INTRODUCTION
Background of Study
The global food supply is becoming increasingly urgent due to rising demand and limited arable land, necessitating the effective use of nutrient fertilizers Farming, as described by Johnston, is a complex biological activity that significantly impacts the environment Just as human survival relies on a stable food supply, plants require essential nutrients for growth, with fertilizers playing a crucial role in modern agriculture According to the FAO, total fertilizer nutrient consumption was 183.2 million tons in 2013, projected to rise to 186.9 million tons in 2014, and is expected to reach 200.5 million tons by the end of the decade, reflecting a steady annual growth of 1.8 percent.
2018 Figure 1.1 outlines estimated global demands for total fertilizer nutrients from
2014 to 2018 compared to actual consumption over the preceding six years
Table 1.1 highlights the increasing global demand for nitrogen fertilizers, which rose from 111.4 million tons in 2013 to 113.1 million tons in 2014, reflecting a growth rate of 1.5% This demand is projected to reach approximately 119.4 million tons by 2018, with an annual growth rate of 1.4% Overall, the total increase in nitrogen demand between 2014 and 2018 is estimated at 6.3 million tons, with 58% allocated to Asia, 22% to the Americas, 11% to Europe, 8% to Africa, and 1% to Oceania.
Figure 1.1: Global consumption of (N+P2O5+K2O) fertilizers as per FAO [3]
Table 1.1: Global demand for fertilizer nutrients (2014 - 2018) in thousand tons [3]
Improper use of any type of fertilizer—natural, inorganic, or organic—can negatively impact the environment Research by Al-Zahrani et al indicates that plants absorb only 30% of conventional fertilizers, with the remainder lost through volatilization, leaching, or fixation, leading to excess nutrients in air and water Excessive fertilizer application can also harm seedlings, which require minimal nutrients during early growth stages Additionally, conventional fertilizers are typically applied multiple times throughout the crop cycle, increasing labor costs and contributing to nutrient surpluses in the environment.
Excess nutrients that plants do not absorb can lead to loss and threaten seedlings and young roots during germination Additionally, if nutrient levels drop below effective thresholds, it negatively impacts normal plant growth.
Figure 1.2: Effect of conventional fertilizer and controlled-release fertilizer on nutrient requirement
Plants require varying amounts of nutrients at different growth stages, starting with smaller quantities in the early phases and increasing as roots, stalks, and stems develop This fluctuation in fertilizer needs throughout the growth cycle highlights the importance of creating fertilizers with a nutrient release profile that aligns closely with the plant's nutrient uptake requirements.
Controlled-release fertilizers (CRF) represent an innovative green technology that minimizes nutrient loss from volatilization and leaching By modifying the kinetics of nutrient release, CRFs ensure that plants receive a timely supply of nutrients that aligns with their metabolic requirements This approach maintains optimal nutrient concentrations, enhancing overall plant health and growth.
Nutrient Concentration in the soil
Figure 1.3: The “ideal fertilizer”: nutrient release is synchronized with the nutrient requirements of a given crop (taken from Trenkel et al [9])
Urea, the leading nitrogen fertilizer, significantly enhances plant growth and improves soil conditions, making it a preferred choice for many controlled-release fertilizer (CRF) applications due to its high nitrogen content and cost-effectiveness The pioneering study on CRF application was conducted by Ortil in 1962, while Al-Zahrani categorized CRFs into three distinct groups.
Reservoir formulations, often referred to as coated fertilizers, utilize an inert carrier to encapsulate the fertilizer core, allowing for controlled nutrient release through diffusion Examples include sulfur-coated urea (SCU) and polyethylene-coated urea Chemically controlled-release fertilizers consist of low solubility organic nitrogen compounds, such as urea-formaldehyde, produced synthetically Additionally, matrix (or monolithic) controlled-release fertilizers (CRFs) feature active ingredients dispersed within a matrix, where they dissolve and diffuse to the external surface.
Research has focused on developing innovative controlled release fertilizers (CRFs) that align nutrient release with the specific uptake cycles of plants This synchronization ensures that nutrients are available when plants need them most, enhancing growth and efficiency.
The application of controlled release fertilizers (CRFs) varies significantly based on the growth periods of different plant species, such as 120 days for cabbage, 100-120 days for short-duration rice, 140-160 days for long-duration rice, and 60-65 days for corn Developing CRFs requires considerable time, often taking 3-6 months or more to assess their release characteristics under both laboratory and field conditions Consequently, modeling is utilized to analyze CRF nutrient release mechanisms, making it an essential tool in the design and application of CRFs.
Modeling and simulation (M&S) play a crucial role in understanding behaviors without real-time testing, particularly in chemical engineering within an optimization framework By employing mathematical models to describe phenomena and processes, M&S aids in generating predictive data essential for decision-making in product and process innovation, as well as design and operations This technique is extensively utilized in research and development of controlled-release fertilizers (CRFs) Many researchers have explored both the phenomena and mathematical models governing nutrient release processes, with a focus on diffusion through coated layers Regression modeling, a statistical approach for estimating relationships between predicted and independent parameters, has been widely adopted in this context, as evidenced by studies conducted by Kochba, Gandeza, Medina, Wilson, and Zheng However, it is important to note that these models were primarily linked to specific coating materials.
Figure 1.4: History of CRFs modeling
1980 Boersma, mathematical model, SCU, diffusion
1987 Glasser, Polymer coated granuler, 1D, diffusion is time dependence
1990 Kochba, semi- empirical, 1st order, ignore geometry size,
1991 Gandeza, empirical, effect of temperature in soil.
1992 Lu and Lee, Fick's law, spherical coordinate, LCU
1996 Byung-Su Ko, sigmoidal release = sum of individual releases
1997 Al-Zahrani, unsteady state, well-mixing core.
2003 Shaviv et al., three stages release, single granule, fick' law
2003 Shaviv et al., extend to population of granules
2004 RYUSEI Ito, Model - sigmoidal pattern
2005 RYUSEI Ito, Design multi- layer coated granule Calculate average diffusivity
2007 Lu, pseudo-steady state, Fick's law, neglect lag period, "trial and error"
2011 Wang, empirical, release relationship betwwen two temperature at 25 and 100 C
2013 Xiao Yu Kinetic models (exponential and double exponential)
2015 Trinh et al., multi-layer diffusion, FEM
2015 Trinh et al., release in soils, imperfect coating
2015 Ahmad, mechanistic model for P release, thickness change over time
History of modeling of CRFs
In 1980, Jarrell and Boersma proposed a mathematical model to analyze urea release from sulfur-coated urea (SCU) particles, evolving from simple to complex methodologies Researchers applied various approaches, including Fick’s first law of diffusion for straightforward analytical solutions and Fick’s second law alongside mass balance over the coating's spherical shell However, all these models relied on the assumption of a pseudo-steady state.
