmC

C 1PVmC P0

where P is partial pressure of N2 (Pa); P0 is saturated vapour

pressure of liquid N2 (Pa); Vm is the gas volume adsorbed by

monolayer of sample (ml); V is the total gas volume adsorbed

by sample (ml); C is a constant incorporating the character-

istics of the adsorption.

2.4. Static release experiment in water

The experimental apparatus for determining the static release

of urea in water is shown in Fig. 1. Samples of about 12 g of the

http://dx.doi.org/10.1016/j.biosystemseng.2013.04.001http://dx.doi.org/10.1016/j.biosystemseng.2013.04.001

Fig. 1 e Experimental device for testing static release. 1

Urea sample, 2 sample pipe, 3 thermometer, 4 water, 5

vessel, 6 magnetic stirring rod, 7 magnetic stirring

apparatus.

b i o s y s t em s e n g i n e e r i n g 1 1 5 ( 2 0 1 3 ) 2 7 4e2 8 2276different types of urea (common urea, P-urea, B-urea and S-

urea) were poured into a pipe which was 100 mm long and

10 mm inside diameter with one end closed. After the urea

recrystallised at room temperature the pipe was placed hori-

zontally with 1 l water in the apparatus.

The components of the apparatus were assembled

together according to Fig. 1. The speed of magnetic stirrer was

10 revolutions s1 that should have ensured that the con-centration of the solution was uniform and not affecting

diffusion. At given time intervals the concentration of urea at

3 different positions in the centre of the vessel was deter-

mined. The water temperature was controlled at 25 C and thepipe was turned 90 every 15 min to keep the area of theinterface constant. All the results were based on three

replicates.

2.5. The model of urea static release rate in water

The experiment was designed according to the procedure of

Higuchi (1963). As the solubility of urea was quite large

(120.17 g urea can dissolve in 100 g water at 25 C) and there intotal about 1% urea in the water, the device could fit the hy-

pothesis of Higuchi as an infinite-trap. In this condition,

when the interface between urea and water moves, the con-

centration gradient in the pipe can be ignored and the urea

concentration in the pipe can be assumed uniform and

assumed to be equal to that in the whole vessel.

Urea release data were analysed using the equation by

Peppas (1985):

Qt k tn (1)where Qt is the fraction of active agent released at time t, k a

constant incorporating the characteristics of the carrier-active

agent system, and n the diffusion exponent, indicative of the

transport mechanism.Another model double-exponent equation (Peppas &

Sahlin, 1989) was proposed:

Qt k1 tm k2 t2m (2)where Qt is the fraction of active agent released at time t, k1 is

the diffusion constant, k2 is the dissolving-erosion constant

and m is the diffusion exponent. The first item k1tm indicates

the cumulative release rate by the diffusion, and the second

item k2t2m indicates the cumulative release rate by dissolution

of the auxiliary frame by water.

2.6. Slow-release effect of S-urea in practical condition

Some experiments were carried out to test the slow-release

effect of S-urea in soil at room temperature: 0.5 g common

urea and S-urea were placed respectively in a flowerpot with

200 g light clay soil (d 2 mm, 100% soil field capacity), ureagranuleswereplacedabout10mmdeepand50mmhighon the

soil, and a perforated film was placed over the flowerpot to

reduce volatilisation of water. After 24 h incubation, 50 ml of

water was sprinkled evenly on the sample and this produced a

leachate, the concentration of urea and total N in the leachate

were tested. The leaching process was repeated at intervals of

24hand thewholeexperimentwascarriedoutover twoweeks.

2.7. Determination of urea and total N concentration

The concentration of urea in water was determined according

to method for the determination of urea residues in canned

mushrooms for export in the Specialised Standard of Peoples

Republic of China (SN/T 1004-2001). The complex compound

of urea and p-dimethylaminobenzaldehyde were detected

using a spectrophotometer (UV-2550, Shimadzu Company,

Japan) operating at a wavelength of 440 nm, and the concen-

tration of urea was calculated according to following formula:

Ct 3:7487 A 0:0171mg ml1

where Ct was the concentration of urea, Awas the absorbency

value and the coefficient of determination (R2) was 0.999.

