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Article

A Novel Large-Particle Slow-Release Fertilizer Improves Nutrient Use Efficiency and Yield of Cassava by Boundary Layer Limitation

by
Cuicui He
1,†,
Hua Wang
1,†,
Guichun Li
2,
Jie Huang
1,
Dengfeng Wang
1,
Xindao Qin
1,
Wen Zhang
1,*,
Dongming Wu
1,*,
Yuanda Jiu
1,
Min Zhao
1,
Yi Xie
1,
Qingmian Chen
1,
Rongfei Zhou
1 and
Minggang Xu
3
1
Tropical Crops Genetic Resources Institute, Environment and Plant Protection Institute, Analysis and Test Center, Chinese Academy of Tropical Agricultural Sciences, Key Laboratory of Agriculture for Germplasm Resources Conservation and Utilization of Cassava, Haikou 571101, China
2
Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agricultural Sciences, Beijing 100081, China
3
The College of Resources and Environment, Shanxi Agricultural University, Taiyuan 030031, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Agronomy 2025, 15(2), 261; https://doi.org/10.3390/agronomy15020261
Submission received: 3 December 2024 / Revised: 6 January 2025 / Accepted: 20 January 2025 / Published: 21 January 2025
(This article belongs to the Topic High-Efficiency Utilization of Water-Fertilizer Resources and Green Production of Crops)
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Abstract

:
Cassava is a crucial food and economic crop in tropical regions globally. In response to challenges in fertilizer use efficiency for cassava cultivation, which is traditionally compromised by extensive leaching and broad root zone distribution, a novel large-particle slow-release fertilizer (LPF) was developed in this study. This fertilizer was synthesized through solution polymerization using non-metallic minerals and seaweed extract. Compared to conventional SFs that release 99% of nutrients within 1 min, the LPF prolonged the release duration to 51 min under optimal synthesis conditions: drying temperature of 80 °C, total extrusion force of 40 t, drying air pressure of −0.40 bar, auxiliary mineral proportion of 50%, and water content of 15%. Microbeam characterization (e.g., FTIR) and kinetic modeling revealed that the superior performance of LPF resulted from mineral crystal enrichment in the outer layer of fertilizer granules, facilitating intra-particle diffusion processes and imposing boundary layer limitations on nutrient release (e.g., N, P, and K). Field experiments validated the slow-release performance of the fertilizer. Notably, soil treated with LPF exhibited superior nutrient retention in the topsoil layer (0–20 cm) both horizontally and vertically. Even with two-thirds of the nutrient content relative to conventional fertilizers, LPF also displayed significant improvements in crop yield, partial factor productivity, and agronomic efficiency by 33.56%, 200.01%, and 513.84%, respectively. These results indicate that LPF presents a promising solution for sustainable cassava cultivation.

