Review on materials & methods to produce controlled release coated urea fertilizer

Journal of Controlled Release 181 (2014) 11–21

Contents lists available at ScienceDirect

Journal of Controlled Release

j ourna l homepage: www.e lsev ie r .com/ locate / jconre l

Review

Review on materials & methods to produce controlled release coatedurea fertilizer

Babar Azeem ⁎, KuZilati KuShaari, Zakaria B. Man, Abdul Basit, Trinh H. ThanhDepartment of Chemical Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia

⁎ Corresponding author. Tel.: +60 16 6171584.E-mail address: [email protected] (B. Azeem).

http://dx.doi.org/10.1016/j.jconrel.2014.02.0200168-3659/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t
a r t i c l e i n f o
Article history:Received 8 January 2014Accepted 21 February 2014Available online 1 March 2014

Keywords:Controlled release coated ureaSlow release ureaUrea coatingUrea release mechanismControlled release fertilizer

With the exponential growth of the global population, the agricultural sector is bound to use ever larger quanti-ties of fertilizers to augment the food supply, which consequently increases food production costs. Urea, whenapplied to crops is vulnerable to losses from volatilization and leaching. Current methods also reduce nitrogenuse efficiency (NUE) by plants which limits crop yields and, moreover, contributes towards environmentalpollution in terms of hazardous gaseous emissions and water eutrophication. An approach that offsets thispollution while also enhancing NUE is the use of controlled release urea (CRU) for which several methods andmaterials have been reported. The physical intromission of urea granules in an appropriate coating material isone such technique that produces controlled release coated urea (CRCU). The development of CRCU is a greentechnology that not only reduces nitrogen loss caused by volatilization and leaching, but also alters the kineticsof nitrogen release, which, in turn, provides nutrients to plants at a pace that is more compatible with theirmetabolic needs. This review covers the research quantum regarding the physical coating of original ureagranules. Special emphasis is placed on the latest coating methods as well as release experiments andmechanisms with an integrated critical analyses followed by suggestions for future research.

© 2014 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111.1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111.2. Controlled release fertilizers (CRFs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.2.1. Classification of CRFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.2.2. Mechanism of controlled release . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.2.3. Advantages and disadvantages of CRFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

1.3. Why controlled release coated urea (CRCU)? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132. Materials and methods for CRCU production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.1. CRCU from sulfur based coating materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.2. CRCU from polymer based coating materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.3. CRCU from superabsorbent/water retention coating materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.4. CRCU from bio-composite based coating materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.5. Commercially available controlled release coated urea (CRCU) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3. Conclusion and suggestions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

1. Introduction

1.1. Background

There has been exponential growth in the earth's population thathas now reached approximately 7.0 billion [1] and is expected to

http://crossmark.crossref.org/dialog/?doi=10.1016/j.jconrel.2014.02.020&domain=pdf

http://dx.doi.org/10.1016/j.jconrel.2014.02.020

mailto:[email protected]

http://dx.doi.org/10.1016/j.jconrel.2014.02.020

http://www.sciencedirect.com/science/journal/01683659

Fig. 1. Classification of controlled release fertilizers.

12 B. Azeem et al. / Journal of Controlled Release 181 (2014) 11–21

approach 9.5 billion by 2050. Global food requirements have alsorisen and the expected per capita food requirement is likely to dou-ble by 2050 [2]. Meanwhile, arable lands diminish due to industrial-ization, urbanization, desertification and land degradation fromheavy flooding [3]. These intimidating factors threaten global food secu-rity and demand a robust response. Multidimensional steps have alreadybeen taken worldwide to meet the challenge of food security with mod-ifications to improve agricultural systems. To meet the increasing fooddemands, the agricultural sector is bound to employ enormous quantitiesof fertilizers that have thus far demonstrated undesirable environmentalimpacts. Hence, it is of paramount importance to develop systems thatboost production and alleviate environmental problems [4]. Controlledrelease fertilizers may be one such solution as they are believed toenhance crop yield while reducing the environmental pollution causedby the hazardous emissions (NH3, N2O etc.) from current fertilizerapplications [5].

1.2. Controlled release fertilizers (CRFs)

Controlled release fertilizer (CRF) is a purposely designed ma-nure that releases active fertilizing nutrients in a controlled, delayedmanner in synchrony with the sequential needs of plants for nutri-ents, thus, they provide enhanced nutrient use efficiency alongwith enhanced yields [5]. An ideal controlled release fertilizer iscoated with a natural or semi-natural, environmentally friendlymacromolecule material that retards fertilizer release to such aslow pace that a single application to the soil can meet nutrient re-quirements for model crop growth [6]. The terms, controlled releasefertilizer (CRF), and slow release fertilizer (SRF), are generally con-sidered analogous. Nevertheless, Trenkel [7] and Shaviv [5] defineddifferences between both. In the case of SRFs, the pattern of nutrientrelease is nearly unpredictable and remains subject to changes in soiltype and climatic conditions. To the contrary, the pattern, quantity,and time of release can be predicted, within limits, for CRFs. Howev-er, in this study, we use the term “Controlled release fertilizers”(CRFs) for both types. A rigorous literature review reveals that thehistory of CRFs' development and evolution has roots in the early1960's [8]. Initially, sulfur and polyethylene were used as coatingmaterials in the preparation of SRFs. This journey eventually includ-ed numerous polymer materials, natural coating agents, multifunc-tional super-absorbent materials, and even nano-composites. Manyof the CRFs have also been prepared on commercial scale so far.

1.2.1. Classification of CRFsThe CRFs have been classified in a diverse manner according

to the literature. A comprehensive classification has been basedon the opinions of Shaviv [5], Trenkel [7], Liu [9] and Rose [10] asportrayed in Fig. 1. Comprehensively, CRFs were classified into threemajor categories:

1. Organic compounds that are further sub-divided to natural organiccompounds (animal manure, sewage sludge etc.) and syntheticallyproduced organic-nitrogen, low solubility compounds. The lattergroup generally includes condensation products from urea andacetaldehyde. These compounds are further subdivided into biologi-cally decomposing compounds, e.g. urea formaldehyde (UF), andchemically decomposing compounds such as isobutyledene-diurea(IBDU) or urea acetaldehyde/cyclo diurea (CDU).

2. The second major category includes renowned water solublefertilizers with physical barriers that control nutrient release. Theseappear either as granules/cores coated with a hydrophobic polymer,or as a matrix of active fertilizer nutrients dispersed on a continuumvia hydrophobic material that encumbers fertilizer dissolution.However, controlled release matrices are less common comparedto coated CRFs, which is why this paper is focused on controlledrelease coated fertilizer that contain only urea. Coated granular CRFs

are subcategorized to those coated with organic polymer materials(e.g. thermoplastics, resins etc.) and those coated with inorganicmaterials (including sulfur and otherminerals). The controlled releasematrix material can be either hydrophobic e.g. polyolefin, rubber etc.,or gel forming polymers sometimes referred to as a hydrogels. Ahydrogel is hydrophilic and the dissolution of fertilizer dispersedthrough hydrogel material is impeded by its ability to retain highamounts of water (swelling).

3. Lastly are inorganic low solubility compounds that include metalammonium phosphates, e.g. KNH4PO4 and MgNH4PO4, and partiallyacidulated phosphate rock (PAPR).

A classification of CRFs can also be based on the mode of controlrelease, i.e. diffusion, erosion or chemical reaction, swelling and osmosis.Blaylock [11] however, classified CRFs as only two major types: thosecoated with low solubility compounds and those coated with watersoluble materials.

1.2.2. Mechanism of controlled releaseIt is vital to become acquainted with the mechanism of controlled

release, the direct measure of the effectiveness of a CRF. Generally, thecontrolled release mechanism is difficult to conceive as it depends onnumerous factors such as the nature of the coating material, the typeof CRF, agronomic conditions and much more. Different mechanismsare cited in the literature and these are still under development. Liu[9] and Shaviv [5] proposed a release mechanism for coated fertilizerscalled the multi-stage diffusion model. According to this model, afterapplying the coated fertilizer, irrigation water penetrates the coatingto condense on the solid fertilizer core followed by partial nutrientdissolution (Fig. 2). Subsequently, as osmotic pressure builds withinthe containment, the granule consequently swells and causes twoprocesses. In the first, when osmotic pressure surpasses threshold mem-brane resistance, the coating bursts and the entire core is spontaneouslyreleased. This is referred to as the “failure mechanism” or “catastrophicrelease”. In the second, if themembranewithstands the developing pres-sure, core fertilizer is thought to be released slowly via diffusion forwhichthe driving force may be a concentration or pressure gradient, or combi-nation thereof called the “diffusion mechanism”. The failure mechanismis generally observed in frail coatings (e.g. sulfur or modified sulfur),

Fig. 2.Diffusionmechanism of controlled release; (a) Fertilizer corewith polymer coating,(b) Water penetrates into the coating and core granule, (c) Fertilizer dissolution and os-motic pressure development, (d) Controlled release of nutrient through swollen coatingmembrane.

13B. Azeem et al. / Journal of Controlled Release 181 (2014) 11–21

while polymer coatings (e.g. polyolefin) are expected to exhibit thediffusion release mechanism.

A pictographic representation of both mechanisms is given in Fig. 2.The controlled release of nutrients also depends on ambient tempera-ture and moisture with the release rate increasing at higher tempera-tures with greater moisture content [10]. The coated fertilizer releasemechanism is basically a nutrient transfer from the fertilizer–polymerinterface to the polymer–soil interface, driven by water. The governingparameters for the release mechanism are: (i) diffusion/swelling; (ii)degradation of the polymer coating, and (iii) fracture or dissolution. Asimilar release mechanism was presented by Guo [12], Liang [13], Liu[14], and Wu [15].

Fig. 3. Post dressing transformation of urea in soil.

