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1Department of Materials Science and Engineering, Nelson Mandela African Institution of Science and Technology, P.O. Box 447, Arusha, Tanzania
2Department of Chemical Engineering, Hanyang University, 1271 Sa 3-dong, Sangnok-gu, Ansan-si, Gyeonggi-do 426-791, Republic of Korea
3Chemistry Department, University of Dar es Salaam, P.O. Box 35091, Dar es Salaam, Tanzania
Received 5 May 2014; Revised 22 July 2014; Accepted 5 August 2014; Published 28 August 2014
Copyright © 2014 Siafu Ibahati Sempeho et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Owing to the high demand for fertilizer formulations that will exhaust the possibilities of nutrient use efficiency (NUE), regulate fertilizer consumption, and lessen agrophysicochemical properties and environmental adverse effects instigated by conventional nutrient supply to crops, this review recapitulates controlled release fertilizers (CRFs) as a cutting-edge and safe way to supply crops’ nutrients over the conventional ways. Essentially, CRFs entail fertilizer particles intercalated within excipients aiming at reducing the frequency of fertilizer application thereby abating potential adverse effects linked with conventional fertilizer use. Application of nanotechnology and materials engineering in agriculture particularly in the design of CRFs, the distinctions and classification of CRFs, and the economical, agronomical, and environmental aspects of CRFs has been revised putting into account the development and synthesis of CRFs, laboratory CRFs syntheses and testing, and both linear and sigmoid release features of CRF formulations. Methodical account on the mechanism of nutrient release centring on the empirical and mechanistic approaches of predicting nutrient release is given in view of selected mathematical models. Compositions and laboratory preparations of CRFs basing on in situ and graft polymerization are provided alongside the physical methods used in CRFs encapsulation, with an emphasis on the natural polymers, modified clays, and superabsorbent nanocomposite excipients.
Controlled release fertilizers (CRFs) are fertilizer granules intercalated within carrier molecules commonly known as excipients to control nutrients release thereby improving nutrient supply to crops and minimize environmental, ecological, and health hazards . In that sense, CRFs usage is an advanced way to supply crop’s nutrients (cf. conventional ways) due to gradual pattern of nutrient release, which improves fertilizer use efficiency (FUE) . In other words, depending on the thickness of the coatings within the formulation, CRFs enable nutrients to be released over an extended period leading to an increased control over the rate and pattern of release , consequently the excipients play a role in regulating nutrients release time and eliminate the need for constant fertilization and higher efficiency rate than conventional soluble fertilizers .
Occasionally the terms controlled release fertilizers (CRFs) and slow release fertilizers (SRFs) have been used interchangeably, yet they are different. Typically, the endorsed differences between slow-release and controlled-release fertilizers are not clear [4, 5]. However, the term CRF is generally applied to fertilizers in which the factors dominating the rate, pattern, and duration of release are well known and controllable during CRF preparation [5, 6]. SRFs on the other hand are characterized by the release of the nutrients at a slower rate than is usual but the rate, pattern, and duration of release are not well controlled [5, 6]; they may be strongly affected by handling conditions such as storage, transportation, and distribution in the field, or by soil conditions such as moisture content, wetting and drying, thawing and freezing, and biological activity [7–9]. Thus, while in SRFs the nutrient release pattern is fully dependent on soil and climatic conditions and it cannot be predicted (or only very roughly) ; with CRFs, the release pattern, quantity, and time can be predicted within certain limits. For example, the classification of sulphur-coated urea (SCU) is subject to debate  due to a significant variation in the release patterns between different batches of fertilizer [5, 6, 11]. As a result, SCU is considered to be SRF despite being debated.