The sigmoidal release pattern, highlighted by Shaviv, has garnered significant attention and consists of three distinct stages: an initial lag phase with no observable release, a steady-state release phase, and a final stage characterized by a gradual decline in release.
Recent developments in release models for sigmoidal patterns have primarily focused on 1D coordinate systems, which consider factors like granule size and coating thickness of natural fertilizers However, this approach limits the ability to accurately describe the effects of non-uniform thickness and population variations, both of which significantly influence nutrient release due to production processes A 2D coordinate system is more appropriate for representing actual fertilizer granules While diffusion through the coating layer is often cited as controlling the release process, environmental factors—such as variations in water and soil composition—are frequently overlooked, leading to inconsistent results in plant growth across different soil environments Additionally, existing models fail to account for imperfections in coating thickness and often neglect the role of water absorption in the coating, which is related to the lag period of nutrient release These limitations are summarized in Table 1.2.
In summary, fertilizers are essential for modern agriculture, particularly as the need for higher crop yields grows amidst limited arable land However, the environmental impact of fertilizers is a significant concern Controlled-release fertilizers (CRFs) may offer solutions to improve nutrient use efficiency (NUE) while minimizing fertilizer losses To achieve optimal results, it is crucial to develop more precise nutrient release models, which can shorten the time needed for experimentation and help create CRF designs that meet the specific nutrient needs of different crops.
Table 1.2: Limitations of previous models in cited literature
Problem Statement
To enhance the understanding of controlled release rates and patterns, a detailed mathematical model is essential for accurately predicting nutrient release in both laboratory and field settings This improved model will serve as a valuable design tool for technologists.
Most studies on urea diffusion have primarily focused on the coating itself, neglecting the impact of water absorption and the nutrient release into the soil, which is complicated by mass transport dynamics In contrast, a 2D model offers a more accurate representation of real fertilizers, particularly addressing the imperfections in coating thickness that can lead to premature nutrient release The limitations of a 1D model in depicting non-uniform coatings highlight the need for a 2D-coordinate model that incorporates multilayer effects and environmental factors such as water and soil interactions Attention to non-uniform coatings is crucial for improving predictive accuracy and optimizing coating application designs.
Research Objectives
This study aims to develop and validate models for two key processes: water penetration into coatings and urea release from the core through the coating layer, addressing both constant and decay release stages Additionally, the research seeks to utilize these models to predict the diffusivity of coatings and to design specific geometrical parameters for controlled release formulations (CRF).
Scope of Research
This study aims to enhance understanding of the urea release process by examining all three stages of release Utilizing computer simulations, the research models the transport of water penetration and nitrogen release Additionally, both experimental and simulated methods are employed to investigate the complete release process from coated granules.
Agrium's commercial controlled-release fertilizer (CRF) is chosen for experimental validation due to its long-lasting release properties and market availability The study comprises two main components: nitrogen release and water absorption Nitrogen release tests are conducted over four months with three replicates, collecting samples at 2-5 day intervals These samples are analyzed using a UV-Vis spectrometer to assess nitrogen concentration and reconstruct the CRF’s release pattern Additionally, water absorption tests are performed to evaluate the dynamics of water absorption into the fertilizer coating.
To efficiently predict the release behavior of Controlled Release Fertilizers (CRF), a 2D model utilizing COMSOL Multiphysics significantly reduces the experimental time required for analysis This model is based on mass conservation equations, concentration-dependent diffusivity, and an interfacial IAR model for nutrient diffusion in soil Employing the Finite Element Method (FEM) allows for accurate simulation of nutrient release and water absorption due to its ability to handle complex geometries and varying material properties The model has been validated through laboratory experiments and literature data, demonstrating its reliability across a diverse range of granule sizes and release durations Additionally, it can predict nitrogen release in various environments, enhancing its applicability The incorporation of a non-uniform coating further improves the model's predictive capabilities, making it a valuable tool for understanding nutrient dynamics in different soil conditions.
The proposed model efficiently predicts the optimal coating material for controlled-release fertilizers (CRF), significantly reducing experimental time by one-third By obtaining coating diffusivity, researchers can accurately design geometrical parameters to achieve a nutrient release pattern that aligns with the specific uptake needs of plants This article focuses on the Sj rice variety to illustrate the calculation process and optimize the selection of suitable parameters.
Thesis Organization
This thesis holds six chapters, namely, introduction, literature review, research methodology, results & discussion, and conclusion A description of each chapter now follows
First chapter is a brief introduction and background leading to the current research This includes problem statement and research objectives
Chapter Two provides a comprehensive literature review that underpins this research, starting with the history of fertilizers and the environmental drawbacks of Controlled Release Fertilizers (CRF) It then outlines existing modeling approaches in the field, highlighting the research gap that this study aims to address.
Chapter Three is crucial for guiding the research direction, detailing the methodology that connects modeling and experimental validation It provides a comprehensive overview of the models and solvers utilized from COMSOL Multiphysics, along with the relevant theories and governing equations for each model The chapter outlines the boundary and initial conditions essential for nutrient release stages Additionally, it applies the nutrient release model to optimize coating parameters and design the geometry of controlled-release fertilizers (CRFs) The final section focuses on experimental methodology, emphasizing the characterization and validation processes for each model, which supply necessary input data for simulations It also describes the execution of nitrogen release tests and water penetration assessments in the coating, including the presentation of experimental results.
Chapter Four is crucial as it presents the findings from the nitrogen release and water penetration models of this study It includes model verifications and comparisons with existing literature, alongside an in-depth discussion of the results and the mechanisms involved in the release process Additionally, the chapter concludes with a demonstration of the design CRF parameters.
Chapter five concludes by summarizing the project's achievements and offers a proposal for future work aimed at improving the model's capabilities to optimize nutrient release design, ultimately supporting farmers and industrial producers.