1. Introduction

Cassava (Manihot esculenta) serves as a crucial staple food source, industrial raw material, and biomass energy resource for approximately 800 million people worldwide, contributing significantly to food security and economic development [1]. According to global statistics, cassava cultivation occupies about 19.6 million hm2, yielding 250 million tons annually, with production primarily distributed in 100 tropical countries, including Nigeria, Thailand, Vietnam, and China [2]. Despite cassava’s resilience in drought-prone and nutrient-poor soils, fertilization remains an essential agricultural practice to optimize its yields [3,4]. For instance, cassava cultivation typically requires approximately 140–210 kg/hm2 of organic fertilizer and 15–30 kg/hm2 of chemical fertilizer annually [5,6]. However, the cassava’s extensive root system and the severe issue of nutrient leaching result in lower nutrient use efficiency, potentially exacerbating risks of eutrophication, soil degradation, and increased greenhouse gas emissions [3]. Theoretically, optimal cassava cultivation requires fertilization with N:P:K ratios ranging from 2:1:2 to 4:1:4. Nevertheless, due to limited availability of specialized slow-release fertilizers, farmers predominantly utilize standard 15:15:15 compound fertilizers, resulting in nutrient utilization rates of merely 27.3%, 2.1%, and 19.7% for N, P, and K, respectively [7]. This inefficiency is not unique to cassava but is also prevalent in other crop systems, raising global concerns [8]. Conventional fertilizers exhibit notably poor efficiency, with 80–90% of phosphorus and 40–70% of nitrogen lost to the environment rather than absorbed by crops [9]. In addition, the mismatch between crop nutrient demands and fertilizer nutrient release rates compromises nutrient use efficiency in cassava cultivation [7,10]. Research indicates that cassava exhibits greater nutrient requirements during the potato-setting stage, whereas traditional chemical fertilizers predominantly release nutrients during the seedling stage [11]. These challenges necessitate the immediate development and implementation of effective remedial strategies.
A promising approach to address the challenges of nutrient utilization and leaching in cassava cultivation is the development of slow-release fertilizers (SFs) [12,13]. Unlike conventional chemical fertilizers, SFs are prepared with hydrophobic inorganic or organic materials that act as barriers to mitigate the “burst effect” of nutrient release, allowing for improved synchronization between nutrient release patterns and crop nutrient demands [14]. Moreover, SFs offer the advantage of reduced labor inputs through their single applications, in contrast to the multiple applications required for conventional fertilizers [15,16]. Previous studies have demonstrated that the use of SFs improves maize yield and fertilizer use efficiency while reducing NH3 volatilization by promoting net photosynthesis and soil enzymatic activities associated with carbon–nitrogen metabolism [16]. However, there is a notable absence of SFs specifically targeted to address the low nutrient utilization efficiency in cassava cultivation [17]. As a woody crop with an extensive root system, cassava cultivation in tropical environments presents distinct challenges compared to herbaceous and grass crops in temperate regions. Tropical soils typically exhibit lower nutrient retention capacity due to intense weathering and high precipitation rates [18,19]. Therefore, ideal SFs for cassava cultivation must possess features including controlled nutrient release, prolonged residual effects, and environmental compatibility. However, traditional SFs are often associated with complicated manufacturing processes, resulting in higher production costs. Moreover, they frequently utilize coating materials such as polyolefins and polyurethanes, which degrade slowly and may pose increased environmental risks [20].
To overcome the deficiencies of traditional SFs, this study synthesizes a novel slow-release fertilizer utilizing non-metallic minerals and seaweed extract, following the principles of artificial soil-like materials [21,22]. The selection of these components is supported by several key considerations. Firstly, the non-metallic minerals employed, namely attapulgite powder, bentonite, and diatomite, feature a fibrous reticular structure with extensive nanoscale channels. This structure imparts advantageous physical and chemical properties, including enhanced specific surface area, adsorption capacity, suspension stability, controlled release capabilities, water retention, and reduced specific gravity [17]. Such properties potentially enhance nutrient retention in tropical latosols. Complementarily, the seaweed extract, rich in amino acids, polysaccharides, phenolic substances, and organic acids, forms stable agglutinations with non-metallic minerals through polar group interactions, enhancing the structural integrity of the fertilizer [9]. Secondly, both non-metallic minerals and seaweed extract contain essential trace elements, including Si, Al, Mg, Fe, K, Ca, and Mn, which can potentially serve as supplementary micronutrient sources [23,24]. Moreover, the organic matter content in seaweed extract provides a valuable carbon source for latosols, which are typically characterized by poor productivity and low organic matter content [25,26]. Thirdly, the non-metallic minerals exhibit plasticity under wet conditions and minimal shrinkage during drying, ensuring stable nutrient release performance during the wet–dry cycles characteristic of tropical regions [26,27]. Given the abundant reserves, cost-effectiveness, and environmental compatibility of non-metallic minerals and seaweed extract [28], this fertilizer formulation shows promise for improving nutrient utilization efficiency and promoting cassava growth in tropical latosols. To address the limitations of traditional small-particle slow-release fertilizers, which often exhibit compromised slow-release performance due to thinner boundary layers, limited nutrient storage capacity, and structural instability [16,29], this study focuses on the development of large-particle slow-release fertilizers according to the International Scientific Center of Fertilizers (CIEC) guidelines. Large-particle slow-release fertilizers have demonstrated superior fertilizer longevity, reduced solubility, improved nitrogen utilization efficiency, minimized nutrient loss, and enhanced root activity [30,31].
Considering these factors, we prepared a novel large-particle slow-release fertilizer incorporating non-metallic minerals and seaweed extract, synthesized through a solution polymerization method combined with orthogonal testing. To elucidate the nutrient slow-release mechanism of the synthesized fertilizer, a comprehensive analytical approach was employed, encompassing three kinetic models, microbeam characterization techniques (e.g., FTIR), and field fertilizer experiments. The primary purposes of this study are (a) to develop a novel fertilizer with remarkable slow-release capacity, cost-effectiveness, and environmental compatibility, thus addressing the prevalent issue of low fertilizer use efficiency in cassava cultivation and (b) to establish a theoretical foundation for the synthesis of next-generation large-particle slow-release fertilizers. The findings of this study are expected to contribute significantly to the advancement of cassava cultivation practices, potentially facilitating improved crop yields while minimizing environmental impacts.