1.2.3. Advantages and disadvantages of CRFsControlled release fertilizers generate savings in fertilizer quanti-

ty and the labor of application frequency because only a single appli-cation is required for the growth season. They also inhibit nutrientloss, seed toxicity, hazardous emissions, leaf burning, dermal irrita-tion, and inhalation problems. In addition, they improve soil quality,handling properties and germination rates. On the other hand, theyare expensive and pose marketing issues. Furthermore, some of thecoating materials used to produce CRFs are non-biodegradable andtoxic to the soil. In most cases, the release pattern is also uncertainin field applications. Some CRFs also drastically change the soil'spH, which is undesirable. Storage facilities also need modificationto avoid pre-mature nutrient release caused by moisture absorbancethrough fissures that result due to the attrition of granules [5,8–10,16].There are number of issues preventing widespread adoption of CRFsin their current state. The application of CRFs is limited by a lack ofdata regarding the release kinetics of CRF in various types of soiland environmental conditions of interest to the agriculture industry.Current CRFs are vulnerable to changes in temperature, ambientmoisture, bioactivity of the soil, and wetting and drying cycles ofthe soil. Changes in any of these conditions will make the releaserate of the fertilizers unpredictable and will negatively affect the ef-ficiency of the fertilizer release, especially if the release rate has beencalibrated for a specific kind of crop. In addition, CRFs do not responddirectly to the plant's demand for nutrients and release nutrients atthe same rate regardless of whether a plant is demanding more nu-trients or none at all.

1.3. Why controlled release coated urea (CRCU)?

Urea is the most widely used fertilizer globally because of its highnitrogen content (46%), low cost, and ease of application [7,17]. There-fore, the development of CRCUhas been a subject of interest for decades[18]. When applied to the soil, urea undergoes a series of biological,chemical and physical transformations to produce plant availablenutrients as follows [7].

NH2ð Þ2COþ 2H2O→Urease

NH4ð Þ2CO3 ð1Þ

NH4ð Þ2CO3 þ 2Hþ→Ammonification

2NH4þ þ CO2 þH2O ð2Þ

2NH4þ þ 3O2→

Nitrosomonas=nitrosococus bacteria2NO2

− þ 2H2Oþ 4Hþ þ Energyð3Þ

2NO2− þ O2→

Nitrobacter bacterium=nitrification2NO3

− þ Energy

ð4Þ

NO3−→

Microorganisms=O2deficient soil N2 þ N2O ð5Þ

NH4þ→

Urease enzyme=Basic soil pHNH3 gð Þ þ Hþ ð6Þ

The 2nd and 4th reactions produce required nutrients for plants.Since plants need only a small quantity of food during early growth,excess nutrients are lost due to leaching. In the 5th and 6th reactions,the nitrogen is lost through hazardous gaseous emissions. Theproduction of a more suitable CRCU is therefore needed to solvethese problems. A pictorial representation of these transformationsis shown in Fig. 3.

Hitherto, various perspectives on slow/controlled release fertilizershave been discussed in different reviews and book chapters. Ussiri [19]presented fertilizer management strategies to boost plant nutrient-useefficiency and reduce nitrous oxide emissions. Davidson [16] wrote onthe control releaseminutiae of nutrients N, P, K, Mg, Zn, and the commer-cial availability of CRFs alongwith somedescription of crops that use CRFsfor nutrition. Trenkel [7] published a review on different options toenhance the nutrient use efficiency of plants, including the use ofCRFs, urease inhibitors, nitrification inhibitors as well as economic andlegislative aspects concerning these materials. A similar review on theimprovement of nutrient-use efficiency with an additional segment forthe controlled release of phosphorus fertilizers was presented by Chien[4]. The use of controlled release nitrogenous fertilizers, specifically forvegetable crops, was discussed by Guertal [20]. Akiyama [21] employed


a beta analysis to evaluate how successfullymodified fertilizers alleviatednitrous oxide emissions. Similarly, Yan [22], Puosi [23], Liu [9], Blaylock[11], Shaviv [5], Sartain [24], Tharanathan [25] and Rose [10] shed lighton different aspects of coated urea and other controlled release fertilizers.

Despite the extensive literature on CRFs, there remains copious roomto stretch research frontiers further for controlled release coated urea(CRCU) products. This paper attempts to elaborate on the knowledgepool within the context of CRCU. Our major focus is on the coatingmaterials, coating methods, release experiments and a critical analysisof the controlled release mechanism for CRCU. We limited our review tocontemporary and 21st century research. However, to establish apprecia-tion for the historical evolution of CRCU, a few papers from the distantpast were included. To enhance the reader's grasp of the subject, this re-view has been divided into sections according to types of coating mate-rials used to produce controlled release coated urea (CRCU).

2. Materials and methods for CRCU production

2.1. CRCU from sulfur based coating materials

Initially, sulfur was an attractive candidate for urea coating due toits several advantages. It is generally claimed that the foremost sig-nificant work on urea coating was accomplished by Blouin et al. forthe Tennessee Valley Authority (TVA), USA. With a goal to encasegranular urea and impart controlled release characteristics, Blouin[6] provided a strong platform that established a cost effective, up-scale production process for the sulfur coating of urea. Initially,urea granules were impregnated by a petroleum by-product (e.g.petrolatum, motor oil, soft wax, etc.) to act as an impervious sealantand sub-coating. A vacuum was then applied to cause the sealantmaterial to penetrate the granules more thoroughly. The sealantwas considered amobile component that prohibited urea dissolutionby filling small channels via capillary action. In turn, the urea wastumbled in a second rolling drum and spray-coated with molten sul-fur. Finally, the sulfur coated urea (SCU) was subjected to a thirdcompartment wherein plasticizers (e.g. polyethylene or polyvinylacetate) adhered to the sulfur shell to aid the spreading and fusionof the sulfur layer and decrease crack formation. For some products,the addition of a plasticizer was proposed as a substitute using inex-pensive, finely divided powders (for example, talc or vermiculite) torender a uniform sulfur layer and decrease the incidence of layercracking. To achieve a comparative study, urea was coated withpetrolatum-only or sulfur-only. A 24-hour dissolution test in waterwas done to evaluate coating effectiveness. The authors found thatthe oil-only coating was absolutely ineffective to withstand waterpermeability. The sulfur-only coating was mildly effective, whereasa combination of both gave effective controlled release results. Thecoating shell with an oil to sulfur ratio of 3:21 withstood water themost with only 1% dissolution in 24 h. Despite the controlled releaseadvantages, this study still had a challenge to address. The presenceof the sealant sub-coating could not negate the need for a uniformsulfur coating. If the sulfur coating was not sufficiently uniform toavoid fissuring, the urea substrate dissolved within minutes, evenin the presence of the sealant sub-coating.

In 1968, Rindt et al. [18] reported that the addition of plasticizers onlymoderately reduced thewater permeability of the sulfur coating and thatthe sulfur solidification period was extended while its tackiness wasworsened by plasticizers. This problem was addressed by applying amicrocrystalline wax coupled to microbicides. This involved a three stepprocess by which molten sulfur was initially sprayed on a rolling bed ofurea granules in anundulating drum, afterwhichmoltenwaxwas pouredon the sulfur coated granules. They believed the wax coating was subjectto attack by soil microorganisms. Hence, 0.5–2% of microbicides (e.g.pentachlorophenol or coal tar) was added to the wax to combat bacterialattack. Lastly, in order to enhance flowwhile avoiding tackiness from thewax, a conditioner was dusted onto the cooled coated granules. The

addition of about 1% diatomaceous earth (kaoline clay or vermiculite)was used for this purpose. Twenty-four hour and longer dissolutiontests revealed dissolution rates of 3.5–42% and 0.8–1.1%, respectively.The higher dissolution rate for the 24 h trial was attributed to smallerparticle size.

The aforementioned work was up-scaled to plant capacity(300 lb/h) by the same authors [8]. They then followed the sametechnique for a coating and dissolution study that focused on evalu-ating optimal parameters for a more effective sulfur coating. Theydiscovered that coating thickness was reciprocal to dissolution rate.Urea particle size distribution also had an inverse effect; i.e. smallergranules dissolved earlier than larger counterparts. The higher disso-lution of small particles was due to granule sphericity as the surfaceto volume ratio of smaller spheres is greater compared to largerspheres. Therefore, with an equal amount of coatingmaterial appliedto both small and large granules, smaller granules received thinnercoating which then granted quicker dissolution. The effect of higherair pressure permitted a finer coating which also enhanced dissolu-tion rates. As for coating effectiveness, their ‘seven day dissolutiontest’ became a reference point thereafter for other researchers. Thistest measured the amount of urea released by a 250 g coated sampleimmersed in 250 ml of water at 100 °F for seven days.

Another study by Tsai at the University of British Columbia, aimed todevelop a process for coating urea with sulfur using a spouted fluidizedbed [26]. A ‘sulfur-only’ coating was applied and optimal process condi-tionswere evaluated to attain reasonably controlled release characteris-tics. Ureawas coatedwithmolten sulfur concurrently with fluidizing airin a spouted bed under certain conditions of temperature and pressure.Optimized conditions eventually comprised 80 °C, fluidizing air flow at0.65 m3/min, and pressurized atomizing air at 208 kPa. Their seven-daytrial found 30% urea dissolution. Akin to Tsai [26], in 1997 Choi [27] alsostudied urea coating with sulfur and derived parameters that predomi-nantly affected the coating performance of a spouted bed. Urea coatingwith molten sulfur was carried out in batch as well as continuous oper-ations and then followed by the seven-day dissolution test. Nitrogenpressurized molten sulfur was introduced with pre-heated atomizingair at the base of the spouted bed and sprayed onto the urea fluidizedbed concurrently followedby drying andwithdrawal. He recommendeda spray angle of forty degrees and the use of multiple spouted beds aswell as an extended coating period for better results in terms of coatinguniformity, which directly affects the controlled release characteristicsof coated urea. The TVA dissolution test findings saw aminimal dissolu-tion of 32.8% at seven days.