LITERATURE REVIEW
Overview
This chapter highlights the essential function of fertilizers in agriculture while addressing their negative environmental impacts from overuse To mitigate these issues, alternatives such as controlled-release fertilizers (CRFs) have been introduced A comprehensive literature review on CRFs, including their release mechanisms and production methods, is presented Additionally, the significance of modeling in analyzing the behavior of controlled release is examined, underscoring the research gap that this project aims to address.
Fertilizers and Their Role on Plant Growth
In the nineteenth century, scientists began to uncover the complexities of plant growth, leading to various theories that explained the requirements for healthy development Through meticulous observation and experimentation, it became evident that soil plays a crucial role in providing nutrients for plants, with humus being recognized as a key factor in promoting plant health Today, it is widely accepted that essential elements can exist in different forms, either as inorganic ions or derived from organic materials, contributing significantly to the growth of green plants.
The nutritional status of a plant is determined by the combination of concentration and lability of essential elements, rather than just their presence By 1890, scientists identified that plants require key elements including carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), sulfur (S), potassium (K), calcium (Ca), magnesium (Mg), and iron (Fe); deficiencies in these elements can lead to poor growth or plant death By the early 1900s, ten of the now-known sixteen essential elements were recognized, although a scientific system to establish their essentiality was lacking, leading to assumptions based on their importance Between 1922 and 1954, additional elements such as manganese (Mn), copper (Cu), zinc (Zn), molybdenum (Mo), boron (B), and chlorine (Cl) were confirmed as essential A comprehensive list of all sixteen essential elements, along with their discoverers and the discoverers of their essentiality, is provided in Table 2.1.
Table 2.1: Chronology of Discoveries of Essential Nutrient Elements [29]
Element Discoverer Year Discoverer of essentiality Year
Ca Davy 1807 von Sachs, Knop 1860
Mg Davy 1808 von Sachs, Knop 1860
Mo Hzelm 1782 Arnon and Stout 1939
B Gay-Lussac and Thenard 1808 Sommer and Lipman 1926
Cl Scheele 1774 Stout 1954 a Element known since ancient times
The concentrations of essential nutrients required for normal plant growth and development vary significantly, ranging from 1.0 to 1 million among thirteen key elements Carbon, hydrogen, and oxygen, sourced from air and water, are combined through photosynthesis, a process unique to green plants containing chlorophyll During this energy-capturing process, which occurs in the presence of light, water molecules (H2O) are split; one part combines with carbon dioxide (CO2) to create carbohydrates (CH2O), while the other part is released as oxygen (O2).
2 n2H O light chlorophyll nCH O nO nCO + + + ® + (2.1)
Table 2.2: Average concentrations of mineral nutrients in plant dry matter required for adequate growth (adapted from Jones et al [29])
Mineral Nutrient mmol/g dry wt mg/kg (ppm)
Percent Relative Number of Atoms
Nitrogen, which constitutes 98% of its presence in soils, is primarily linked to organic material The total nitrogen content in soils varies, ranging from 0.02% in subsoil to 2.5% in peat Soil nitrogen exists in three primary forms: organic matter, ammonium (NH4+), and nitrate (NO3-).
) ions fixed on exchange sites of clay minerals, and (3) ammonium and nitrate (NO3 -
) ions in soil solution The forms of nitrogen that are important to plant nutrition are ammonium (NH4 +
) Both nitrate and ammonium are taken up by plants and constitute the major crop fertilizer forms of nitrogen [29]
Nitrogen fertilizers undergo various transformations in the soil, primarily through mineralization, which converts organic nitrogen into inorganic forms, and nitrification, which changes ammonium into nitrate Additionally, nitrogen is incorporated into the soil via symbiotic fixation, where nitrogen gas (N2) is converted into ammonia or ammonium Plants primarily absorb nitrogen in the forms of ammonium and nitrate, but nitrogen can be lost from the soil through processes such as plant uptake, leaching, volatilization, and denitrification, where nitrate is converted back into nitrogen gas Factors influencing these transformations include aerobic and anaerobic conditions, soil pH, temperature, and the presence of chemical inhibitors or certain fungicides and pesticides.
The application of nitrogen fertilizers in large quantities significantly affects soil pH, particularly through urea and ammonium-based fertilizers, which lead to notable acidification Nitrate is the primary nitrogen form absorbed by plants, primarily due to the rapid conversion of ammonium in the soil This nitrification process occurs in two steps: first, bacteria such as Nitrosomonas spp oxidize ammonium into nitrite, and then Nitrobacter bacteria convert nitrite to nitrate, with the second step occurring more rapidly than the first.
Under typical conditions, nitrite accumulation in soil is minimal unless hindered by certain environmental or agricultural practices The process of nitrification necessitates oxygen and produces hydrogen ions, leading to gradual soil acidification Consequently, lime is needed to counteract this acidity and restore soil balance.
Nitrogen content in plant leaf tissues typically ranges from 1.0% to 6.0% of dry weight, with high levels potentially causing growth stimulation that leads to deficiencies in other nutrients due to dilution During early growth, petiole nitrate measurements can reach 8,000 to 12,000 ppm, dropping to 3,000 to 8,000 ppm in mid-season, primarily concentrated at the base of main stems and in the petioles of new leaves As all nitrogen forms are mobile within plants, deficiency symptoms first manifest in older leaves, resulting in slow growth, weak and stunted plants, small leaves, and a light green to yellow foliage color, with premature leaf drop Severe nitrogen deficiency can lead to necrosis, reduced root growth and branching, and an increased root-to-shoot ratio, ultimately diminishing both yield and quality.
Plants are more tolerant of excess nitrate than of ammonium, which can become toxic if not converted into carbon-containing nitrogen compounds after absorption High levels of ammonium can hinder potassium uptake by competing for root binding sites, leading to toxicity conditions This toxicity manifests as restricted and often discolored root growth, resulting in vascular tissue breakdown and impaired water uptake.
To enhance agricultural production, it is essential to either expand cultivated land or boost crop yields per unit area While recycling agricultural residues and waste can help maintain soil fertility, it does not enhance it Improving soil fertility requires the addition of external nutrients, which can be effectively achieved through the use of fertilizers.