CRFs use is associated with several economic, agronomical, and environmental returns. Economically, CRFs supply nutrients to the crops for the entire season through a single application thereby saving spreading costs and reduce the demand for short-season manual labour required for topdressing operations . Agronomically, CRFs usage is associated with the improvement of plant growth conditions, such as reduction of stress and specific toxicity resulting from excessive nutrient supply in the root zones. Similarly, CRFs increase the availability of nutrients due to the controlled release of nutrients into a “fixing” medium during the fixation processes in the soil as well as supplying nutrients in the forms preferred by plants; in that way the synergistic effect between nutrients in the CRFs is enhanced . From the environmental perspective, CRFs improves NUE and in so doing reduces losses of surplus nutrients (over plant needs) to the environment . Consequently, high levels of fertilizer accumulation in the environment are minimized, thereby lessening several environmental problems associated with conventional fertilizer use such as eutrophication which causes O2 depletion, death of fish, unpleasant odour to the environment, and aesthetic problems [7, 12, 13].
2. Classification of CRFs
Several classifications of CRFs have been proposed. In this review, we will attempt to discuss a few of them. Based on Shaviv’s grouping , CRFs may be classified as follows.
2.1. Organic-N-Low-Solubility Compounds
These can be subdivided into biologically decomposing compounds usually based on urea-aldehyde condensation products, such as urea-formaldehyde (UF), urea-triazone (UT), crotonylidene diurea (CDU), and chemically decomposing compounds, such as isobutylidene-diurea (IBDU). Succinctly, UF is prepared by reacting excess urea under controlled conditions of pH, temperature, U-F ratio, and reaction time. UT solution is based on the reaction of urea-ammonia-formaldehyde. CDU is prepared by reacting urea with acetaldehyde under the catalysis of an acid. IBDU is prepared by reacting liquid isobutyraldehyde with solid urea [5, 7, 10, 14].
2.2. Fertilizers in Which a Physical Barrier Controls the Release
These can be subdivided into granules coated by hydrophobic polymers or as matrices in which the soluble active material is dispersed in a continuum that restricts the dissolution of the fertilizer. The coated fertilizers can further be divided into fertilizers with organic polymer coatings that are either thermoplastic or resins and fertilizers coated with inorganic materials such as sulphur or mineral based coatings. The materials used for preparation of matrices can also be subdivided into hydrophobic materials such as polyolefins and rubber and gel-forming polymers (hydrogels) which are hydrophilic in nature. Broadly, the use of coated fertilizers in agricultural practices is quite common as compared to the use of matrices. For instance, sulphur-coated urea (SCU) was developed at the Tennessee Valley Authority laboratories and manufactured commercially for almost 30 years [7, 15]. Its preparation is based on coating preheated urea granules with molten sulphur. The CRF alkyd-type resin-coated fertilizer (Osmocote) was first produced commercially in California in 1967. It is a copolymer of dicyclopentadiene with a glycerol ester . In fact, these formulations control the rate of nutrient release offering multiple environmental, economic, and yield benefits . Gel-based matrices are still being developed .
2.3. Inorganic Low-Solubility Compounds
This type of CRFs includes fertilizers such as metal ammonium phosphates (e.g., MgNH4PO4) and partially acidulated phosphate rocks (PAPR). Besides, the biologically and microbially decomposed N products, such as UF, are commonly referred to in the trade as slow-release fertilizers and coated or encapsulated/occluded products as controlled-release fertilizers [5, 7]. Essentially, Zhang et al.’s writings provide a deep detailed account on the subject in a much more broad sense .
3. Preparation of CRFs Formulations
Slowing the release of plant nutrients from fertilizers can be achieved by different methods and the resulting products are known as slow- or controlled-release fertilizers. With controlled-release fertilizers, the principal method is to cover a conventional soluble fertilizer with a protective coating (encapsulation) of a water-insoluble, semipermeable or impermeable-with-pores material. This controls water penetration and thus the rate of dissolution and ideally synchronizes nutrient release with the plants’ needs. The most important manufactured materials include (i) materials releasing nutrients through either microbial decomposition of low solubility compounds, for example, organic-N low-solubility compounds, such as urea-aldehyde condensation products, or chemically decomposable compounds, for example, IBDU [5, 6]; (ii) materials releasing nutrients through a physical barrier, for example, sulphur-coated urea (SCU) ; (iii) materials releasing nutrients incorporated into a matrix, which itself may be coated, including gel-based matrices, which are still under development [5, 6, 17]; materials releasing nutrients in delayed form due to a small surface-to-volume ratio, for example, super-granules, briquettes, tablets, spikes, plant food sticks , and others [19–21].