Ayub [28] prepared sulfur coated urea in a 2-D spouted bed andevaluated the effects of spouting air temperature, atomizing air, andliquefied sulfur flow rates on the quality of sulfur coated urea in termsof the dissolution rate. He posited that the dissolution rate was a func-tion of spouting air temperature but the rate remained unaffected bythe atomizing air flow rate. For example, dissolution was 100% and95.61% at spouting air temperatures of 69 °C and 82.5 °C at atomizingair flow rates of 1.0 and 1.4 m3/h, respectively. The major dependenceof coating quality on spouting air temperature was determined interms of sulfur's behavior at different temperatures. At lower spoutingair temperatures, the exterior of the sulfur particles solidified prior tocoating the urea's surface. To the contrary, sulfur particles close tomelt-ing point solidified after coating the urea granules resulting in a moreuniform coating layer. The seven-day dissolution trial resulted in amin-imal level of 95.61% at the highest spouting air temperature (82.5 °C),with a sulfur flow rate of 33.9 g/min and atomizing air flow rate of1.4 m3/h. Orthorhombic (Beta) sulfur is amorphous in nature andmost suitable for encasing other polymer materials to enhance coat-ing longevity. Whereas monoclinic (Alpha) sulfur is crystalline andsubject to cracks and fissures which reduce coating life. Moreover,beta sulfur readily converts to alpha sulfur at about 60 °C. To retardthe transformation from the amorphous to crystalline phase andthereby strengthen sulfur coating against cracking and deformation,


Liu [29] produced a dicyclopentadiene-modified (DCPD-modified) sulfurcoated urea in afluidized bed. TheDCPD-modified sulfurwas obtained bysimply mixing DCPD and sulfur at elevated temperatures for 1–6 h. Toevaluate release characteristics, a certain amount of the coated urea waspoured into a deionized water beaker kept at constant temperature andsealed with polyethylene film to avoid evaporation. A certain volume ofwater was periodically taken from beaker at regular intervals to analyzenitrogen concentration via spectrophotometry. The reduced watervolume in the beaker was replaced with fresh water. The seven-dayrelease rate of the sulfur-only coated urea was about 83% while therelease rate for DCPD-modified sulfur coated urea was 53.5%, thus, givinga comparatively far better result.

Another technique was introduced by Detrick [30] for urea coat-ing with sulfur as an inner coat to a secondary polymer coating.Here, the innovation permitted monomers to react on the surfaceof sulfur coated granules to form a polymer over-coating. The sulfurcoating was then protected by the secondary polymer coat whichproved more resistant to tackiness and mechanical degradationscaused by impacts, abrasion, handling, transportation, storage etc.It presented good controlled release characteristics when comparedto sulfur coated urea with an outer polymer coating. Urea granuleswere initially preheated in a fluidized bed and then spray coatedwith sulfur in a heated rotary drum. The resultant granules werethen sprayed with diethylene glycol triethanolamine polyol anddiisocyanate monomers in a second rotating drum with multiplenozzles. The seven-day dissolution rate was 38%. Detrick expandedthis study to present another approach [31] that surpassed the previ-ous in terms of enhanced controlled release attributes for sulfurcoated urea. A triple layer coated urea was produced with an innerlayer formed by on-surface polymerization of certain monomers(4,4-diphenylmethane diisocyanate, triethanolamine and diethyleneglycol polyols). This was followed by a second layer of molten sulfurand a third outer layer produced by on-surface polymerization of theaforementioned monomers. The controlled release period more thandoubled that of the previous technique's result. Further work on ureacoating with sulfur can be consulted in the following U.S. Patents:3567613 (Fleming), 4636242 (Timmons), 3697245 (Dilday), 4676821(Gullett), 4142885 (Heumann), 4857098 (Shirley), 5219465 (Goertz),5405426 (Timmons) and 5454851 (Zlotnikov).

Sulfur has been used to produce CRCU for decades. It also acts as asecondary plant nutrient and fungicide. It further possesses acidicproperties that neutralize soil alkalinity. It is also a relatively cheapmaterial that reduces the caking tendency of many fertilizers.When contrasted with many polymer materials used for urea coat-ing, it is biodegradable [6,26]. On the other hand, the crystalline na-ture of sulfur leads to the development of microscopic pores andcracks that induce significant brittleness [18,31], and is also proneto higher friability when subjected to elevated temperatures in thesoil. Due to its inherently augmented surface tension, sulfur coatingappears to possess low wettability and adhesion to the urea sub-strate [26]. The sulfur-only coating is, therefore, not an effective seal-ant and requires additional conditioning materials that become vitalfor its application to urea granules which poses economic con-straints. Since sulfur shells left in the soil are not immediately inte-grated, an excessive amount of sulfur may build up and react withwater to acidify the soil [30].

Themechanism for the controlled release of sulfur coated urea com-prises two steps: the burst effect and then continual release by diffusionas mentioned in Section 1.2.2.

2.2. CRCU from polymer based coating materials

Following the affair with sulfur, polymeric materials were widelyused to coat urea since sulfur coatings were easily disrupted by mi-croorganisms whereas polymer coatings were not. The nutrient re-lease from polymer coating is affected by diffusion as a function of

coating thickness and soil temperature. However, polymer spray coatinginvolves organic solvents that not only inflict additional costs of the leansolvent and solvent recovery, but also cause hazardous environmentalemissions. Hence, the use of aqueous polymeric solutions was initiatedto counter these issues. Donida [32] studiedurea coating using a commer-cially available aqueous polymeric material called Eudragit L30-D55®(methacrylic acid copolymers) in a two dimensional spouted bed withtop spray orientation. The coating's composition is given in Table 1.Eudragit L30-D55® is mixed with water in addition to: talc, esthearatesof magnesium, triethyl citrate, polyethylene glycol, and titanium dioxideto produce the CRCU. Higher atomizing air pressure and fluidizing airtemperature produced a uniform coating film due to the production ofsmaller droplets and improved spreading of the suspension, respectively,resulting in a homogeneous layer. The coating thickness also impartedcontrolled release characteristics as it increased in thickness at a highercoating suspension rate and atomizing air pressure, but decreased withincreased fluidizing air flow rate and temperature. However, elutriationwas also caused at elevated air temperatures due to the pre-mature dry-ing of droplets before contacting the granules' surface.

The impermeable film on urea granules formulated by EudragitL30-D55® was not effective for soils with a pH N5.5. Also, low tem-peratures and high flow rates caused a rough coating surface. There-fore, the need arose to optimize processing conditions usingdifferent coating compositions as mentioned in Table 1 [33]. In thiscase, nutrient release was measured by a static capture system inwhich filter paper soaked in H2SO4 was used to capture evolved ni-trogen and an empirical process was used tomeasure its loss. The op-timal fluidizing air temperature was 74 °C; the optimal suspensionflow rate was 11 ml/min; and the optimal atomizing air pressurewas 68.95 kPa; which produced a controlled release of evolved nitro-gen at 3–57%. The authors extended this research by using vinasse asthe solvent instead of water for the aqueous polymeric suspension[34]. Vinasse is an effluent of the ethyl alcohol industry that preventspollution when used as an ingredient of the coating solution. Addi-tionally, it also contains the plant nutrients nitrogen, potassium, cal-cium and magnesium. Therefore, Rosa [34] used vinasse instead ofwater to prepare a coating suspension from Eudragit with the samecomposition (Table 1) as previously reported [33]. The equipment,as well as the coating method and nitrogen volatilization measure-ments were also the same. The coating process was successfully car-ried out with the use of vinasse as a solvent and achieved a decreasein nitrogen volatilization up to 57%.

The permeability of water and urea in the coating film is a factorthat governs release rate, release time, and release pattern. Lan [35]studied the effects of various process parameters on the film's struc-ture and permeability by coating urea granules in a Wurster type,fluidized bed apparatus. Polyacrylic acid latex with 40% solids wasused as the coating solution. At elevated fluidizing air temperatureand atomizing gas pressure, the coating film had a porous structureattributed to the poor spreading and pre-mature drying of droplets.Similarly, higher spraying rates resulted in reduced dewatering ca-pacity by forming large pores on the coating's surface leading topoorly controlled release.

Wu [36] reported that thicker coating layersmay damage soil qualityif they are not degraded in parallel with nutrient release. With this inmind, urea coating with polyurethane is costly but its thinner coatinglayer was said to reduce coating cost by coating greater quantities ofurea granules with less material. Coating was done in a rotating drumso that isocyanate, polyols and wax were added to urea granules for acertain period. The reaction between isocyanate and polyols formed a10–15 μm thick polyurethane layer on the granules while paraffinacted as lubricant to facilitate the process. Water dissolution and soilincubation experiments revealed a 10% dissolution over the first tendays with 70–80% dissolution in thirty days followed by total releaseby forty–fifty days. The release mechanism was the same as mentionedby Shaviv [5].

Table 1Composition of coating suspension.