The concentration of total N was determined according to

method for the determination of total N in water in The Spe-

cialised Standard of Peoples Republic of China (GB 11894-89).

The NO3 was also detected using a spectrophotometer (UV-2550, Shimadzu Company, Japan) at wavelengths of 220 nm

and 275 nm.

2.8. Statistical method

The statistical results and the nonlinear fit of Eq. (1) and Eq. (2)

were calculated using Origin software (Origin 8.725, Originlab

Company, USA).3. Results and discussion

3.1. Morphology and physical structure

Figure 2 shows SEM images of the surface of common urea

and S-urea. The surface of common urea was even and its

http://dx.doi.org/10.1016/j.biosystemseng.2013.04.001http://dx.doi.org/10.1016/j.biosystemseng.2013.04.001sandroidHighlight

Fig. 2 e SEM images of themorphology and physical structure. (A and B) the surface of common urea, (C and D) the surface of

S-urea.

b i o s y s t em s e ng i n e e r i n g 1 1 5 ( 2 0 1 3 ) 2 7 4e2 8 2 277molecules ordered forming a uniform layer (Fig. 2A and B). The

bentonitemolecules inside S-urea formed an irregular surface

like a membrane with disordered mesh (Fig. 2C and D). Inor-

ganic molecules may connect with each other mainly by

electrostatic attraction using its double-electronic layer

(Bhattacharjee & Elimelech, 1997; Cai et al., 2009). If bentonite

is dispersed homogeneously by full blending it should form a

lattice structure in three-dimensional space. OP should

dissolve in melting urea and extend its long chain in the so-

lution, in this way it cross-linked the bentonite molecules and

strengthened the lattice frame (Fig. 2D).3.2. Analysis of the physical and chemical character ofS-urea

The IR spectra of S-urea sample were similar to that of com-

mon urea (Fig. 3A). The peaks at 3447 and 3343 cm1 of com-mon urea aswell as of S-urea could be assigned to asymmetric

and symmetric stretching vibration of NH2. The peak at

3250 cm1 of both the two types of urea can be assigned toOeH vibration of absorbed water. The peak at 1688 cm1 canbe assigned to carbonyl (C]O) and 1613 cm1 peak can beassigned to NeH bending vibration and CeH stretching vi-

bration (mainly NeH bending vibration domain) of O]CeNH2(He et al., 2007; Xie et al., 2011). In the finger print zone of

1500e400 cm1, all the peaks were similar.The peaks of 3430 and 1641 cm1 for bentonite can be

assigned to OeH vibration of absorbed water. The peak at

1041 cm1 can be assigned to SieO vibration (Chen, Yang, Luo,& Lu, 2002). It showed that there might be no chemical reac-

tion during the mixing process and the mixture of molecules

could connect with each other mainly by some physical

attraction such as the Van der Waals force, hydrogen bond

and electrostatic attraction.The results from X-ray diffraction of common urea and S-

urea samples are shown in Fig. 3B. Both the samples had the

similar diffraction angle (2q). S-urea had a sharp peak at

22.28, it was a little less intense than that of common urea at22.33, it showed that S-urea had a tighter arrangement ofmolecules than common urea using the Bragg calculation

(Ding & Liu, 1998; Zheng, Zhang, Cai, Fu, & Wang, 2005). That

might be because the bentonite was inserted between the gap

in the urea crystals and this influenced its process of

recrystallisation.