According to Liu et al. , intercalation of nutrients into the excipients is normally achieved by two methods. In the first method, the compound to be loaded is added to the reaction mixture and polymerized in situ whereby the compound is entrapped within the gel matrix, whereas in the second method, the dry gel is allowed to swell in the compound solution and after equilibrium swelling, the gel is dried and the device is obtained. This involves graft-polymerization [23–26]. The benefits and drawbacks are that for the former method, the entrapped compound may influence the polymerization process and the polymer network structure; while for the latter, the loaded compound always accumulates on the surface during the drying of the loaded hydrogel, which consequently leads to a “burst effect”; moreover, the loading amount may be low if the compound affects the water absorbency strongly.
Typical physical methods for encapsulating fertilizers include spray coating, spray drying, pan coating, and rotary disk atomization. Special equipment for these methods are rotary drum, pan or ribbon or paddle mixer, and fluidized bed . The details of these methods are beyond the scope of this paper.
4. Nutrient Release in the CRFs Context
In this perspective, with regard to the European Standardization Committee (CEN), nutrient release (of course from the excipients) can be manifest by the transformation of a chemical substance or rather fertilizer nutrients into a plant-available form (e.g., dissolution, hydrolysis, degradation, etc.), whereas slow release is the release wherein the rate of a nutrient release from the fertilizer is slower than that from a fertilizer in which the nutrient is readily available for plant uptake . CEN’s declaration alleged that “fertilizer should be described as CRFs if at room temperature the nutrients released exceed 15% in 24 hours, or no more than 75% released in 28 days, or at least about 75% released at the stated release time” [5, 7] giving different release patterns. That is to say, CRFs that do not meet these three CEN’s criteria are nonpertinent for the subject of controlled release formulations since the patterns will not comply with the standard ones, namely, linear and sigmoidal release patterns (Figure 1).
Figure 1: The effect of temperature on the release rate of Meister.
As mentioned above, release patterns can be classified into linear and sigmoidal release types [5, 28]. Examples of linear-release formulations presenting nutrient release between 30 and 270 days at 25°C for Meister formulation are given in Figure 2, whereas for sigmoidal-release formulations presenting nutrient release between 40 and 200 days at 25°C for Meister are shown in Figure 3 .
Figure 2: Linear release pattern.
Figure 3: Sigmoidal release pattern.
Actually, the characteristic features of CRFs encompass the release pattern (i.e., shape, lag, lock off); release duration; differential release between N, P, and K; effect of temperature on release; effect of the medium/environmental conditions on release [5, 6]. In most cases, the energy of activation of the release, , is calculated on the basis of estimates of the rate of the release (% released per day) during the linear period obtained from the release curves .
As far as CEN’s definition of release is concerned, the example from Meister formulation described above should comply with the criterion that at least about 75% of nitrogen should be released at the stated release time for this CRF to be approved. As a matter of fact, Kanno  indicated that at the end of 160 days the nitrogen intake reached 79% of the applied N (Figure 4) and so conforming to CEN’s conditions.
Figure 4: Relationship between dissolved N and fertilizer derived N uptake of paddy rice.
In point of fact, establishing nutrient release profiles requires data from both field testing and laboratory testing. In the laboratory, release of nutrients from the excipients is done using water and soil matrices [5, 30]. Field testing involves net bags placed in the ploughed layer of soil in the actual field [5, 30]. Industrial methods involve extract at 25°C, 40°C, and 100°C . However, Du et al.  provide a new procedure where release characteristics are tested in three different systems, namely, (i) free water (which he termed common procedure); (ii) water saturated sand packed in columns; (iii) sand at field capacity moisture.