Ref. Weight % composition of coating suspension

32 Eudragit (16.7%), polyethylene glycol (0.75%), triethyl citrate (0.5%), esthearateof Mg (1%), titanium dioxide (1.8%), pigment (0.2%), talc (2.75%), water (76.3%)

33 Eudragit (25%), polyethylene glycol (0.75%), triethyl citrate (0.5%), esthearate ofMg (3%), titanium dioxide (1.8%), pigment (0.2%), talc (3%), water (65.75%)

34 Eudragit (25%), polyethylene glycol (0.75%), triethyl citrate (0.5%), esthearate ofMg (3%), titanium dioxide (1.8%), pigment (0.2%), talc (3%), vinasse (65.75%)


Due to higher costs and process complexity along with issues ofenvironmental pollution caused by polymers, research frontiers shiftedtowards developing low cost, easily fabricable and environmentallyfriendly materials [37]. Although the price of starch based coating waslow, so was nutrient release longevity compared to polymer coatingformulations, and furthermore, they were occasionally incompatiblewith crop metabolic needs [38]. Yang [38] employed waste polystyrene(thermocol) as a coatingmaterial mixed with wax and polyurethane assealants for a more cost effective and controlled release urea fertilizer.Initially, polyurethane was prepared by dissolving and agitatingdiphenylmethane diisocyanate in ethyl acetate and castor oil, afterwhich polyurethane was mixed with ethyl acetate-dissolved-polystyrene. Urea granules were then spray coated with this solutionin a Wurster fluidized bed followed by oven drying at 40 °C for 24 h toremove excess ethyl acetate. Nitrogen release was measured in stillwater at 25 °C using the Kjeldahl method. The release rate slowed withgreater coating thickness and the addition of wax to the coating solutiondid not have a significant effect. To the contrary, polyurethane effectivelyenhanced controlled release characteristics.

The development of controlled release and environmentally safe ureafertilizer was also studied byMathews in 2010 [39]. Here, the conceptualadvantage of the swelling capacity of certain polymers that retainedstrength enough towithstand osmotic pressure and avoid the burst effectduring gelation, finally materialized. Urea was coated with a newly syn-thesized poly[N-isopropyl acrylamide]-co-polyurethane (PNIPAm-PU)and the controlled release of urea—monitored by mass spectroscopy—was observed as a function of the soil's temperature, pH, and moisture.The coating solution was first synthesized with NMR characterization tovalidate the claimed structure. The Amino Terminated Poly N-IsopropylAcrylamide (NH2-PNIPAm) was synthesized by the radical polymeriza-tion of N-Isopropyl acrylamide (NIPAm) with potassium persulfate asthe initiator and 2-aminoethanethiol hydrochloride as the chain transferreagent in aqueous media. The next step involved the preparation ofIsocyanate Terminated Polyurethane (NCO-PU-NCO) by degassingPoly(1,4-butylene adipate)diol end-capped (PBAG) at high temperatureallowing this to react with 4, 40-methylene bis(phenyl isocyanate)(MDI) in a tri-necked flask equipped with a stirrer. In the third step,PNIPAm-PU was synthesized by the reaction of NH2-PNIPAm withNCO-PU-NCO at 90 °C. Urea granules were then dip coated in thissolution, followed by centrifugal separation and vacuum drying. Theproposed release pattern was similar to that mentioned by Yong [40].In this study, the coating solution was set to vacuum drying; a timeconsuming, lengthy process.

To enhance nitrogen uptake efficiency of tea plants while studyingcontrolled release behavior, Han [41] developed three different controlledrelease fertilizers. Urea granules were coated with Ca–Mg phosphate,polyolefin, and polyolefin plus dicyandiamide (DCD). The granules wereplaced in a concrete mixer with a smooth inner surface and the DCD,dissolved in dilute phosphoric acid, was sprayed onto the granules. Thecoated granules were then placed in a Ca–Mg phosphate powder andsprayed with wax sealant. Polyolefin and polyolefin plus DCD coatedurea granules were also prepared in the same fashion. Pot and fieldexperiments were done to study controlled release and nitrogen uptake

by tea plants using all three coated urea samples. The polyolefin plusDCD coated granules produced the best results in terms of controlledrelease while maintaining optimal soil nitrogen concentration over thelong term.

Petchsuk et al. [42] reported the feasibility of poly(lactic acid-co-ethylene terephthalate) as a coating material. In a comparative study,urea granules were coated with commercial polylactic acid (PLA) andPLA, plus poly(lactic acid-co-ethylene terephthalate) which weresynthesized by the authors. After dissolving this polymer in chloroform,it was sprayed on urea granules in a rotating mixing machine followedby two drying steps (hot air and heating gun). Controlled releaseproperties were evaluated by monitoring the urea concentration byrefractive indexing in a rotating bottle of water containing the coatedurea granules. In addition, they also employed a scanning electronmicroscopic morphological study. The authors posited that controlledrelease was a function of the percent of coating applied, which, inturn, directly depended on the molecular weight, nature, concentrationand frequency of the polymer coating. They also determined thatcontrolled release was markedly affected by the coating's surfacemorphology.

1-Naphthylacetic-acid (NAA) has been reported to be a plant growthregulator (PGR) for the rooting of cuttings, fruit-set inhibition, fruitshedding, and the initiation offlowering [43]. In 2012, Qiu [43] preparedCRCU with dual attributions of controlled release and PGR. To preparethe coating material, the monomers N-butyl methacrylate (BMA),methyl methacrylate (MMA) and 2-hydroxyethl acrylate (HEA) wereadded to a four-necked flask equipped with a stirrer, nitrogen supply,and condenser. Benzoyl peroxide (BPO) (0.45%) dissolved in 5 ml ofethyl acetate was then added to the monomers and the mix wasagitated under nitrogen with ethyl acetate for ~6 h until a non-stickymaterial was produced. The non-reacted monomers were then precipi-tated by n-hexane and separated. The final poly(BMA-MMA-HEA) wasair dried at 80 °C. The solid coatingmaterial (PBMHs-NAA) thus obtained,along with the paraffin wax, was dissolved in ethyl acetate and thesolution was then sprayed onto a fluidized bed of urea granules at 75 °Cfor 25 min. The controlled release property of the coated fertilizer wasdetermined by water dissolution followed by a urea concentration assayvia UV spectrophotometry. The initial release rate was high (100% ineight days) due to the sticky adhesion of the granules which caused arough surface. The addition of 10% paraffin wax, however, prolongedthe dissolution: 1.54% at 24 h, and 78.77% at 28 days.

Polymer coatingmaterials have also been used to affect the controlledrelease of urea when urea acted as a constituent of compound fertilizers.For example, the combination, polyvinyl chloride/polyacrylamide/naturalrubber/polylactic acid, was employed to coat a compound fertilizercontaining urea, phosphorous, potassium, calcium, magnesium andcopper [44]. Polyethylene and paraffin wax were used for the NPKcompound fertilizer [45] and a polysulfone/cellulose acetate/polyacrylo-nitrile based coating by Tomaszewska [46–48]. However, the scope ofour review does not cover coating materials for compound fertilizers asour focus is urea and those materials used as coating to enhance itscontrolled release.

Costa et al. [49] studied the coating of urea granules with polyhy-droxybutyrate (PHB) and ethyl cellulose (EC) using simple immersionand manual spraying with a pulverizer and triggler. PHB and EC wereinitially dissolved in chloroform and acetone, respectively, as the coatingsolution. Adjuncts were also employed to facilitate interface interactionsbetween the coating solution and urea granules. The urea dissolutionrate in distilled water was measured via indirect enzymatic conversionof urea to ammonia with a spectrophotometer to determine concentra-tions. The optimal coatingmaterial allowed complete urea release within5 min, which was incompatible with set standards for agricultural use.

Polymer materials offer a number of advantages when used ascoatingmaterials for the controlled release of urea. They are biologicallyinert against microbial attack and provide a supply of nutrients consis-tent with crop metabolic needs over longer periods of time. They are


also able to retain both micro and macronutrients within the helicalpolymer chainmatrix [34]. Despite these advantages, polymermaterialsdo have limitations. The coating processes are quite complex andinvolve a number of chemicals. The overall process does not attractcommercial attention because of the high cost as most polymer mate-rials require the use of organic solvents to formulate a coating solution.This not only increases costs due to solvents and their recovery, but alsoposes adverse environmental impacts in terms of hazardous emissions.Furthermore, many, if not all polymer coatings, are non-biodegradableafter total nutrient release and present a new type of soil pollutionthat is undesirable. Hence, most controlled release urea fertilizersproduced thus far have not been effectively admitted to commercialproduction. Even so, certain polymers have been employed to producecontrolled release urea on a commercial scale; a glimpse of these isgiven in the next sections.

2.3. CRCU from superabsorbent/water retention coating materials

Superabsorbent polymer materials (SPMs) have recently caught theattention of research circles because of interesting properties that favorCRCUproduction. These SPMsare 3-dimensional cross-linkedhydrophilicpolymerswith an ability to imbibewater that is hundreds of times higherthan their own weight and which cannot easily be removed even underextended pressure [12,14,50–55]. They find attractive use in agriculturaland horticultural applications due to reduced water consumption andirrigation frequency, especially in drought prone areas and are thusconsidered economical. The advantages of SPM produced CRCUs includesoil improvement through aeration, abatement of soil degradation, allevi-ation of water evaporation losses, reduction of environmental pollutionthrough volatilization and leaching, and a decrease in crop morbiditydue to increased nutrition through enhanced nutrient retention periods[12,14,50–56,92].

Yong's study [40] triggered new vistas of research for the productionof multifunctional controlled release coated urea fertilizers with attri-butes of controlled release and improved water retention propertiesthat are very beneficial, especially in regions with limited watersupply. The most frequently used SPMs are classed as cross-linkedpolyacrylates/polyacrylamides, hydrolyzed cellulose-polyacrylonitriles/starch polyacrylonitriles graft copolymers, and cross-linked copolymersof maleic anhydride. The general methods employed in most studies forSPMs as a coating material are based on either solution polymerizationor inverse-suspension polymerization. The solution polymerizationinvolves the blending of NH3-neutralized acrylic acid (AA) or acrylamide(AM) basedmonomers in aqueous solution, followed by the addition of awater-soluble cross-linking N,N′-methylenebisacrylamide (MBA) andpotassium/ammonium persulfate as initiators. The blending is continuedat increased temperature until a rubbery product is obtained whichis then dried, ground and sieved for coating purposes. For inverse-suspension polymerization, the surfactant and dispersant are mixed toform a water-in-oil phase in which AA/AMmonomers are blended witha cross-linker and initiator as described above. The resultant micro-spherical product is dried to form a free flowing powder that requiresno grinding or sieving.