The pore size distribution of common urea and S-urea were

obtained by N2 adsorption and desorption experiment and the

specific surface area was calculated using BET equation. As

showninFig.4A, theshadedarea indicatesthefinalvolumeofN2adsorbed by the sample, the larger shaded area the larger the

surface area. The surface area of commonureawasmuch larger

than that of S-urea. With increasing N2 pressures more gas

comes into contact with the smaller pores and is adsorbed, the

shadedareaunderthedifferential relativepressure indicates the

amount of the pore sizes and their proportion. It indicates that

the pore size of S-urea is distributed mainly in a smaller size

range. The specific surface areas of the two forms of urea were

calculated and this is shown in Fig. 4B; common urea was

1.698m2 g1 and S-ureawas 0.046m2 g1. This result shows that

S-urea has a more compact structure. This reinforces the con-

clusions that the twoadditives occupy thepotential space inside

urea and form a compact lattice structure with network con-

nections that should greatly decrease the specific surface area.3.3. The effect of bentonite and OP on the release of urea

The static release experiment was carried out with the sam-

ples of common urea, P-urea, B-urea and S-urea respectively

using the apparatus shown in Fig. 1. The releasing process of


Fig. 3 e Physical and chemical analysis of the structure. (A)

IR spectra of common urea, S-urea and bentonite. (a)

Common urea; (b) S-urea; (c), bentonite. (B) XRD analysis of

common urea and S-urea. (a) Common urea; (b) S-urea.

Fig. 4 e BET analysis of common urea and S-urea. (A) The

distribution of the pore size of common urea and S-urea. (a)

Common urea; (b) S-urea. (B) Specific surface area of

common urea and S-urea.

b i o s y s t em s e n g i n e e r i n g 1 1 5 ( 2 0 1 3 ) 2 7 4e2 8 2278each kind of sample was tested to investigate the affect of

each of the auxiliary materials, bentonite and OP. Figure 5A

shows the release rate of each type of urea at the specified

intervals of time. The time for common urea to be released

entirely was less than half an hour, the time for P-urea was

about 1 h and that of B-ureawas near 9 h. The release rate of P-

urea was about half that of common urea, but it was still more

rapid than the others. OP can dissolve and extend the length of

its chain in the dissolving urea, making a physical connection

with it, but both OP and urea dissolve easily in water. There-

fore, the release rate from P-urea was similar to that of com-

mon urea as the water infiltrated into its structure. B-urea had

a longer release time than P-urea, this could be because the

bentonite could not dissolve in water and the lattice frame-

work connected by bentonite increased the path length for the

penetration of water. Furthermore, this action could be more

effective as the bentonite particles absorbed water and

swelled (Slade, Quirk, & Norrish, 1991). The final result

comparing the release times of P-urea and B-urea showed thatthe structure of the bentonite was the main factor slowing

down the release of urea.

The release rateofS-ureawas slower thanthat ofB-urea, 75%

of B-urea was released for about 7 h and S-urea with the same

amount was released after almost 14 h, approximately double

time compared of B-urea and almost 28 times that of common

urea. Thismeant that S-urea could have a longer residence time

insoilandthenitrogenreleasedcouldhavemorechanceofbeing

usedbyplants therebyreducingthepotential forwaterpollution.

This result also showed that OP played a very important role

when it was used with bentonite, since it cross-linked the

bentonite particles to form a firm network and strengthened its

lattice structure. These connections were strong and not easily

broken by water because OP was anchored by the bentonite (Li

et al., 2007). Therefore, OP was the auxiliary material that

greatly strengthened the slow release effect of bentonite.3.4. The effect of bentonite amount on the release of S-urea

Bentonitewas themain additive that could influence the release

rate of S-urea, thestatic release experimentwascarried outwith


Fig. 5 e Thecharacteristicofslowrelease. (A)Timedependent

release of urea with different auxiliary addition. (B) Time

dependent release of urea with the amount of bentonite.

Fig. 6 e Release kinetics of S-urea. (A) Correlation results of

equation Peppas. (B) Correlation results of double-

exponent equation.