5. Mechanism of Nutrients Release from CRFs Formulations
Consistent experimental data with reference to release phenomena of nutrients from polymer coated CRFs are indispensably beneficial for better agronomic and environmental results . Agric , after a period of laboratory testing of Meister CRFs, obtained the results described in Tables 1 and 2 for both linear and sigmoid patterns. The designed formulation which is marketed as Meister has its mechanism proposed by the company and the summary is given in Figure 5. The mechanism is based on three significant steps, namely, water adsorption, dissolution of urea, and leaching.
Table 1: Linear release pattern.
Table 2: Sigmoid release pattern.
Figure 5: Release of nitrogen from polyolefin coated urea in water at 25°C.
In addition to that, Guo et al.  proposed the mechanism of nitrogen release from urea-formaldehyde (UF) slow-release fertilizer granules based on three steps. Step one: the coating materials become swollen by absorbing water from the soil and so get transformed into hydrogels which contribute to increasing the orifice size of the 3D network of the coating materials so that it benefits the diffusion of the fertilizer in the core of the gel network. As a result, a layer of water between the swollen coatings and the UF granule core is formed. Step two: water slowly diffuses into the cross linked polymer network and dissolves the soluble part of UF; consequently the soluble part of the fertilizer gets slowly released into the soil through the swollen network with the dynamic exchange of the water in the hydrogel and the water in the soil. Step three: the soil microorganisms penetrate through the swollen coatings and assemble around the UF granule thereby degrading the insoluble part of nitrogen in UF granule into urea and ammonia which in turn is slowly released into the soil via dynamic exchange. Such steps have also been described as lag period, linear stage, and decay period by other researchers .
This mechanism can be adapted to effectively explain the release behaviour in other CRF formulations. Different mathematical mechanistic models based on empirical and mechanistic approaches plus empirical and semiempirical models have been proposed for prediction of the nutrient release using chemophysical parameters as will be discussed in the coming sections. Nevertheless, most mechanisms reveal that nutrients release from CRFs is mainly controlled by diffusion mechanism with respect to temperature, thickness of the coating material, type of nutrient, and the presence or absence of the relevant soil microorganisms.
6. Predicting Nutrient Release from CRFs
Profoundly, a number of empirical and semiempirical mechanistic mathematical models have been put forward in order to provide realistic theoretical assumptions connected to the patterns of nutrients release mechanisms based on the nature and the properties of the delivery systems (DS) , and in that case, release models have been used as tools for improving the CRFs’ design methodology leaving behind conceivable breakthroughs in assessing prospective hazards such as leaching or volatilization losses and effects such as “bursting” or “tailing effect” [7–9]. Such conceptual approaches include the diffusion model, zero order kinetics model, first order kinetics model, Higuchi model, Korsmeyer-Peppas model, Hixson-Crowell model, Weibull model, Baker-Lonsdale model, Hoffenberg model, sequential layer model, Couarraze model, and Peppas-Sahlin model. In particular, most of the proposed release models assume that the release of nutrients from coated CRFs is either controlled by the rate of solute diffusion from the fertilizers or by the rate of water/vapour penetration into the CRF through the coating .
6.1. Diffusion Model
Considering a mathematical model developed for urea release from sulphur-coated granules under soil conditions [7, 33], the assumption was that urea diffuses from the granule through pores or holes caused by erosion of the coating and that the transport is influenced by temperature and soil water content; thus, diffusion occurs through the coating. This model was verified using Fick’s first law as where is the mass of urea diffusing out of the granule, is the effective diffusion coefficient of urea in water, is the cross-sectional area through which diffusion occurs, and is the urea concentration. The subscript is the value for the internal pore coating or outside segments . The predictive power of this model is certainly restricted to the fact that particle flux is directly proportional to the spatial concentration gradient. Nonetheless, it is not the spatial concentration gradient that causes particle movement, that is, particles do not push each other . That is to say, particles do exhibit random motion on the molecular level and this random motion ensures that a tracer will diffuse thereby decreasing the concentration gradient .