With this background, Guo [12] prepared slow release membraneencapsulated, double coated urea granules with an inner shell of cross-linked starch and an outer layer of acrylic acid and acrylamide. Soil incu-bation experiments determined nitrogen release by using the Kjeldahlmethod of distillation. Results indicated 10%, 15%, and 61% release rateson days two, five, and thirty, respectively. Coated urea, thus obtained,has cross-linked starch as inner coating layer with a copolymer of cross-linked acrylic acid and acrylamide as an outer coating. The slow releasemechanism involves the absorption of water by the coating materialwhich causes it to swell and transform to a hydrogel. The core ureathen dissolves in the hydrogel's water and diffuses slowly through agrid like system of the swollen hydrogel via mass transfer of water fromwithin the hydrogel to water in the soil. Another double coated urea

with an inner coating of urea-formaldehyde and an outer layer of cross-linked poly(acrylic acid)/organo attapulgite composite was prepared byLiang [53]. He reported a dried CRCU released of 3.9%, 7.5%, and 75%(wt.%) at two, five, and thirty days of soil incubation, respectively. Therelease mechanism was similar to Guo's study with a slight difference:water, after diffusing through the outer coating, slowly penetrated theurea-formaldehyde layer to dissolve the urea which then escaped slowlyby a dynamic exchange between hydrogel free water and soil moisture.The coating's thickness and the solubility of urea-formaldehyde werecharacterized as controlling factors for the slow release. Hence, higherthickness and lower solubility produced the best slow-release outcome.

Liang [50] also prepared double coated urea granules with an innerlayer of polystyrene and outer coating of cross-linked poly(acrylicacid)-containing urea. The urea release of the polystyrene coating wassaid to follow the same mechanism suggested by Shaviv [5]; i.e. athree stage release mechanism: first came a lag period in which waterpenetrated the coating without urea release; then a constant releaseperiod followed when urea dissolved and flowed through the coating(burst effect); and finally, there came a stage of decline until the releaseof urea ultimately ended. The presence of the second coating layer inthis study waived the burst effect in which more than 70% of the ureawas released. Hence, the outer coating not only enhanced slow releasebut further facilitated effective irrigation due to water retention.

Some investigators have prepared controlled release urea fertilizersby either blending with superabsorbent materials or polymerizationwith a superabsorbent mixture. However, these slow release formulaehave thus far experienced the undesirable “burst effect” that hampersthe controlled release property. Some polymer shells also remain inthe soil for a long time after nutrients have been completely released.Hence, an approach to enhance their biodegradability to avoid hazard-ous emissions and other effects was presented by Ni [54]. He preparedCRCU with an attapulgite matrix as the fertilizer core with two layersof coating: ethyl cellulose joined to a plasticizer as the inner coat, anda sodium carboxymethylcellulose (CMC) plus hydroxyethylcellulose(HEC) based hydrogel as the outer coat. Attapulgite is a type of octahe-dral layeredMg–Al–silicate absorbentmineral with hydroxyl groups onits surface. It is almost inert towards salts (like urea), so it is preferred asa substrate for superabsorbent composite materials [55]. After 24 h ofsoil incubation, the urea release rate was 8.7%. During this phase,water diffused gradually into the granules as slower release was facili-tated by the hydrophobic ethylcellulose coating. During the secondstage, from day two to five, there was consistent release caused by thediffusion of nutrients outwardly followed by dynamic mass transfer tothe external atmosphere. In the last phase, from day two onwards, thesolution's concentration within was lowered as bulk water wasabsorbed. During this stage, attapulgite absorbed the remaining nutri-ents which further enhanced the slow release of urea.

Another recent study by Yang et al. [57], addressed the issue ofpolymer biodegradability with double coated urea granules producedwith biodegradable biopolyurethane derived from liquefied corn Stoveras the inner coating, and a superabsorbent material based on chickenfeather meal modified with acrylic acid as the outer coating. For theinner coating, urea granules were placed in a rotary drum and thecoating solution was poured on rotating granules. Different runs weremade to produce different mass coatings. For the outer layer, theacrylic-acid-modified-chicken-feather-meal (MCFM-AA) solution waspoured on previously prepared coated granules followed by an adher-ent (MCFM-AA powder) to produce the final compact product. Releasekinetics was studied in deionized water as well as in soil. The periodicincrements in the mass coating of the inner coating layer caused signif-icant reductions in release rates. For example, N release slowed from1.5 days to 13 and then to 57 days as the mass of the inner coatingincreased from 3.2% to 5.3% and 8.5% (wt.%), respectively.

Tao [51] developed a triple polymer coated slow release urea with aninner coating of polyethylene, that primarily served as a slow releasefilm;an intermediate coating layer of poly(acrylic acid-co-acrylamide) that


served as a superabsorbent water retaining layer; and an outermostcoating of poly (butyl methacrylate) to protect the intermediate layer.The study intended to gain slow releasewhile avoidingwater evaporationlosseswith a goal to lessen irrigation frequency by the use of amultifunc-tional superabsorbent slow release fertilizer. The three-layered coatingoperation utilized fluidized bed equipment that avoided nutrient lossdue to high temperatures in the dip coating, which was easily amenableto up-scaling to pilot or industrial use. Slow release behavior wasmonitored by soil incubation. At a thickness of 25 μm, the release ratewas 4.2%, 38%, and 56% respectively for days 1, 7 and 14 respectively.Similarly, at 50 μm it was 0.15%, 13.5%, and 24%; and at 75 μm it was0.1%, 10.1%, and 10.3% on days 1, 7 and 14, respectively for both trials.The release mechanism was the same as described in Section 1.2.2 inreference to Shaviv's work [5]. Hence, Tao demonstrated that nutrientrelease increased at elevated temperatures while the release rate wassimilar in both soil and water.

In 2012, Wang double coated urea in a pan granulator withk-Carrageenan-sodium alginate (kC-SA) as the inner shell and a cross-linked k-Carrageenan graft, copolymerized with polyacrylic acid andcelite (kC-g-poly AA/celite), as the outer shell [58]. Sodium alginate isan anionic naturalmacromolecule extracted frommarine algae. Similar-ly, k-Carrageenan is an anionic polysaccharide extracted from red sea-weed. The combination of both materials enhanced the mechanicalstrength of the coating layers and the hydrogel's brittleness whichthen eased water super-absorption. After coating, granules with a thinlayer were dried and subjected to a duplicate coating step for betterthickness. The same procedure was repeated to enclose kC-SA coatedurea granules in a superabsorbent outer coating of (kC-g-poly AA/celite)followed by drying at 30 °C to obtain the final product. Soil incubationexperiments revealed 39%, 72%, and 94% nitrogen release on days two,five and twenty-five, respectively. The release mechanism was thesame as depicted by the same author in previous studies [12] and [53].

The use of SPMs to produce CRCU offers a number of advantages, themost prominent being super-absorption of water combined with thecontrolled release of urea. However, the preparation steps are complexand required raw materials are costly. The CRCU products producedthus far offer higher costs which present a major impediment to theircommercialization. Another aspect that prevents their commercializa-tion is the non-biodegradability of some coatingmaterials which causesa new type of soil pollution. However, this remains a relatively newresearch field and scientists are addressing these issues.

2.4. CRCU from bio-composite based coating materials

To obviate effects from the non-biodegradability of certain polymercoatings and to offset higher operational costs, the development of bio-composite based coating materials for controlled release coated ureahave recently caught interest in the research circles, with starch as acontender. Starch naturally occurs as a polysaccharide biopolymer thatis abundantly available from many renewable plant sources. Due to itslow cost, biodegradability, and abundance, several non-food applicationsof starch have been investigated, with starch based controlled releasecoatingmaterials as one of the numerous areas. Since starch is hydrophil-ic, it cannot be used as a coating material on its own for CRCU prepara-tions and requires blending with other materials for effective utilization[37,59–63].

In 2005, Ito [64] prepared dual coated urea granules with an innerlayer of poorly soluble isobutylidendiurea (IBDU) and the outer layerof starch with wax powder in a high shear granulator mixer using asimple blending technique. Through HPLC, he found that the nutrientrelease rate can be modified by adjusting both the fraction of dispersedparticles and the thickness of both inner and outer coatings. With onlyone coating (in the absence of an outer coating), that shell was subjectto a diffusion release mechanism. The dual layer, on the other hand,followed a sigmoidal pattern of controlled release. The sigmoidal releasepattern, as necessitated by some applications, refers to an initial slower

release quantity followed by consistent increases. The proposed releasemechanism followed a dual path. First off, the core nutrient shrunk insize after dissolution in water. Secondly, the concentration of the corenutrient solution kept decreasing until the concentration equilibratedwithin the reservoir. The release rate from a single layer preparationhad soluble particles with a faster diffusion release pattern attributedto the formation of microchannels through which active nutrientsimmediately flowed. However, the dual layer product caused a sigmoidalrelease because of the hindrance offered by the outer, more impermeablelayer.

Suherman [61] prepared a coating solution by mixing starch, acrylicacid, and polyethylene glycol with slow additions of water and continu-ous stirring until a homogeneousmixture was obtained. The urea coatingwas carried out in a fluidized bed with a top spray of the starch basedcoating solution.Water dissolution experiments revealed reduced releaserates with increased starch content of the coating. Higher temperaturesenhanced the release rate because of the pre-mature drying of the coatingdroplets. Also, elevated temperatures reduced the proportion of liquidbridges on the urea granules, thus, leaving uncoated spots that permittedhigher release rates later on.