b i o s y s t em s e ng i n e e r i n g 1 1 5 ( 2 0 1 3 ) 2 7 4e2 8 2 279different amounts of bentonite ranging from1% to 5%. Figure 5B

shows the results of the relationshipbetween the release timeof

ureaandthedifferent amountsof addedbentonite.About75%of

common urea was released within half an hour, the same

amountofS-ureawasreleased forabout 4hwith1%ofbentonite

added, and this time was extended to about 7.5 h and 14 h

respectively for S-urea with 3% bentonite and S-urea with 5%

bentonite. The S-urea had prominent effect on slowing the

releaserateofnitrogenandthiscangreatly increase itsefficiency

of use. The release rate of urea was remarkably reduced by

increasing the amounts of bentonite (from 1% to 5%). Bentonite

was diffused evenly within the melting urea, its particles were

unrestricted and stable and they deposited in the space of urea

molecules. The more the quantity of bentonite used, the more

compact the lattice structure which decreased the specific sur-

face area and pore size (Ding & Liu, 1998). The release rate could

therefore be controlled to accommodatedifferent requirements.3.5. Analysis of slow release kinetics

Figure 6 shows the release kinetics of S-urea (5% bentonite

and 0.15% OP) and the simulation results using the Eq. (1)(Fig. 6A) and Eq. (2) (Fig. 6B). The parameters of Eq. (1) were

characterised by the values of k was 7.5939 and n was 0.8687

(with R2 0.9988), indicating that the release of S-urea did notagree with the Higuchi model. That suggests that the release

of S-urea should not vary with the square root of time, as with

Fickian diffusion, but as with anomalous diffusion (Peppas,

1985; Peppas & Sahlin, 1989). Using Eq. (2) supported this

point and differentiated the mechanism from the Fickian

diffusion and dissolving-erosion diffusion by the values of

k1 0.5650, k2 7.1202 and m 0.4421 (with R2 0.9988). Theratio k2t

2m divided k1tm was 39.52 at 13 h (75% of the total

amount of urea), this showed that the dissolution of the

auxiliary frame eroded by water was the main factor con-

trolling the release (Kaunisto et al., 2011; Peppas & Sahlin,

1989).

The data for all the types of urea shown in Fig. 5A were

correlated using Eq. (1) and Eq. (2). The kinetic parameters are

shown in Table. 1.

The correlation results of Eq. (1) showed that the release

mode of all the types of urea tested was not Fickian diffusion

but anomalous diffusion. It indicated that, even including the

sample of common urea, all of the release process was

controlled not only by the concentration diffusion but also by

some other mechanism that may be associated with the


Table 1 e Kinetic parameters of Fig. 5A using Eq. (1) andEq. (2) respectively.

Type ofsample

Kinetic parametersfor Equation (1)


n R2 Ratioa R2

Urea 0.8245 0.9472 0.37b 0.9968

P-urea 0.9104 0.9955 2.62b 0.9988

B-urea 0.9647 0.9954 8.93c 0.9952

S-urea 0.8687 0.9988 39.52c 0.9988

a : k2t2m divided k1t

m.

b : Time for 90% urea release.

c : Time for 75% urea release.

b i o s y s t em s e n g i n e e r i n g 1 1 5 ( 2 0 1 3 ) 2 7 4e2 8 2280porous structure of the urea. The results also indicated that

additives may affect the release process of urea by strength-

ening its original structure.

The cumulative release rate of Fickian diffusion and

dissolving-eroding diffusion were calculated using Eq. (2). The

coefficients of determination (R2) all approached 0.99, showing

that this equation may describe the release process well and

could be used to explain the mechanism. However, the true

process is probably more complex and this does not agree

with the hypothesis referred to by Lee (2011); some results

expressed that the value of the release quantity of urea were

even calculated as being negative. If using the absolute value

to denote the degree of these two release modes, namely the

ratio of k2t2m divided k1t

m, it increased regularly according to

the sequence commonurea, P-urea, B-urea and S-urea (shown

in Table. 1). This further confirmed the conclusion that the

dissolving-eroding effect becomes more and more prepon-

derant with more compact structures reinforced mainly by

bentonite.

Dealing with the data shown in Fig. 5B in the same way, a

similar result can be found in Table. 2. It also showed that

increasing the proportion of bentonite could strengthen the

structure of urea, at the same time increasing the influence of

dissolving-eroding diffusion. The ratio of S-urea (1%) was less

than P-urea as the structure of the latter was not more

compact, this abnormal result might because that OP diffused

into the water and its chain structure increased the local

viscosity, and this could affect the diffusion process of urea.