According to the latest FAO report, global fertilizer consumption is projected to grow by 1.8% annually through 2018, with East and South Asia accounting for 60% of nitrogen fertilizer usage Fertilizers play a crucial role in replenishing nitrogen in soil after crop harvests, contributing significantly to agricultural productivity since the postwar era However, excessive nitrogen use can lead to environmental issues, including water resource damage For 2018, the total fertilizer requirement is estimated at 200.5 million tons of N + P2O5 + K2O, with nitrogen usage alone expected to reach approximately 119.4 million tons, making up about 59% of total fertilizer demand.
Global agricultural authorities recognize that achieving production targets necessitates a significant increase in fertilizer usage While fertilizers are crucial for agricultural development, they can negatively impact the environment, leading to issues like heightened soil acidity and nutrient loss through leaching and evaporation To mitigate these adverse effects, various alternatives have been suggested.
Legumes offer a nitrogen supply without direct financial costs, yet they require energy for the fixation process While it may seem that using legumes is more environmentally friendly than synthetic fertilizers, this is not always the case For instance, in well-nourished crops, 400 kg of carbohydrates are consumed to fix every 100 kg of atmospheric nitrogen After the growing season, the decomposition of plants can lead to significant nitrate release, risking leaching during winter rains Reports indicate that leaching losses can reach up to 130 kg of nitrogen per hectare from red clover in England, highlighting that in certain situations, legumes may be less environmentally sustainable than fertilizers that can be tailored to meet crop nitrogen needs.
Alternative nitrogen sources such as waste manure and sewage sludge offer an eco-friendly option, but they have a significant drawback due to their low nutrient concentration, which is approximately 1/40 that of conventional fertilizers This results in high transport and spreading costs per nutrient unit, making them less viable for use over long distances from the source.
Fertilizers continue to play a crucial role in agricultural development, despite the growing interest in alternative sources Organic manures provide a slow release of nutrients, ensuring a steady supply over time, but their unpredictable release rates, influenced by weather and soil conditions, limit precise control In contrast, inorganic fertilizers allow for better management of nutrient supply through timely applications The emergence of controlled-release fertilizers (CRFs) addresses these challenges by combining the benefits of both organic and inorganic options, offering a consistent nutrient supply tailored to the plant's developmental stages while enabling predictable release behavior Further insights into CRFs can be found in sections 2.4 and 2.5.
Figure 2.1: Example of nitrogen utilization from legumes (adapted from [32]).
Adverse Impacts of Fertilizers on the Environment
Applying fertilizer to soil helps supplement nutrients for plants, but not all of these nutrients are absorbed by crops The remaining nutrients can either stay in the soil, leach into water, or volatilize into the atmosphere Excessive application of fertilizers can lead to leaching, contaminating surface and groundwater, and contribute to health and environmental issues due to the accumulation of chemicals in ecosystems Understanding the environmental impacts of fertilizers is crucial for sustainable agricultural practices.
The primary concern regarding chemical fertilizers is their contribution to groundwater contamination, as nitrogen fertilizers convert to nitrates that easily dissolve in water and persist in groundwater for decades This accumulation poses significant environmental risks, including associations with serious health issues such as gastric cancer, goiter, birth malformations, hypertension, and testicular cancer A particularly alarming effect of nitrate contamination is methemoglobinemia, or 'Blue Baby' syndrome, which occurs when infants consume formula mixed with nitrate-laden water, leading to dangerously low blood-oxygen levels and potentially fatal outcomes Additionally, nitrogen pollution in groundwater plays a role in the creation of marine "dead zones," further exacerbating environmental degradation.
The application of nitrogen fertilizers in agriculture significantly contributes to greenhouse gas emissions, particularly nitrous oxide (N2O) and nitric oxide (NO), which together account for approximately 60% of emissions from this sector These gases are naturally produced in soil through microbial processes such as nitrification and denitrification The release of nitrous oxides is particularly concerning due to their impact on global warming and ozone layer depletion, which can lead to increased ultraviolet radiation exposure for humans and animals The rising use of nitrogen fertilizers for high-demand crops like corn has exacerbated nitrous oxide emissions While nitrogen fertilizers are crucial for profitable crop yields, adopting more efficient nitrogen management practices can substantially reduce these emissions, lower production costs, and alleviate nitrogen contamination in surface and groundwater.
Figure 2.2: Impacts of nitrogen fertilizers on the environment
Table 2.3: U.S Agricultural Greenhouse gas Emissions by Source [35]
Source Total Emissions Agricultural Emissions
CO 2 from fossil fuel consumption 0.6 7
Fertilizer runoff from land accumulates in lakes, rivers, and eventually oceans, leading to increased concentrations that pollute aquatic environments This pollution causes algal blooms, which deplete oxygen in subsurface waters, creating anaerobic conditions that asphyxiate fish and crustaceans The resulting ecological disruption impacts local communities that rely on these water sources for food.
Surface-applied ammonium and urea fertilizers can lead to significant NH3 volatilization, especially in calcareous and alkaline soils This process diminishes the economic efficiency of agricultural systems by lowering crop yields and increasing fertilizer costs Moreover, ammonia emissions from fertilized fields can adversely affect nearby ecosystems, harming local vegetation Some of the emitted NH3 may oxidize into nitric acid, which, when combined with sulfuric acid from industrial activities, contributes to acid rain This acid rain not only damages vegetation but also acidifies lakes, resulting in aluminum toxicity that threatens fish and plant life.
Controlled-Release Fertilizers and Slow-Release Fertilizers
To improve nutrient use efficiency in agriculture, various fertilizers are available, including controlled-release fertilizers (CRFs) and slow-release fertilizers CRFs are specifically formulated to release essential nutrients in a controlled manner, aligning with the plants' nutrient requirements over time This synchronization not only enhances nutrient use efficiency but also boosts crop yields significantly.
An ideal controlled-release fertilizer features a natural or semi-natural macromolecule coating that slows nutrient release, allowing a single application to fulfill a crop's nutrient needs for an entire season While these fertilizers extend nutrient availability, their release rates are significantly influenced by soil conditions, such as microbial activity and moisture levels.