2. Materials and Methods

2.1. Materials

The chemical regents, including urea, potassium dihydrogen phosphate, and potassium chloride, were purchased from Sinopharm Group Chemical Reagent Co., LTD (Shanghai, China). The non-metallic minerals, comprising attapulgite powder, bentonite, and diatomite, were sourced from Tuoyi New Materials Co., LTD (Guangzhou, China). All reagents utilized were of analytical grade. Seaweed extract was prepared using an enzymatic hydrolysis approach [32]). Specifically, the seaweed was washed, dried, and crushed to obtain a fine powder. The resultant seaweed powder was evenly mixed with water at a ratio of 1:10 to 1:20. A complex enzyme preparation, composed of neutral protease, hemicellulase, and pectinase in a mass ratio of 2:3:5, was then added to facilitate the hydrolysis process.

2.2. Preparation of Slow-Release Fertilizer

The synthesis of the slow-release fertilizer was accomplished using a solution polymerization method in combination with orthogonal testing [20]. First, through a pot experiment, the optimal nutrient ratio was determined to be N-P2O5-K2O = 19-10-19, using urea, potassium dihydrogen phosphate, and potassium chloride as raw materials. Second, the optimal ratio of non-metallic minerals was determined based on previous studies, with a ratio of 85:10:5 for attapulgite powder, bentonite, and diatomite, respectively. Third, the seaweed extract was prepared using the enzymatic hydrolysis approach described in Section 2.1. Subsequently, the large-particle slow-release fertilizer was produced using a ball press (YQ290, Huaxiang, Xuzhou, China) and a fertilizer granulator (DZ-40, Juxing, Wenling, China). Specifically, 115.7 kg of pre-mixed minerals (attapulgite powder: bentonite: diatomite = 85:10:5) was poured into the fertilizer material tank along with 17.4 kg of water, followed by stirring and pressing with a pressure hammer for 10 min. This process was repeated 3–5 times and allowed to stand for 30 min to ensure thorough hydration of the pre-mixed minerals. Subsequently, 5.5 g of the hydrated material was extruded in the sphere fertilizer grinder and mixed with 11.2 g of pre-mixed chemical fertilizer (N:P:K = 19:10:19) and 1.0 g of seaweed extract. The mixture underwent a polymerization reaction under vacuum negative pressure drying, using γ-polyglutamic acid solution as a binding agent. The resultant fertilizer polymerizate was then molded into granules using a squeezing granulator equipped with a suitable grinding apparatus. To produce the final large-particle slow-release fertilizer, the granules were coated with palm oil and polyols and subsequently dried using low-temperature negative-pressure technology (K11 vacuum drying oven, Yixiang, Nanjing, China). We synthesized three distinct slow-release fertilizers, namely (i) SPF, a small-particle slow-release fertilizer (approximately 3 mm in diameter); (ii) LPF, a large-particle slow-release fertilizer with an equal ratio of nitrogen, phosphorus, and potassium (approximately 25 mm in diameter); and (iii) LPFR, a large-particle slow-release fertilizer containing two-thirds of the nutrient content of LPF. Detailed nutrient compositions for these fertilizers are provided in Table S1 of the Supplementary Materials.