On the other hand, a low concentration of bentonite (1%) didTable 2 e Kinetic parameters of Fig. 5B using Eq. (1) andEq. (2) respectively.

Type ofsample



n R2 Ratioa R2

Urea 0.8245 0.9472 0.37b 0.9968

SeU(1%) 0.9979 0.9928 0.60c 0.9936

SeU(3%) 0.7548 0.9976 7.41c 0.9976

SeU(5%) 0.8687 0.9988 39.52c 0.9988

a k2t2m divided k1t

m.

b Time for 90% urea release.

c Time for 75% urea release.not increase the compact of structure as much as 5%

bentonite did and it can flocculate with OP and decrease the

affect of viscosity (Xiao & Cezar, 2003).3.6. Slow-release effect of S-urea under practicalconditions

The result of Fig. 7 shows that during the first 4 days, the

quantity of urea and total N in leaching solution of S-urea

was much less than that of common urea; it was about only

half of the latter. Also the remaining nutrition in soil of S-

urea provided a steady release velocity during the next 10

days as common urea did. This result shows that S-urea

might have some slow release effect under practical condi-

tions and this affect may last longer in soil than it does in

water. It greatly increases the feasibility of using S-urea in

agricultural production. The reason might be that OP can

connect between the soil particles during the release process

as it did with bentonite, in this way the local circumstances

in the soil around the fertiliser might be changed and it could

affect the diffusion of water and change the process of

nutrition.Fig. 7 e Slow-release effect of S-urea in soil. (A) The

leaching loss of urea under practical conditions. (B) The

leaching loss of total N under practical conditions.


b i o s y s t em s e ng i n e e r i n g 1 1 5 ( 2 0 1 3 ) 2 7 4e2 8 2 2814. Conclusions

A new type of slow-release urea was produced and tested. IR

analysis showed that the two additives used, bentonite andOP,

did not react and no new chemical bond appeared. The addi-

tives connected mainly by the Van der Waals force, hydrogen

bond and electrostatic attraction. A lattice structure came into

being after the urea recrystallised at room temperature.

Bentonite accumulated in the space of urea, or inside its crys-

tals, and linked together as the SEM image showed. OP can

strengthen this connection by setting up a bridge between the

bentonite aggregates forming a network. The new type of urea

had a lower specific surface area and its pore size was distrib-

uted in a smaller range and its larger stacking density could

increase the path length for water. Kinetic simulation of the

results using the Peppas and the double-exponent equations

showed that the release rate of this type of urea was mainly

affectedbydissolving-erodingprocesswhichmaybecontrolled

by compactness of the lattice structure and this trend may be

strengthened by increasing the amount of bentonite.

Acknowledgement

WethankDr. FanghuaLi forhisvaluablediscussionsandcareful

revisions. This work was supported by the National Agriculture

Transformation Fund of China (No. 2010GB2C300185) and the

Directional Project of Chinese Academy of Sciences.r e f e r e n c e s

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A novel slow-release urea fertiliser: Physical and chemical analysis of its structure and study of its release mechanism1. Introduction2. Materials and methods2.1. Materials2.2. Preparation of bentonite-urea (B-urea), organic polymer-urea (P-urea) and slow-release urea (S-urea)2.3. Physical and chemical analysis of structure2.4. Static release experiment in water2.5. The model of urea static release rate in water2.6. Slow-release effect of S-urea in practical condition2.7. Determination of urea and total N concentration2.8. Statistical method

3. Results and discussion3.1. Morphology and physical structure3.2. Analysis of the physical and chemical character of S-urea3.3. The effect of bentonite and OP on the release of urea3.4. The effect of bentonite amount on the release of S-urea3.5. Analysis of slow release kinetics3.6. Slow-release effect of S-urea under practical conditions

4. ConclusionsAcknowledgementReferences

A Novel Slow-release Urea Fertiliser Physical and Chemical Analysis of Its Structure and Study of...

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