In 2012, K2S2O8 modified starch (ST) was prepared by gelatinizingstarch with water at 80 °C followed by cooling and mixing withK2S2O8 at 60 °C for 45min [62]. Themodified starchwas graft polymer-izedwith natural rubber (NR) latex bymixing and stirring at 60 °C for 3h in the presence of Teric®16A16 to produce NR-g-ST. The NR-graft-polymerized starch was then used to encase urea granules to makeCRCU. Coating was done by simple immersion of urea granules intothe graft polymer blend followed by drying. The urea release rate inwater, as determined by UV–vis spectrophotometer, was 21% in 24 h.The diffusion mechanism of release was followed by nutrients so thatonly the core's shell remained; the core being hydrophobic natural rub-ber and a shell of starch. The hydrophilic nature of starch is associatedwith the presence of hydroxyl functional groups [59]. Various studiesattempted to transform this hydrophilic nature to hydrophobic by theaddition of different chemicals and additives. In most cases, consequentcontrolled release achievements did not correspondwith cropmetabol-ic needs, and thus, failed to meet standards (10-12 weeks) set by thescientific community.

Lignin is a cheap and natural macromolecular compound that isabundantly available as awastematerial frompulp and paper industries[65]. Moreover, lignin is renewable, biodegradable, amorphous, and arelatively hydrophobic bio-polymer compared to other polymers [66].Perez [65] prepared a lignin based controlled release urea formulationby mixing urea and lignin in a glass reactor immersed in a thermostaticsilicon oil bath. The mixture was heated and the resultant urea-ligninmatrix was cooled to give a glass like structure that was later milled ina crusher to obtain the desired size range of controlled release particles.This study also included urea coating with ethyl cellulose in a Wursterfluidized bed. Ethyl cellulose predominantly possesses high physicaland chemical stabilities with good film forming properties and isrelatively less toxic. A 5% ethanol solution of ethyl cellulosewas sprayedonto a fluidized bed of urea granules at 60 °C followed by air drying inthe same chamber at 70 °C. Different runs were made to producedifferent coating thicknesses for the analysis. Both the lignin basedcontrolled release urea particles and the ethyl cellulose coated granuleswere subjected to water leaching experiments to evaluate release rates.The patterns produced very slow releases in the early stage followed bya constant release leading to a period of decaying release. The coating'sthickness, as reported by many others, had an inverse effect in terms ofcontrolled release. The comparative study revealed that ethyl cellulosecoated granules were better than the lignin based slow release ureaformulation because of its coating uniformity which retarded waterdiffusion through the coating layer.

Mulder [66] also employed soda flax lignin (Bioplast) coupled withacronal as a plasticizer and alkenyl succinic anhydride (ASA) as ahydrophobizing or cross-linking agent to produce CRCU. The urea


granules were spray coated in a rotary pan coater with 25% bioplastdispersion as well as plasticizer and cross-linking agents at 70 °C.Refractive index measurements were made to evaluate the amount ofnitrogen released in water. Coating thickness and uniformity playedkey roles in the inhibition of urea dissolution. Coating uniformity isincreased by spraying the coating suspension in three stages. In thefirst step, a handsome quantity of suspension solids should engulf theurea granules in order to avoid the dissolution of urea during theprocess and which also allows the urea to become part of coatingmaterial. In the second and third steps, relatively small quantities ofsolids should be sprayed to fill fissures and micropores to contributetowards coating uniformity. Higher coating thickness granted bettercontrol release properties and the hydrophobizing action of ASA alsoplayed a key role in impeding urea dissolution. Furthermore, coatingfilms with a plasticizer remained intact in water for two weeks andthe cross-linker aided the coating layers and significantly reduced therelease rate but still could not meet set market standards. It was,therefore, suggested to chemically modify cellulose to enhance controlrelease properties.

Considering the swellability and biodegradability of konjac flour,Yong [40] prepared a controlled release urea fertilizer and studied itseffect on various process parameters. A pallet of urea was initially heatmolded to cake and then soaked in coating material. The coatingconsisted of compound polyether added to water, silicon oil and acatalyst that was fluffed uniformly under heat for 10 min. Heating andwhisking continued with a further addition of toluene diisocyanateand konjac flour until the solution turned white. This was then spreadon the caked urea. The coated urea was oven dried at 60–80 °C, thesetting temperature of the coating material. To study controlled releasebehavior, sodium hyposulfite titration experiments were used. Coatedsamples were buried in soil in beakers at constant temperature withan additional 500 ml of water. During the first 8 weeks, only a 20%release was observed which then rose to 70–80%. This is because thekonjac flour initially absorbed water and swelled, which, in turn,inhibited urea release by narrowing exhaust channels. Later, thegelation of konjac flour occurred followed by microbial attack whichdisintegrated the material and assured rapid urea release. Soil burialtests at 70–90 °C proved that the coating material was biodegradable.In another study, 5% acetone solution of ethyl cellulose and celluloseacetate phthalate at 30 °C were used to coat urea beads in a Wursterfluidized bed unit at temperatures ranging from 32 to 51 °C [67]. Soilincubation tests were done in a soil filled flask mounted on an orbitalshaker kept rotating at 120 rpm. Released ureawas analyzed by conduc-tivity which indicated the release rate for coating with ethyl cellulosewas higher than that of cellulose acetate phthalate. However, bothcoating materials were analogous in terms of the release mechanism.The three stage release rate was initially high, followed by a fairlyconstant release preceding a prolonged decline.

Vashishtha [68] posited that thedual advantage of sulfur coatedurea—i.e. controlled release of urea and availability of sulfur as a plantnutrient—can better be achieved when phosphogypsum is used asthe coating material instead of sulfur. This was likely because phos-phogypsum is not only slightly soluble in water but also because itdoes not alter the soil pH (sulfur makes the soil pH acidic). Secondly,to transform sulfur coated urea to a plant available form (sulfateform), common sulfur must undergo bacterial transition whereasphosphogypsum, provides plant available sulfate readily. With thisas a background, Vashishtha [68] employed both dry and wetmethods to prepare phosphogypsum coated urea in a fluidized bed.The only difference between either method was that the wet method (amixture of phosphogypsum with neem oil, linear alkyl benzene, andwater) was used to prepare the coating material; whereas, in the drymethod, the same mixture was prepared without the addition of water.Neem oil and linear alkyl benzenewere used as binder and surfactant, re-spectively. Water dissolution experiments were conducted with twicedistilled water and with magnetic stirring until 100% dissolution took

place. The dissolution rate decreased with increased coating thicknessand the coating layer produced with the wet method was more effectivethan the dry preparation.

2.5. Commercially available controlled release coated urea (CRCU)

Despite high operational costs, CRCUs have been produced and soldon commercial scale. However, most of these products have beenlimited to horticultural and ornamental applications rather than largescale agriculture. The Tennessee Valley Authority (TVA) pioneered thecommercialization of CRCU with a large scale production of sulfurcoated urea. The Arthur Daniels Co. (ADM) was the first to producepolymer coated fertilizers using dicyclopentadiene with glycol ester. Aliterature review is given in Table 2 that provides an overviewof coatingmaterials that have been used to produce CRCU on a commercial scalethus far.

3. Conclusion and suggestions

The coating of urea is required to avoid nitrogen loss throughleaching, volatilization, and denitrification. CRCUs inhibit thisloss and serve to release nitrogen in a mode that is compatiblewith the metabolic requirements of plants. Millions of researchdollars have been spent to develop numerous coating materialsand techniques; even so, the production of CRCU has yet toreach industrial scale. Sulfur alone cannot be effectively used asa coating material to produce CRCU because of its amorphousnature. Many sealants, binders, plasticizers and protective agentshave therefore been used to combat the immediate burst effect,all of which increase process complexity and costs, which iswhy the production of sulfur coated urea has almost been aban-doned. CRCUs based on polymer/superabsorbent materials offerpromising potential in terms of extended controlled release andwater retention, but the complexity of processing, elevatedcosts and the non-environmentally friendly side effects of somematerials prevent industrial scale production. A relatively smallresearch quantum is reported with regard to the production ofCRCUs with starch, lignin and cellulose based coating materials,which are relatively cheaper, biodegradable and renewable.However, their augmented hydrophilicity and limited controlledrelease characteristics are weak points.

In view of this thorough investigation, the authors offer a fewsuggestions:

• The production of CRCU should begin with original industrial gradeurea granules rather than melting, transforming, dissolving orpolymerization to fabricate controlled release matrices with othermaterials.

• Coating material should be selected with a view to its (i) affinitywith urea; (ii) its ability to permeate water and urea solution; (iii)its capability to impede immediate urea escape from the coatingsurface; and (iv) its ability to release urea in a manner that meetsa crop's metabolic requirements over a specified period of time. Itshould also be biodegradable and cheaper. Apparently, no suchmaterial(s) exist which possess these ideal traits. Nevertheless,bio-composites based on starch/lignin/cellulose can indeed bemodified to significantly achieve such properties.

• The coating process should enable industrial production of CRCUwithout changing the spherical geometry of urea granules. For thisreason, a fluidized bed coater, pan coater or rotary drum coatermay be employed. Due to its excellent heat and mass transfercharacteristics in addition to its easy operation,fluidized bed coatingis a good candidate for industrial scale production. However, bear inmind that when using the fluidized bed, coatingmaterials should becompatible with effortless spraying of the fluidized bed of ureagranules.

Table 2Coating materials used to produce CRCU on commercial scale.