Controlled-Release Fertilizers (CRFs) and Slow-Release Fertilizers (SRFs) are often viewed as similar, yet Trenkel and Shaviv highlight key differences between them SRFs exhibit an unpredictable nutrient release pattern influenced by varying soil and climatic conditions, while CRFs allow for a more predictable release of nutrients in terms of timing and quantity, albeit within certain limits For the purposes of this study, the term "Controlled-Release Fertilizers" encompasses both CRFs and SRFs.
The development of Controlled Release Fertilizers (CRFs) dates back to the early 1960s, initially utilizing sulphur and polyethylene as coating materials Over time, the evolution of CRFs has incorporated a variety of materials, including polymers, natural coating agents, multifunctional super-absorbent substances, and nano-composites Many of these CRFs have been produced on a commercial scale, showcasing their significant advancements in agricultural technology.
Current literature presents a diverse classification of Controlled Release Fertilizers (CRFs), with a comprehensive system based on the insights of Shaviv, Trenkel, Liu, and Rose This classification delineates three major categories of CRFs, as illustrated in Figure 2.3.
1 Organic compounds that are further sub-divided into natural organic compounds (animal manure, sewage sludge etc.) and synthetically produced organic-nitrogen, low solubility compounds The latter group generally includes condensation products from urea and acetaldehyde These compounds are further subdivided into biologically decomposing compounds, e.g., urea formaldehyde (UF) and chemically decomposing compounds such as isobutyledene-diurea (IBDU) or urea acetaldehyde/cyclo diurea (CDU)
2 The second major category includes well-known water-soluble fertilizers with physical barriers that control nutrient release These are produced either as granules/cores coated with a hydrophobic polymer, or as a matrix of active fertilizer nutrients dispersed on a continuum using hydrophobic materials that encumber fertilizer dissolution However, controlled release matrices are less common compared to coated CRFs, which is why this dissertation is focused on the Controlled-Release Coated Fertilizer that contains only urea Coated granular CRFs are subcategorized into those coated with organic polymer materials (e.g., thermoplastics, resins etc.) and those coated with inorganic materials (including sulfur and other minerals) Nowadays, coating materials include non-degradable and degradable, especially bio-degradable Controlled release matrix material can be either hydrophobic, e.g., polyolefin, rubber, etc., or gel forming polymers sometimes referred to as hydrogels A hydrogel is hydrophilic wherein the dissolution of the dispersed fertilizer through hydrogel material is impeded by its ability to retain high amounts of water (swelling)
3 Lastly are inorganic low solubility compounds that include metal ammonium phosphates, e.g., KNH4PO4 and MgNH4PO4, and partially acidulated phosphate rock (PAPR)
Another classification of CRFs can also be based on the mode of the control release, i.e., diffusion, erosion or chemical reaction, swelling and osmosis Blaylock
[41], however, classified CRFs as only two major types: those coated with low solubility compounds and those coated with water soluble materials
Figure 2.3: Classification of controlled-release fertilizers
Understanding the controlled-release mechanism is crucial for evaluating the effectiveness of controlled-release fertilizers (CRFs) This mechanism is complex and influenced by various factors, including the coating material, type of CRF, and agronomic conditions Current literature discusses multiple mechanisms, many of which are still evolving Liu and Shaviv introduced the multi-stage diffusion model for coated fertilizers, which illustrates that after application, irrigation water penetrates the coating, leading to nutrient dissolution As osmotic pressure increases within the granule, it can result in either the coating bursting when pressure exceeds the membrane's resistance, releasing the core's nutrients.
Hydrophilic matrices Organic coating Inorganic coating
Non-degradable biodegradable materials can undergo a "failure mechanism" or "burst release," where they spontaneously release their contents Alternatively, if the membrane can withstand the increasing pressure, the core fertilizer is gradually dissolved and released through diffusion, driven by either a concentration gradient, a pressure gradient, or a combination of both.
The diffusion mechanism is a key process in coated fertilizers, where both diffusion and failure mechanisms occur simultaneously, though their ratios differ based on the coating materials used Frail coatings, such as sulfur or modified sulfur, typically exhibit a failure mechanism, while polymer coatings, like polyolefin, are designed to demonstrate a diffusion release mechanism Figure 2.5 illustrates these potential release scenarios effectively.
Nutrient release from coated fertilizers is influenced by the coating characteristics, such as diffusivity, and the geometry of the granules, including thickness and radius Additionally, environmental factors like ambient temperature and moisture content significantly affect the release rate, which tends to increase with higher temperatures and greater moisture levels The primary mechanism for nutrient release involves the transfer of nutrients from the fertilizer-coating interface to the coating-soil interface, facilitated by water Key parameters governing this release mechanism include diffusion and swelling, degradation of the polymer coating, and fracture or dissolution Similar mechanisms have been discussed in studies by Guo, Liang, Liu, and Wu.
Figure 2.4: A demonstration of an entire nutrient release process (obtained from
Fertilizer release into soil gradually and consistently
Water penetrate in through the coating
Nitrogen dissolve into solution inside the granule
Nitrogen move out through the polymer coating
Controlled-release coated fertilizers utilize a polymer coating to encapsulate the fertilizer core As water infiltrates the coating and granule, pressure accumulates, leading to a burst release of nutrients Subsequently, these nutrients are gradually released through a diffusion mechanism, ensuring a steady supply for plants.
2.4.3 Advantages and Disadvantages of CRF/SRFs
Controlled-release fertilizers offer significant savings by reducing the quantity of fertilizer needed, minimizing application frequency, and lowering labor costs, as only one application is necessary throughout the growing season Additionally, they mitigate nutrient losses and prevent issues such as seed toxicity, hazardous emissions, leaf burning, dermal irritation, and inhalation problems These fertilizers also enhance soil quality and handling properties while safeguarding seed germination from adverse effects.
Table 2.4: Advantages and disadvantages between controlled-release fertilizer and conventional fertilizer
Nevertheless, they are expensive and pose marketing issues In addition, some coating materials used to produce CRFs are non-biodegradable and toxic to the soil
The release pattern of Controlled Release Fertilizers (CRFs) remains uncertain in field applications, as they can significantly alter soil pH and require modified storage facilities to prevent premature nutrient release due to moisture absorption Widespread adoption of CRFs is hindered by insufficient data on their release kinetics across different soil types and environmental conditions Additionally, CRFs are sensitive to temperature fluctuations, ambient moisture, soil bioactivity, and wetting and drying cycles, leading to unpredictable nutrient release rates that can compromise fertilizer efficiency, particularly when tailored for specific crops Furthermore, CRFs do not adjust to the plant's nutrient demands, releasing nutrients at a constant rate regardless of the plant's actual needs.