2.3. Determination of Nutrient Content, Microstructure, and FTIR Characteristic

The total nitrogen content of the fertilizer was measured using the reduction digestion method [25]. Phosphorus content was determined via the quinoline phosphomolybdate mass method, while potassium content was assessed using the tetraphenylboron mass method [26]. A microstructural analysis of the slow-release fertilizers was conducted using an intelligent ultra depth-of-field 3D digital microscope (Smartzoom 5, Zeiss, Jena, Germany) to capture images of both the surface and internal morphologies [31,32]. FTIR spectroscopy was performed using a Tensor 27 spectrometer (Bruker, Karlsruhe, Germany) over the wavenumber range of 400–4000 cm−1. Prior to analysis, all fertilizer samples were finely ground and pelletized with potassium bromide (KBr) using the KBr disk method. The samples were subsequently dried in a vacuum oven to ensure optimal spectral quality [33].

2.4. Analysis of Slow-Release Performance and Mechanism of the Studied Fertilizers

To assess the slow-release performance of these fertilizers, a hot water extraction method at 100 °C was employed to simulate nutrient release rates. The experimental protocol utilized a 1:20 ratio of fertilizer samples to hot water. Samples were collected at predetermined time intervals, and the nutrient content was then analyzed. N, P, and K in the fertilizer were determined using the Kjeldahl method, quinomolidone precipitation method, and potassium tetraphenylborate gravimetric method, respectively. To explore the underlying slow-release mechanisms, three kinetic models were applied to describe the release kinetics, namely the pseudo-first-order model (Equation (1)), the pseudo-second-order model (Equation (2)), and the intra-particle diffusion model (Equation (3)) [26,33].
Q t = Q e ( 1 − exp ( − K 1 t ) )
Q t = K 2 Q e 2 t / ( 1 + K 2 Q e t )
Q t = K p t 1 / 2 + C p
where Qe and Qt (mg/kg) represent the quantity of nutrients (e.g., N, P, and K) released from the three slow-release fertilizers at equilibrium and at time t (min), respectively; K1 (1/min) denotes the kinetic rate constant of the pseudo-first-order model, K2 (kg/(mg·min)) and Kp (mg/(kg·min1/2)) are the release rate constants for the pseudo-second-order and intra-particle diffusion models, respectively; and Cp (mg/kg) is a constant associated with the thickness of the boundary layer.

2.5. Fertilizer Efficiency Experiment for Cassava Growth

The fertilizer efficiency experiment for cassava growth was conducted in 2019 at the Innovation Base of the Institute of Tropical Crop Variety Resources (109.50° E, 19.59° N), Chinese Academy of Tropical Agricultural Sciences, Hainan Province. The study area features a tropical marine monsoon climate. The soil is categorized as Latosol derived from granite. A split-block design was implemented with four treatments, each occupying a 10 m × 10 m plot, specifically, (1) CK, the control treatment without fertilization; (2) SPF, application of the small-particle fertilizer only; (3) LPF, the large-particle fertilizer treatment with equivalent nitrogen, phosphorus, and potassium content to that of SPF; and (4) LPFR, the large-particle slow-release fertilizer treatment containing two-thirds of the nitrogen, phosphorus, and potassium content of LPF. Each treatment was replicated three times, with plots of equal size (10 m × 10 m). The experimental plots underwent management for 3 years prior to sample collection in December 2022. Detailed fertilization information is provided in Table S1.
Soil quality changes are slow and difficult to observe in the short term. Thus, to better evaluate the effects of SF on soil quality and crop growth, soil samples were collected in December 2022 at the cassava planting period. Five soil samples were randomly taken from each plot at a depth of 0–20 cm and immediately transported to the lab. The samples were fully homogenized and sieved through a 2 mm mesh prior to analysis. The fresh weight and nutrient content of cassava in both the above- and below-ground parts were measured. Soil pH was determined by the potentiometric method with a soil-to-water ratio of 1:2.5 (w/v). The soil organic matter content was analyzed using the high-temperature external heat potassium dichromate oxidation-volumetric method. The soil total nitrogen was quantified by the Kjeldahl nitrogen determination method. Soil alkaline hydrolysis nitrogen was assessed using the alkali hydrolysis diffusion method. Soil available phosphorus was determined by the HCL-ammonium fluoride extraction molybdenum antimonic resistance colorimetric method, while soil available potassium was measured by ammonium acetate extraction and flame photometry.