Commercial name Composition of coating material Company/provider Ref

SCU Sulfur + wax + diatomaceous earth + coal tar Tennessee Valley Authority (TVA) USA [69]Meister[7, 10, 15, 20, 27, 70, 270]

Polyolefin + inorganic powder Chisso Co. Kitakysya Japan [69–75]

LP30 ~ 180, LPS40 ~ 200LPSS 100

Polyolefin Chisso-Asahi Fertilizer Corporation [76–79]

CRU Polymeric material Agrium Inc. Calgary [80]CU & CUS Polymeric material Chisso-Asahi Fertilizer Corporation [81]PCF Polyurethane-like Haifa Chemicals Co. Ltd. [82,83]Zn-coated urea Zinc oxide Indo-Gulf

Fertilizers, Jagdishpur (UP), India[84]

Agrium PCU Polymeric material Agrium US Inc. [85–88]Kingenta PCU Polymeric material Shandong Kingenta Ecological Engineering Co. Ltd. China [85]

[89,90]Humate coated urea Humic acid [69]Duration type 5 Polymeric material [69]PCU Polyolefin [91]


• The granulation process can be used to produce controlled releaseurea granulates. Analytical grade urea can be either blended ormade to physically react with a suitable material (with the sameaforementioned attributes) that best constructs a controlled releaseformulation which can then be converted to appropriate granularsizes and shapes as needed. For this purpose also, the fluidized bedgranulator can successfully be employed.

Acknowledgment

The authorswould like to offer their utmost appreciation to UniversitiTeknologi PETRONAS for providing a conducive work environment andstate-of-the-art research facilities. The research grants extended to usby the Ministry of Higher Education, Malaysia (MOHE) (LRGS Fasa 1/2011) for ongoing research projects are also decidedly acknowledged.The linguistic expertise shared by Mr. Zaheer Hussain, Lecturer, NationalUniversity of Modern Languages (NUML), Lahore-Pakistan, is also highlyacknowledged.

References

[1] P. Gerbens-Leenes, S. Nonhebel, W. Ivens, A method to determine land requirementsrelating to food consumption patterns, Agric. Ecosyst. Environ. 90 (1) (2002) 47–58.

[2] M.E. Brown, B. Hintermann, N. Higgins, Markets, climate change, and food securityin West Africa, Environ. Sci. Technol. 43 (21) (2009) 8016–8020.

[3] C. Jie, et al., Soil degradation: a global problem endangering sustainable develop-ment, J. Geogr. Sci. 12 (2) (2002) 243–252.

[4] S. Chien, L. Prochnow, H. Cantarella, Recent developments of fertilizer productionand use to improve nutrient efficiency and minimize environmental impacts, Adv.Agron. 102 (2009) 267–322.

[5] A. Shaviv, Controlled release fertilizers, IFA International Workshop onEnhanced-Efficiency Fertilizers, Frankfurt, International Fertilizer Industry Associa-tion, Paris, France, 2005.

[6] Blouin, G.M.,Method of making sulfur-coated fertilizer pallet having a controlled disso-lution rate, 1967, Google Patents.

[7] M.E. Trenkel, Slow-and controlled-release and stabilized fertilizers: an option for en-hancing nutrient use efficiency in agriculture, IFA, International fertilizer industryassociation, 2010.

[8] G.M. Blouin, D.W. Rindt, O.E. Moore, Sulfur-coated fertilizers for controlled release.Pilot-plant production, J. Agric. Food Chem. 19 (5) (1971) 801–808.

[9] L. Liu, et al., A review: controlled release systems for agricultural and food applications,in: N. Parris, L.S. Liu, et al., (Eds.), New Delivery Systems for Controlled Drug Releasefrom Naturally Occurring Materials, ACS Symposium series, 992, 2008, pp. 265–281.

[10] R. Rose, Slow release fertilizers 101, in: R.K. Dumroese, L.E. Riley, T.D. Landis (Eds.),Technical, 2002.

[11] A. Blaylock, J. Kaufmann, R. Dowbenko, Nitrogen fertilizer technologies, West. Nutr.Manag. 6 (2005) 8–13.

[12] M. Guo, et al., Preparation and properties of a slow-release membrane-encapsulatedurea fertilizer with superabsorbent and moisture preservation, Ind. Eng. Chem. Res.44 (12) (2005) 4206–4211.

[13] R. Liang, M. Liu, L. Wu, Controlled release NPK compound fertilizer with the functionof water retention, React. Funct. Polym. 67 (9) (2007) 769–779.

[14] M. Liu, et al., Preparation of superabsorbent slow release nitrogen fertilizer byinverse suspension polymerization, Polym. Int. 56 (6) (2007) 729–737.

[15] L. Wu, M. Liu, Preparation and properties of chitosan-coated NPK compound fertil-izer with controlled-release and water-retention, Carbohydr. Polym. 72 (2) (2008)240–247.

[16] D. Davidson, F.X. Gu, Materials for sustained and controlled release of nutrients andmolecules to support plant growth, J. Agric. Food Chem. 60 (4) (2012) 870–876.

[17] T. Zheng, et al., Superabsorbent hydrogels as carriers for the controlled-release ofurea: experiments and a mathematical model describing the release rate, Biosyst.Eng. 102 (1) (2009) 44–50.

[18] D. Rindt, G. Blouin, J. Getsinger, Sulfur coating on nitrogen fertilizer to reduce disso-lution rate, J. Agric. Food Chem. 16 (5) (1968) 773–778.

[19] D. Ussiri, R. Lal, The role of fertilizer management in mitigating nitrous oxide emis-sions, Soil Emission of Nitrous Oxide and its Mitigation, Springer, 2013, pp. 315–346.

[20] E. Guertal, Slow-release nitrogen fertilizers in vegetable production: a review,HortTechnology 19 (1) (2009) 16–19.

[21] H. Akiyama, X. Yan, K. Yagi, Evaluation of effectiveness of enhanced‐efficiency fertil-izers as mitigation options for N2O and NO emissions from agricultural soils: meta‐analysis, Glob. Chang. Biol. 16 (6) (2010) 1837–1846.

[22] X. Yan, et al., Recent advances on the technologies to increase fertilizer use efficien-cy, Agric. Sci. China 7 (4) (2008) 469–479.

[23] F. Puoci, et al., Polymer in agriculture: a review, Am. J. Agric. Biol. Sci. 3 (1) (2008) 299.[24] J. Sartain, et al., New tools for the analysis and characterization of slow-release

fertilizers, ACS Symposium series, ACS Publications, 2004.[25] R. Tharanathan, Biodegradable films and composite coatings: past, present and

future, Trends Food Sci. Technol. 14 (3) (2003) 71–78.[26] B.S. Tsai, Continuous spouted bed process for sulphur-coating urea, 1986.[27] M. Choi, A. Meisen, Sulfur coating of urea in shallow spouted beds, Chem. Eng. Sci.

52 (7) (1997) 1073–1086.[28] G. Ayub, S. Rocha, A. Perrucci, Analysis of the surface quality of sulphur-coated urea

particles in a two-dimensional spouted bed, Braz. J. Chem. Eng. 18 (1) (2001) 13–22.[29] Y.-H. Liu, et al., Urea particle coating for controlled release by using DCPD modified

sulfur, Powder Technol. 183 (1) (2008) 88–93.[30] Detrick, J.H., Process for producing improved sulfur-coated urea slow release fertilizers,

1997, Google Patents.[31] Detrick, J.H. and G.L. Hargrove, Polymer-sulfur-polymer coated fertilizers, 2002,

Google Patents.[32] M.W. Donida, S.C. Rocha, Coating of urea with an aqueous polymeric suspension in a

two-dimensional spouted bed, Dry. Technol. 20 (3) (2002) 685–704.[33] G.S. da Rosa, S.C. dos Santos Rocha, Effect of process conditions on particle growth

for spouted bed coating of urea, Chem. Eng. Process. Process Intensif. 49 (8)(2010) 836–842.

[34] G.S. da Rosa, S.C.d.S. Rocha, Use of vinasse to produce slow‐release coated urea inspouted bed, Can. J. Chem. Eng. 91 (2012) 589–597.

[35] R. Lan, et al., Experimental modeling of polymer latex spray coating for producingcontrolled-release urea, Particuology 9 (5) (2011) 510–516.

[36] S.Wu, et al., Synthesis andperformance of polyurethane coatedurea as slow/controlledrelease fertilizer, J. Wuhan Univ. Technol. Mater. Sci. Ed. 27 (1) (2012) 126–129.

[37] Y. Niu, H. Li, Controlled release of urea encapsulated by starch-g-poly (vinylacetate), Ind. Eng. Chem. Res. 51 (38) (2012) 12173–12177.

[38] Y.-c Yang, et al., Improving the quality of polymer-coated urea with recycled plastic,proper additives, and large tablets, J. Agric. Food Chem. 60 (45) (2012) 11229–11237.

[39] A.S. Mathews, S. Narine, Poly [N‐isopropyl acrylamide]‐co‐polyurethane copolymersfor controlled release of urea, J. Polym. Sci. A Polym. Chem. 48 (15) (2010) 3236–3243.

[40] W. Yong, L. Jian, C. Xiaoyao, Biodegraded and polyurethane drape-formed ureafertilizer, J. Wuhan Univ. Technol. Mater. Sci. Ed. 20 (2) (2005) 12–14.

[41] W.Y. Han, et al., Nitrogen release dynamics and transformation of slow releasefertiliser products and their effects on tea yield and quality, J. Sci. Food Agric. 88(5) (2008) 839–846.

[42] A. Petchsuk, et al., Controlled-release materials for fertilizer based on lactic acidpolymers, Adv. Mater. Res. 55 (2008) 905–908.

[43] X. Qiu, et al., 1‐naphthylacetic‐acid‐functionalized polyacrylate‐coated urea withdual controlled‐release properties, J. Appl. Polym. Sci. 129 (2012) 559–567.

http://refhub.elsevier.com/S0168-3659(14)00120-5/rf0005

























































































[44] M. Hanafi, S. Eltaib, M. Ahmad, Physical and chemical characteristics of controlledrelease compound fertiliser, Eur. Polym. J. 36 (10) (2000) 2081–2088.

[45] S. Al-Zahrani, Utilization of polyethylene and paraffin waxes as controlled deliverysystems for different fertilizers, Ind. Eng. Chem. Res. 39 (2) (2000) 367–371.