Figure 2.6: A comparison of the supplied nutrients between conventional and controlled-release fertilizers.
Coating Materials for CRF Production
The ongoing development of coating materials for Controlled Release Formulations (CRFs) aims to optimize release characteristics while minimizing production costs Recent research trends have increasingly focused on environmentally friendly materials, highlighting their importance in the evolution of coating technologies.
2.5.1 CRF from Sulfur Based Coating Materials
Sulfur has been a popular choice for producing coated release fertilizers (CRF) for decades due to its numerous benefits, including serving as a secondary plant nutrient and fungicide It helps reduce fertilizer caking and has acidic properties that can neutralize soil alkalinity Additionally, sulfur is relatively inexpensive and biodegradable compared to polymer materials However, its crystalline structure can lead to microscopic pores and cracks, resulting in increased brittleness.
High demand for sulfur-coated urea arises from its unique properties, yet its low release rate can lead to increased friability when exposed to high soil temperatures The sulfur coating's high surface tension results in poor wettability and adhesion to the urea substrate, making it an ineffective sealant without additional conditioning materials, which can impose economic challenges Furthermore, sulfur residues in the soil can accumulate, potentially reacting with water to acidify the soil over time Overall, the nutrient release mechanism from sulfur-coated urea involves both immediate and gradual processes.
“burst” and diffusion release, as mentioned in section 2.4.2
2.5.2 CRFs Using Polymer-Based Coating Materials
Polymeric materials have become the preferred choice for coating urea, as they are resistant to microbial disruption, unlike sulfur coatings The nutrient release from polymer-coated granules is influenced by diffusion, which depends on the coating thickness and soil temperature These polymer coatings offer several advantages for controlled urea release, including biological inertness against microbial attacks and the ability to supply nutrients in alignment with crop metabolic requirements over extended periods Additionally, they effectively retain both micro and macro nutrients within their helical polymer chain matrix.
Polymer materials offer several advantages, but they also face significant limitations The complexity of the coating processes, which involve multiple chemicals, contributes to high costs, primarily due to the necessity of organic solvents for formulating coating solutions This reliance on solvents not only elevates expenses related to their recovery but also raises environmental concerns due to hazardous emissions To address these issues, the development of aqueous polymeric solutions has been initiated Additionally, many polymer coatings are non-biodegradable after nutrient release, leading to undesirable soil pollution As a result, the commercial production of controlled-release urea fertilizers has been limited Nevertheless, some polymers have been successfully utilized in the commercial production of controlled-release urea, which will be discussed in the following section.
2.5.3 CRF from Superabsorbent/Water Retention Coating Materials
Superabsorbent polymer materials (SPMs) have gained significant attention in research due to their unique properties that enhance CRF production These 3-dimensional cross-linked hydrophilic polymers can absorb water hundreds of times their weight, retaining it even under pressure Their application in agriculture and horticulture is particularly beneficial in drought-prone areas, as they reduce water consumption and irrigation frequency, making them an economical choice The use of SPM-produced Controlled Release Fertilizer Units (CRCUs) improves soil aeration, mitigates soil degradation, decreases water evaporation losses, reduces environmental pollution from volatilization and leaching, and lowers crop morbidity by enhancing nutrient retention.
The use of smart polymer materials (SPMs) in the production of controlled-release fertilizers (CRFs) offers significant benefits, including enhanced water absorption and regulated urea release However, the complex preparation process and expensive raw materials lead to higher production costs, hindering their commercialization Additionally, the non-biodegradability of certain coating materials contributes to soil pollution, posing further challenges Despite these obstacles, ongoing research in this emerging field aims to address these critical issues.
2.5.4 CRF from Bio-composite-Based Coating Materials
Recent research has focused on developing bio-composite coatings for controlled-release fertilizers to address the issues of non-biodegradable polymer coatings and high operational costs Starch, a naturally occurring polysaccharide biopolymer sourced from renewable plants, has emerged as a promising candidate due to its low cost, biodegradability, and abundance However, because starch is hydrophilic, it cannot be used alone as a coating material for controlled-release fertilizers and must be blended with other materials for optimal effectiveness.
2.5.5 Commercially Available Controlled-Release Coated Fertilizer
Despite their high operational costs, controlled-release fertilizers (CRFs) have been commercially produced and sold, primarily for horticultural and ornamental uses rather than large-scale agriculture The Tennessee Valley Authority (TVA) led the way in CRF commercialization by producing sulfur-coated urea on a large scale Additionally, Arthur Daniels Co (ADM) became the first company to manufacture polymer-coated fertilizers using dicyclopentadiene and glycol ester A comprehensive literature review of the coating materials utilized for commercial CRF production is presented in Table 2.5.
Several companies have marketed thin PCU products as controlled-release nitrogen sources (e.g., ‘‘POLYON’’-coated urea by Pursell, ‘‘ESN’’ by Agrium,
In Malaysia, several companies, including HIF TECH, Nufarm, WASTECH Multigreen, Diversatech Fertilizer, and SK Specialities, offer controlled-release fertilizers (CRFs) primarily for the oil palm plantation industry These CRFs are often customized based on the age of the plants, with claims that their use can reduce fertilizer costs by 30% and increase oil palm yields by 25% Notably, SK Specialities asserts that it was the first CRF producer in Malaysia to employ advanced polymer coating technology using palm oil-based polymers A summary of the commercial CRFs available in the Malaysian market is presented in Table 2.6.