2.6. Statistical Analysis

A statistical analysis was conducted using SPSS 19 software (SPSS Inc., Chicago, IL, USA). One-way ANOVA was performed, followed by Tukey’s test for multiple comparisons. Differences among treatments were considered statistically significant at a threshold of p < 0.05. To decipher the slow-release mechanism of the fertilizers, three kinetic models were employed to fit the nutrient release curves (N, P, and K) using a non-linear modeling approach in Origin 2021 [26,33].

3. Results

3.1. Optimization of Fertilizer Production Process

An orthogonal experiment incorporating six factors and three levels was conducted to optimize the fertilizer production process, using the release time of 80% nutrient content (determined by water extraction at 100 °C) as the primary evaluation criterion. The experimental design is detailed in Table S2. The investigated factors and their respective levels were (A) fertilizer diameter (1.0, 2.0, and 3.0 cm); (B) drying temperature (70, 80, and 90 °C); (C) total extrusion force (120, 80, and 40 t); (D) drying air pressure (0, −0.4, and −0.8 bar); (E) proportion of slow-release materials (30%, 40%, and 50%); (F) and proportion of added adhesives (5%, 10%, and 15%). The results of the orthogonal L18(3)6 experiments are shown in Table 1, and the inter-subject effect test results are displayed in Table S3. From the analysis of the sum of squares (SS), factor A (fertilizer diameter) showed the most significant impact on nutrient release, followed by C (total extrusion force), F (proportion of added adhesive), B (drying temperature), E (proportion of slow-release materials), and D (drying air pressure). The optimal combination for synthesizing large-particle slow-release fertilizers was determined to be A3B2C3D2E3F3, corresponding to a particle diameter of 3.0 cm, drying temperature of 80 °C, total extrusion force of 40 t, drying air pressure of −0.40 bar, proportion of slow-release materials of 50%, and proportion of added adhesives of 15%.

3.2. Characterization of Morphology and Composition of Large-Particle Slow-Release Fertilizer

To elucidate the nutrient distribution within the large-particle slow-release fertilizer, optical microscopy and infrared spectroscopy were employed to investigate the morphological characteristics and functional groups present at both the surface and interior of the fertilizers. As shown in Figure 1, a significant concentration of mineral nutrient crystals (serving as sustained-release material) was observed in the outer layer of fertilizer granules. This arrangement facilitated full coverage of NPK nutrients within the interior of the slow-release fertilizer. Conversely, the interior exhibited a lower density of mineral nutrients, indicating successful preparation of the slow-release fertilizer under optimal synthetic conditions. This structural arrangement was further supported by FTIR spectroscopy results. Both the surface and interior of the slow-release fertilizer showed similar FTIR spectral features, with main absorption peaks observed in the ranges of 3500–2600 cm−1 and 1500–800 cm−1.

3.3. Comparison of Slow-Release Performance Between Large-Particle and Conventional Small-Particle Slow-Release Fertilizers

The slow-release performance of the synthesized large-particle slow-release fertilizer was assessed using the cumulative delivery of N, P, and K as indicators. Figure 2 illustrates the release profiles, demonstrating that nitrogen release for the large-particle slow-release fertilizer reached 80% at 26 min, 90% at 41 min, and 100% at 51 min. Potassium release exhibited a similar trend, achieving 80% at 21 min, 90% at 31 min, and 100% at 51 min. In contrast, conventional small-particle slow-release fertilizers available in the market released 99% of their nutrients within 1 min. These results highlight the superior slow-release performance of the synthesized large-particle fertilizer, which could extend nutrient availability for crop growth and mitigate N and P pollution risks.
To explore the nutrient-release mechanisms of both large-particle and small-particle slow-release fertilizers, three kinetic models were applied, namely the pseudo-first-order model, the pseudo-second-order model, and the intra-particle diffusion model, respectively. As shown in Figure 2 and Table 2, the fit coefficients followed the order of P-R2 (0.902–0.989) > 1-R2 > 2-R2, indicating that the slow-release kinetics were primarily governed by intra-particle diffusion processes rather than chemical sorption involving valency forces through electron sharing or exchange of active groups. The large-particle fertilizer exhibited larger R2 values than the small-particle fertilizer, suggesting a more pronounced intra-particle diffusion process during nutrient release. Conversely, the small-particle fertilizer showed a better fit to the pseudo-first-order model, with higher 1-R2 values than the large-particle fertilizer, indicating that additional physical processes, such as interface diffusion, may contribute more significantly to nutrient release for small-particle fertilizers. The Cp values, which are associated with the thickness of the boundary layer, were higher for the large-particle fertilizer, indicating a thicker boundary layer conducive to improved slow-release performance. In addition, P exhibited a stronger intra-particle diffusion process compared to N and K, as evidenced by larger Cp and P-R2 values and lower Kp values.