Moreover, a study by Jarrell and Boersma  revealed that the diffusion of urea through the sulphur coating occurred in two steps represented in the following models: where , while is the initial mass of urea in the granule, is the concentration of saturated urea solution, is the coating thickness, is the density of solid urea, and is the onset of the period of the decaying rate of release as the solution inside the granule becomes unsaturated.
Similarly, this study is also boundless for the reason that it ignores some important factors and features that are relevant to diffusion of active bioactive substances from an excipient or rather a membrane-coated granule (sphere). It is for that reason that the following Arrhenius type of model pertaining to the diffusion coefficient was suggested [7, 33, 36]: where is the kelvin absolute temperature and stands for the apparent energy of activation for urea diffusion from the excipients. This expression as proposed provides a conceivable explanation for the temperature dependence on the CRFs release rates. On the same side, a similar model for simulating nutrients release from the CRFs in a 1D coordinate system is known ; however, an additional assumption in favour of this model is that the diffusion coefficient is time dependent, thus giving the following expression: where is time, is an initial value at , and is an empirical constant. The time dependence of presents a lag in the curve describing cumulative release with time (i.e., sigmoidal release pattern) which could otherwise not have been obtained by simply applying Fick’s law described before .
6.2. Sequential Layer Model
This model assumes that during the release of an active ingredient from the hydrophilic excipients, significant water concentration gradients are formed in the first place at the matrix/water interface and by so doing there is a creation of water imbibition into the system and as a result, and there occur dramatic physicochemical changes, namely, the exact geometry of the active substance within the excipients, axial and radial direction of the mass transport, and water diffusion coefficient on the matrix. Due to swelling of the excipients following water imbibition phenomenon, the concentration of participating species (i.e., polymer and a chemical substance) significantly changes thereby causing increased dimensions of the system. Consequently, the dissolution of the active ingredient occurs and so it diffuses out of such hydrophilic system following concentration gradients. Essentially, the amount of water available for dissolution is directly proportional to the diffusion coefficient of the active substance within the excipients. In that view, dissolution rate constant, , of the active ingredient-excipient system can be computed and is given as where and are the dry polymer matrix mass at time and , respectively; is the surface area of the device at time [20, 38–45].
6.3. Hopfenberg Model
The primary assumption in this model is that nutrients are released from the surface-eroding excipients possessing some geometries ranging from slabs, spheres, and infinite cylinders displaying heterogeneous erosion. This approach can be mathematically expressed as where is the concentration of the chemical substance dissolved in time , is the total matrix (chemical-excipient) concentration dissolved when the system is exhausted, is the erosion rate constant, is the initial concentration of chemical substance/fertilizer in the matrix, and is the initial radius for a sphere of cylinder or the half-thickness for a slab. The value of is 1, 2, and 3 for a slab, cylinder, and sphere, respectively [20, 38–45].
6.4. Weibull Model
As far as CRFs formulation is concerned, this model accounts for the release of nutrient molecules from the erodible matrix formulations with an assumption that factors influencing the overall release rate are exclusively mass dependent, while other factors stand to be time dependent . The model depicts that a plot of logarithm of the amount of nutrient molecules dissolved in an excipient’ solution versus the logarithm of time will be linear and it is mathematically given as where relates to the time scale of the process corresponding to the ordinate . refers to the lag time before the onset of the release process, is time after release phenomena, is the shape parameter corresponding to the ordinate value when time , and relates to the fraction of the active ingredient in the excipient’ solution at time [20, 38–45].
Despite limitations associated with this model including the inability to sufficiently characterize the release kinetics of the nutrient molecules and the limited use for establishing in vivo/in vitro correlation, the model is known to be grander in the fact that the release half-life can easily be calculated and also the errors associated with it are only single figures, that is, minimum. In fact, the number of single figure errors is known to be higher than other models .
6.5. Korsmeyer-Peppas Model
Based on the CRFs context, this semiempirical model is effective in the determination of the concentration of nutrient molecules released from the excipients’ membranes. Theoretically, the simple expression allied to this model is given as where refers to a constant incorporating structural and geometric characteristics of the given active substance, is the release exponent indicative of the release mechanism, and