Table 2.5: Coating materials used to produce CRF on a commercial scale
Commercial name Coating material Company/Provider Ref
SCU Sulfur + wax + diatomaceous earth + coal tar
Tennessee Valley Authority (TVA) USA
Meister Polyolefin + inorganic powder (polymer composition of natural products, resins and additives)
LP30 - 180 Polyolefin Chisso-Asahi Fertilizer
ESN Polymeric material Agrium Inc Calgary
CU & CUS Polymeric material Chisso-Asahi Fertilizer
MultiCoteÔ Polyurethane-like Haifa Chemicals Co Ltd., [76] Zn-coated urea Zinc oxide Indo-Gulf Fertilizers,
Kingenta PCU Polymeric material Shandong Kingenta
Duration Ò Micro-thin membrane Agrium Advanced
Technologies FLORIKOTE ọ Dual reactive layer polymer coating
POLYON ® Durable ultra-thin polyurethane coating
HANCOTE Resin coated fertilizer Hanfengs
Table 2.6: Commercial CRFs in Malaysian market
Malaysia Distributor Commercial name CRF Producer, Country
WASTECH Multigreen Agrium Agrium Advanced Technology,
HIF TECH SDN BHD AGROBLEN Scotts
SK Specialities, Sarawak SK COTE Ò Single
Modeling and Computational Fluid Dynamics
Modeling and simulation (M&S) enables the prediction of system behavior without real-life testing, particularly in chemical engineering, where it employs high-fidelity mathematical models within an optimization framework This approach supports decision-making in product innovation, design, and operation Effective modeling combines a mathematical model grounded in chemical engineering principles, experimental data for validation, and solution techniques to harness the model's predictive power By clarifying system behavior, models help reduce production and design costs, shorten time to market, and evaluate alternatives, ultimately aiming to enhance operations and identify optimal solutions.
Computational fluid dynamics (CFD) is a numerical method for solving conservation equations related to mass, momentum, energy, and species using programming languages Mastery of partial differential equations and numerical methods is crucial for approximating solutions effectively As CFD software emerged in the late 1980s, tools like ANSYS Fluent, COMSOL Multiphysics, and OpenFOAM became popular in scientific and engineering fields Specialized CFD software, such as AVL FIRE and ANSYS Polyflow, cater to specific physical systems like internal combustion engines and polymers The growth of powerful desktop computers has enabled more complex modeling and simulation of real-world phenomena, incorporating various physical laws, thereby expanding the applications of computational fluid dynamics.
“computational fluid dynamics” or “CFD modeling and simulation” are often now referred to as Multiphysics Modeling and simulation
Computer-based modeling and simulation have become crucial in the research and development processes of science and engineering, both in academia and industry These computational models serve as virtual prototypes that predict the behavior of physical systems, often revealing insights that are difficult to obtain through real-world prototypes The visualization of these models is vital for comprehending complex systems or those where measurements are challenging to acquire Computational Fluid Dynamics (CFD)-based models present a reliable, cost-effective solution for simulating real-world problems, contingent on the assumptions made in the models They are now integral to the initial stages of product research and development, occurring before the design of physical prototypes As the need for accurate representations of real-world physical systems increases, the complexity of the geometry and the assumptions, including appropriate boundary conditions, applied to computational models also escalates.
CFD and multiphysics modeling rely on four key conservation equations: mass, momentum, energy, and species conservation, which encapsulate the fundamental laws of physics These mathematical formulations effectively address most chemical engineering systems, although exceptions exist for complex applications like fuel cells and batteries.
Figure 2.7: Knowledge and skills any CFD engineers need to possess for successful work with CFD and multi physics software tools [82].
Role of Modeling in the Study of the Release of Nitrogen
Controlled-release fertilizers are designed to provide good control over the release in soil according to plant nutrients demand They are expected to provide high use
To enhance the efficiency of numerical methods while minimizing environmental impacts, the synchronization of nutrient supply and plant uptake can be achieved through the use of controlled-release fertilizers (CRFs) that gradually release nutrients Identifying an optimized CRF necessitates accurate predictions of nutrient release rates Additionally, effective nutrient management requires tools that can forecast nutrient release under varying soil and environmental conditions However, significant time is needed to characterize the release performance of CRFs in both laboratory and field settings during the design of coating applications.
Computational modeling plays a crucial role in minimizing experimental work, costs, and time by accurately predicting nutrient release behavior, which aids manufacturers and R&D teams in designing controlled-release fertilizers (CRFs) Additionally, it enables environmentalists to assess potential hazards like leaching and volatilization more effectively For farmers, modeling facilitates the planning of agricultural frameworks by helping them select appropriate fertilizers, application methods, and rates in line with the 4R Nutrient Stewardship management principles Current research focuses on enhancing the understanding of mechanisms that control release rates and patterns, as well as environmental factors such as temperature, moisture, microorganisms, acidity, and soil type Researchers are also working to utilize mechanistic-mathematical models for predicting nutrient release under various conditions, serving as a valuable design tool for technologists.
Various methods have been utilized to develop mathematical models for controlled-release fertilizers, including regression, kinetic, and mechanistic models Regression models establish a statistical relationship between dependent and independent variables, offering a simpler approach compared to other modeling techniques However, their applicability is limited to specific cases and remains valid only within the experimental range in which they were derived.
The regression model lacks consideration of internal mechanisms within a system, as highlighted in the research of Gandeza, Medina, and Wilson The kinetic modeling approach, derived from various theories in the literature, fits observational data to select the most appropriate theory While this model can accurately describe system behavior, it may also lead to misinterpretations Although the kinetic model is theoretically a mechanistic model, it is categorized separately in this thesis to clarify the understanding level of the model Demonstrations of this approach can be found in the works of Kochba and Hara Key theories for describing the kinetics of release include the Higuchi model, Zero-order model, and Ritger-Peppas model.
Mechanistic models, which are grounded in fundamental theories and phenomena within a system, provide a mathematical interpretation of physical processes These models require a comprehensive understanding of the internal dynamics of the studied object Research by Lu and Lee, Al-Zahrani, and Shaviv has demonstrated that hydrophobic coatings, particularly polymer-based ones, enhance the nutrient release characteristics of fertilizers The release patterns from these coated fertilizers can vary, including parabolic, linear, and sigmoidal forms Notably, linear and sigmoidal release patterns align more effectively with plant nutrient uptake compared to parabolic release.
[7, 9, 28] Polymer coated controlled-release fertilizers are less sensitive to soil conditions [83]
2.7.1 Nitrogen Release Behavior Based on Regression and Kinetic Models
Nutrient release from coated controlled-release fertilizers (CRFs) is primarily regulated by diffusion through the coating layer, with recent studies concentrating on predicting this release behavior Researchers such as Gandeza, Medina, Wilson, and Zheng have utilized regression models for this purpose Additionally, Kochba proposed a semi-empirical model, treating the CRF release as a first-order process, akin to the decay of the fertilizer population, which is mathematically represented in equation (2.2).