3.4. Field-Scale Evaluation of Slow-Release Fertilizer Application in Cassava Systems

To assess the performance of the synthesized slow-release fertilizer at the field scale, the effects on soil nutrient transport were investigated in both the horizontal and vertical directions.

3.4.1. Horizontal Soil Nutrient Transport in Response to Slow-Release Fertilizer Application

Compared to the control (CK), soil pH increased in the order of LPFR>LPF>SPF treatments (Table S4), indicating that large-particle fertilizers, especially those with reduced nutrient content, were more effective in improving soil pH than small-particle fertilizers. The LPFR treatment displayed the most significant positive effect on organic matter content and available nutrients (e.g., AN, AP, and AK). However, the difference between LPFR and LPF was statistically significant only for soil AK (p < 0.05), suggesting that large-particle fertilizers with reduced nutrient content may be more effective in increasing soil available nutrients. Figure 3 illustrates the horizontal distribution of soil available nutrients (e.g., AN, AP, and AK) at different depths. In the 0–10 cm layer, the available nutrient levels only increased in the LPF and LPFR treatments, with no significant changes observed in the SPF treatment relative to the control. For instance, the LPF treatment resulted in available N, P, and K contents of 103.7 mg/kg, 83.3 mg/kg, and 190.0 mg/kg, respectively, indicating that large-particle fertilizers are well suited to meet the root development characteristics and nutrient requirements of cassava. In contrast, the nutrients released from the SPF treatments did not appear to be retained in the soil during cassava growth. The absence of significant differences between the LPF and LPFR treatments further suggests that large-particle fertilizers with reduced nutrient content may be a more resource-efficient and cost-effective option. However, the positive effects of large-particle fertilizers on available N and K were observed in the 10–20 cm and 20–30 cm horizontal soil layers, while no significant impact was found for available P. This implies that additional approaches may be necessary to improve soil available p levels.

3.4.2. Vertical Distribution of Soil Nutrients in Response to Slow-Release Fertilizer Application

The vertical distribution of soil available nutrients showed patterns similar to those observed in the horizontal direction. The LPFR treatment exhibited the most significant improvement in nutrient levels, followed by the LPF and SPF treatments, especially in the 0–10 cm soil layer (Figure 4). For instance, the content of available N, P, and K in the LPFR treatment at the 0–10 cm depth was 2.2–3.1 times higher than those in the CK. However, these discrepancies diminished with the increase in soil depth. Notably, for P and K, no significant differences were observed among the three treatments at the 20–30 cm depth. This indicates that the nutrients released from large-particle fertilizers were predominantly retained in the topsoil, potentially reducing nutrient leaching. The retention effect was most pronounced for P in the LPFR treatment at the 0–10 cm depth, followed by K and N. On the contrary, while the SPF treatment had a relatively minor impact on the promotion of soil nutrient levels, its potential for nutrient migration to deeper soil layers should not be overlooked. It is noteworthy that reducing the nutrient content in large-particle fertilizers did not decrease nutrient retention in the 20–30 cm layer, indicating a potential risk of nutrient leaching. However, given the well-developed root system of cassava at this depth, the retention of nutrients in this layer may also be beneficial for cassava growth.