[46] M. Tomaszewska, A. Jarosiewicz, Use of polysulfone in controlled-release NPK fertil-izer formulations, J. Agric. Food Chem. 50 (16) (2002) 4634–4639.

[47] M. Tomaszewska, A. Jarosiewicz, K. Karakulski, Physical and chemical characteristicsof polymer coatings in CRF formulation, Desalination 146 (1) (2002) 319–323.

[48] M. Tomaszewska, A. Jarosiewicz, Polysulfone coating with starch addition in CRF for-mulation, Desalination 163 (1) (2004) 247–252.

[49] Milene M.E. Costa, et al., Use of polyhydroxybutyrate and ethyl cellulose for coatingof urea granules, J. Agric. Food Chem. 61 (42) (2013) 9984–9991.

[50] R. Liang, M. Liu, Preparation and properties of a double-coated slow-release andwater-retention urea fertilizer, J. Agric. Food Chem. 54 (4) (2006) 1392–1398.

[51] S. Tao, et al., Preparation and characterization of triple polymer‐coated controlled‐release urea with water‐retention property and enhanced durability, J. Appl.Polym. Sci. 120 (4) (2011) 2103–2111.

[52] S. Qin, et al., Synthesis and characterization of slow‐release nitrogen fertilizer withwater absorbency: based on poly (acrylic acid‐acrylic amide)/Na‐bentonite, J.Appl. Polym. Sci. 126 (5) (2012) 1687–1697.

[53] R. Liang, M. Liu, Preparation and properties of coated nitrogen fertilizer with slowrelease and water retention, Ind. Eng. Chem. Res. 45 (25) (2006) 8610–8616.

[54] B. Ni, et al., Environmentally friendly slow-release nitrogen fertilizer, J. Agric. FoodChem. 59 (18) (2011) 10169–10175.

[55] L. Xie, et al., Slow-release nitrogen andboron fertilizer froma functional superabsorbentformulation based on wheat straw and attapulgite, Chem. Eng. J. 167 (1) (2011)342–348.

[56] X.-S. He, et al., Characteristics and performance of novel water-absorbent slowrelease nitrogen fertilizers, Agric. Sci. China 6 (3) (2007) 338–346.

[57] Yuechao Yang, et al., Biobased polymer composites derived from corn stover and feath-er meals as double-coating materials for controlled-release and water-retention ureafertilizers, J. Agric. Food Chem. 61.34 (2013) 8166–8174.

[58] Y. Wang, et al., κ-Carrageenan–sodium alginate beads and superabsorbent coatednitrogen fertilizer with slow-release, water-retention, and anticompaction proper-ties, Ind. Eng. Chem. Res. 51 (3) (2012) 1413–1422.

[59] L. Chen, et al., Controlled release of urea encapsulated by starch-g-poly (L-lactide),Carbohydr. Polym. 72 (2) (2008) 342–348.

[60] X. Han, S. Chen, X. Hu, Controlled-release fertilizer encapsulated by starch/polyvinylalcohol coating, Desalination 240 (1) (2009) 21–26.

[61] Suherman, Producing slow release urea by coating with starch/acrylic acid in fluidbed spraying, Int. J. Eng. Technol. IJET-IJENS 11 (06) (2011) 4.

[62] S.-A. Riyajan, Y. Sasithornsonti, P. Phinyocheep, Green natural rubber-g-modifiedstarch for controlling urea release, Carbohydr. Polym. 89 (1) (2012) 251–258.

[63] S. Ariyanti, Z. Man, B.M. Azmi, Improvement of hydrophobicity of urea modifiedtapioca starch film with lignin for slow release fertilizer, Adv. Mater. Res. 626(2013) 350–354.

[64] R. Ito, B. Golman, K. Shinohara, Design of multi-layer coated particles with sigmoidalrelease pattern, Chem. Eng. Sci. 60 (20) (2005) 5415–5424.

[65] M. Fernández-Pérez, et al., Lignin and ethylcellulose as polymers in controlled re-lease formulations of urea, J. Appl. Polym. Sci. 108 (6) (2008) 3796–3803.

[66] W. Mulder, et al., Lignin based controlled release coatings, Ind. Crop. Prod. 34 (1)(2011) 915–920.

[67] S. Lu, et al., Urea release rate from a scoop of coated pure urea beads: unified ex-treme analysis, J. Chin. Inst. Chem. Eng. 38 (3) (2007) 295–302.

[68] M. Vashishtha, P. Dongara, D. Singh, Improvement in properties of urea by phospho-gypsum coating, Int. J. ChemTech Res. 2 (1) (2010) 36–44.

[69] J.K. Kiran, et al., Effects of controlled release urea on the yield and nitrogen nutritionof flooded rice, Commun. Soil Sci. Plant Anal. 41 (7) (2010) 811–819.

[70] X. Yan, et al., Nitrous oxide emission from wetland rice soil as affected by the appli-cation of controlled-availability fertilizers and mid-season aeration, Biol. Fertil. Soils32 (1) (2000) 60–66.

[71] S. Shoji, et al., Use of controlled release fertilizers and nitrification inhibitors to in-crease nitrogen use efficiency and to conserve air and water quality, Commun.Soil Sci. Plant Anal. 32 (7–8) (2001) 1051–1070.

[72] H. Chu, et al., Soil microbial biomass and activities in a Japanese Andisol as affected bycontrolled release and application depth of urea, Biol. Fertil. Soils 42 (2) (2005) 89–96.

[73] S. Haque, Z. Sultana, Efficiency of Meister-10 and urea-Nimin as controlled releasenitrogen fertilizers on rice, SAARC J. Agric. 6 (2) (2008) 31–37.

[74] X.-Z. Wang, et al., Nitrogen cycling and losses under rice-wheat rotations with coat-ed urea and urea in the Taihu lake region, Pedosphere 17 (1) (2007) 62–69.

[75] D. Chen, et al., Evaluation of a polyolefin coated urea (Meister) as a fertilizer forirrigated cotton, Nutr. Cycl. Agroecosyst. 81 (3) (2008) 245–254.

[76] Y. Hara, Application of parametrization using the Richards function to nitrogen releasefrom coated urea and growth of rice seeds, Jpn. Agric. Res. 35 (3) (2001) 155–161.

[77] K. Inubushi, et al., Effects of controlled-release coated urea (CRCU) on soil microbialbiomass N in paddy fields examined by the 15 N tracer technique, Nutr. Cycl.Agroecosyst. 63 (2–3) (2002) 291–300.

[78] W. Cheng, et al., N2O and NO emissions from a field of Chinese cabbage as influ-enced by band application of urea or controlled-release urea fertilizers, Nutr. Cycl.Agroecosyst. 63 (2–3) (2002) 231–238.

[79] F. Zvomuya, et al., Nitrate leaching and nitrogen recovery following application ofpolyolefin-coated urea to potato, J. Environ. Qual. 32 (2) (2003) 480–489.

[80] R. McKenzie, et al., Controlled-release urea for winter wheat in southern Alberta,Can. J. Soil Sci. 87 (1) (2007) 85–91.

[81] Y. Takahashi, et al., Effect of basal side‐dressing of various types of coated ureafertilizer on shoot growth, yield components and seed composition of soybean(Glycine max (L.) Merr.), Soil Sci. Plant Nutr. 52 (3) (2006) 264–273.

[82] C. Du, et al., Prediction of nitrate release from polymer-coated fertilizers using anartificial neural network model, Biosyst. Eng. 99 (4) (2008) 478–486.

[83] M.D. Patil, et al., Performance of polymer-coated urea in transplanted rice: effect ofmixing ratio and water input on nitrogen use efficiency, PaddyWater Environ. 8 (2)(2010) 189–198.

[84] Y.S. Shivay, et al., Relative yield and zinc uptake by rice from zinc sulphate and zincoxide coatings onto urea, Nutr. Cycl. Agroecosyst. 80 (2) (2008) 181–188.

[85] M.L.Wilson, C.J. Rosen, J.F.Moncrief, A comparison of techniques for determining nitro-gen release from polymer-coated urea in the field, HortSci. 44 (2) (2009) 492–494.

[86] B. Golden, et al., Evaluation of polymer-coated urea for direct-seeded, delayed-floodrice production, Soil Sci. Soc. Am. J. 73 (2) (2009) 375–383.

[87] Y. Soon, et al., Effect of polymer-coated urea and tillage on the dynamics of availableN and nitrous oxide emission from Gray Luvisols, Nutr. Cycl. Agroecosyst. 90 (2)(2011) 267–279.

[88] C. Grant, et al., Crop yield and nitrogen concentration with controlled release ureaand split applications of nitrogen as compared to non-coated urea applied atseeding, Field Crop Res. 127 (2012) 170–180.

[89] Q. Zhu, M. Zhang, Q. Ma, Copper-based foliar fertilizer and controlled release ureaimproved soil chemical properties, plant growth and yield of tomato, Sci. Hortic.143 (2012) 109–114.

[90] Y. Ji, et al., Effect of controlled-release fertilizer on mitigation of N2O emissionfrom paddy field in South China: a multi-year field observation, Plant Soil(2013) 1–14.

[91] H. Akiyama, et al., Nitrification, ammonia-oxidizing communities, and N2O andCH4 fluxes in an imperfectly drained agricultural field fertilized with coatedurea with and without dicyandiamide, Biol. Fertil. Soils 49 (2) (2013) 213–223.

[92] Q. Xu, Proceedings of the 2013 International Conference onMaterial Science and En-vironmental Engineering, DEStech Publications, Inc., ISBN: 9781605951256, 2013,p. 233.





















































































































Review on materials & methods to produce controlled release coated urea fertilizer

Documents

Transcript of Review on materials & methods to produce controlled release coated urea fertilizer