0 log (2.2) where K is the decay rate constant that was obtained through experimental fitting,
C 0 is the initial concentration of the urea core
Recent studies have overlooked the impact of geometry, size, and lag periods on nutrient release models Notably, Gandeza developed a semi-empirical model using a quadratic equation to examine how soil temperature affects nutrient release from CNR-polyolefin-coated urea, enabling predictions of release rates across different temperatures Wang further advanced this research by utilizing a regression model that significantly shortened experimental timeframes from days to hours, although it was limited to specific coating materials Shavit introduced a novel controlled-release fertilizer (CRF) using a soluble fertilizer mixed with a thickener, applying Lee's simple kinetic model to validate his findings with a Fickian release factor of 0.5 Additionally, Xiaoyu's experiments on various urea types led to the formulation of both exponential and double-exponential equations, which captured only 60% of the overall release process, highlighting the need for more comprehensive models.
2.7.2 Nitrogen Release Behavior Based on the Mechanistic Model
Significant advancements have been made in the mathematical modeling of nutrient release patterns in aquatic environments, highlighted by the work of researchers such as Lu and Lee, Al-Zahrani, Shaviv, Lu, and Du.
Jarrell and Boersma were pioneers in creating a mathematical model for the release of urea from sulfur-coated urea (SCU) particles, utilizing Fick's first law as the foundation for their model.
This model effectively describes nutrient release from slow-release fertilizers (SCU), though it simplifies the process by using a one-dimensional coordinate system rather than accounting for the spherical shape typical of conventional fertilizer granules In a similar approach, Glasser utilized a one-dimensional diffusion model to forecast nutrient release from polymer-coated granules, incorporating a time-dependent diffusion coefficient (D eff) that resulted in a nonlinear release pattern.
Lu and Lee [22] utilized Fick’s law in spherical coordinates to analyze nitrogen release from latex coated urea (LCU) They derived an analytical solution under the assumption of a pseudo steady state, addressing both constant and decay release stages Their model effectively considered the spherical shape and size of fertilizer granules, although it assumed a uniform coating The study detailed the release rates for each stage of nitrogen release.
Zhang [23] established a binary diffusion system to analyze nitrogen release in water, incorporating a soil retardation factor (S b ) [91] to account for diffusion resistance from polymer-coated urea (PCU) and assess the impact of the coating on release rates Al-Zahrani [5] developed a model for the unsteady state of polymeric membrane particles under well-mixed conditions, comparing it to an analytical solution; however, the lack of experimental validation raised concerns about the model's accuracy in reflecting real system behavior Subsequently, Ni [57] compared experimental results with their analytical solution in scenarios involving small a.
Shaviv [11] developed an analytical solution for the three stages of release based on Fick’s first law, calculating the lag time required for water absorption into the controlled release fertilizer (CRF) to fill the shell's space The model incorporated factors such as coating thickness and granule size, with the release process detailed by equation (2.6).
P S sat - t 0 £ t < t 1 ữữứ ờở ộ- ( - ) ỳỷ ự ỗỗ ử è ổ - 3 1 exp
They also extended the model to the release from a population of granules [24] Their model successfully simulated the release from modified polyolefin (MPO) and polyurethane-like coated fertilizers
Ito [92] conducted a simulation of a two-stage release process from a spherical shell, focusing on the dissolution of the urea core and the subsequent decrease in its concentration The analytical solution for this process was derived from equation (2.7).
The model was utilized to create multi-layered coated fertilizers, although its calculations relied on the average diffusivity of each coating layer for the sake of simplicity in the analytical method.
Research Gap
This comprehensive review highlights the essential role of fertilizers in agriculture and showcases the latest advancements in modeling techniques that predict the release behavior of Controlled Release Fertilizers (CRFs).
1) Nutrients are required to be supplied into soil in order to maintain optimal plant production quality and yield However, fertilizer is largely used at intensive magnitudes that cause excess nutrients to adversely affect the environment and human health The appearance of CRF is a breakthrough green technology that can combine the advantages of organic manures and conventional fertilizer in order to reduce these effects while also increase nutrient use efficiency (NUE)
2) The mission to investigate novel materials for CRFs is on the stage in order to reduce the production cost and to become friendly to the environment while still retaining its longevity release characteristics for an entire crop application With these goals in view, modeling plays an important role to enhance and predict the release characteristics because it helps to reduce the requirement time for experimental tests Even though several models have been proposed, either a regression model, kinetic model, or a mathematical model, most of the cited models were only related to a single solute diffusion mechanism in 1D-coordinate system and the water absorption process was mostly neglected, see Table 2.7 Therefore, a more detailed model is proposed which accounts for the release of nitrogen through multi-layers including the coating and water environments in 2D-axisymmetric coordinates system The proposed model also integrates the effect of urea concentration on the diffusion of urea in the water/soil domain iii) Nutrient release characteristics in soil are lacking in most of the cited literature, which may be due to the complexity of the soil environments as previously mentioned in section 0 Besides that, the release in soil is more important than that in water due to its close reality in the practical application Hence, this research aims to integrate the mass transport equation in soil with the interfacial area ratio (IAR) formula to describe the release behavior of nitrogen from the soil environment
This thesis presents a model for nitrogen release from controlled-release fertilizers (CRF) in a two-dimensional geometric system, utilizing the finite element method to account for diffusion through multiple layers and environmental influences Table 2.8 outlines the limitations of existing models in the literature and highlights the anticipated improvements from the developmental models proposed in this project.
Table 2.8: Summary of the limitation of the models cited in literature and expected achievements by the present research
1 Previous models were based on
Fick’s first or second law, and analytical solutions were obtained in 1D coordinate only
The release models utilize a 2D-axisymmetric coordinate system to enhance information extraction and accurately represent the real granule, particularly addressing the non-uniformity of the coating.
2 The effect of environment was not accounted in the release models
The effect of environment and multi- layers is accounted in the release models Urea diffusivity in environment depends on urea concentration
3 The coating thickness was assumed uniform
The coating thickness is non-uniform in the extended (porous) model
4 Most of the mechanistic models concentrated in the release in water
The current’s model can account for the release either in water or soils Ability to extrapolate the release in soils from the release in water
5 Water absorption (penetration) was mostly blurred
The model for water penetration in coating layer