3.4.3. Effect of Large-Particle Slow-Release Fertilizer on Cassava Yield

As shown in Figure 5, the application of large-particle slow-release fertilizers significantly increased cassava yield in both above-ground and below-ground biomass. The highest below-ground yield was observed in the LPF treatment at 38.80 t/hm2, followed by LPFR at 31.11 t/hm2, SPF at 29.05 t/hm2, and CK at 20.17 t/hm2. The above-ground yields were also highest in the LPF and LPFR treatments, attaining 49.17 t/hm2 and 46.24 t/hm2, while the SPF and CK treatments yielded 34.16 t/hm2 and 26.84 t/hm2, respectively. The ratio of below-ground to above-ground yield was highest in the LPF treatment (0.84), followed by CK (0.77), SPF (0.63), and LPFR (0.61). These results demonstrate that under identical nutrient conditions, large-particle slow-release fertilizers significantly enhance cassava growth, with consistent improvements in both below-ground and above-ground biomass production. Notably, while the LPFR treatment did not significantly increase below-ground yield compared to SPF, it resulted in a significant increase in above-ground biomass. This suggests that soil and fertilizer nutrients may prioritize above-ground growth, with below-ground biomass accumulation occurring under conditions of sufficient nutrients. In addition, compared to the SPF treatment, both the LPF and LPFR treatments reduced the reliance on inherent soil fertility while increasing the contribution of the fertilizer and improving the fertilizer use efficiency. This indicates that the observed yield improvements were mainly attributed to enhanced land productivity resulting from large-particle fertilizer application. Given the comparable agronomic efficiency between the LPFR and LPF treatments, the former, with its reduced nutrient content, may present a more cost-effective solution for optimizing cassava growth while minimizing input costs.

4. Discussion

4.1. Advantages of Large-Particle Slow-Release Fertilizer Synthesis

Recent developments in large-particle slow-release fertilizer formulations have aimed to address the incongruity between nutrient release rates and crop nutrient uptake requirements [30,31]. The synthesis of large-particle slow-release fertilizers using non-metallic minerals and marine algae in this study offers several advantages over conventional fertilizers. Firstly, the reduced specific surface area of large-particle fertilizers minimizes the consumption of coating materials during encapsulation, significantly reducing production costs [17,34]. For instance, the coating material requirement for a 12 mm particle size controlled-release fertilizer is approximately 1/44 of that for a 3 mm particle size, with 90% of the incremental cost of coated slow-release fertilizers attributed to encapsulation materials [9,23]. Additional studies have also reported that super-large-particle slow-release fertilizers can effectively reduce environmental N losses, increase fruit yields, save on labor costs, and maximize net economic benefits [34]. Secondly, the substantial volume of large-particle fertilizers allows for a broader range of material and process selection [17]. This encompasses the utilization of diverse raw materials, such as mineral substances and humic acid, and the application of various production methods, including film coating, extrusion, and drum fluidized bed processes [7,16]. Thirdly, the slower nutrient release rate of large-particle slow-release fertilizers extends their effective duration, reducing fertilization frequency and associated costs [30,31]. Microbeam characterization (e.g., FTIR) and kinetic modeling demonstrate that the novel LPF exhibits enrichment of mineral crystals in the outer layer of fertilizer granules, facilitating intra-particle diffusion processes and imposing boundary layer limitations on nutrient release (e.g., N, P, and K) (Figure 2). The non-metallic minerals used, such as attapulgite powder, bentonite, and diatomite, are known for their fibrous reticular structures, abundant nanoscale channels, and large specific surface areas [17]. Seaweed extract, which is rich in amino acids, polysaccharides, phenolic substances, and organic acids, can agglutinate with non-metallic minerals through polar groups to form a stable fertilizer structure [9,25,26]. The consistent quality of each granule ensures precise nutrient application, thereby facilitating the advancement in precision agricultural machinery and equipment. This approach addresses the low fertilizer use efficiency associated with one-time fertilization in cassava cultivation, enhances nutrient provision during the later stages of crop growth, and mitigates potential environmental risks posed by membrane coating materials.

4.2. Nutrient Release Performance of Large-Particle Slow-Release Fertilizer

The nutrient release kinetics of slow-release fertilizers are significantly influenced by environmental conditions, with water playing a crucial role in determining release rates [35,36]. Increased water temperatures have been shown to accelerate nutrient release [37]. Consequently, a standardized protocol has been established for the expedited assessment of nutrient release rates from slow-release fertilizers at varying water temperatures. In this study, 100 °C water was employed to evaluate the nutrient release rate of the slow-release fertilizer. The findings revealed that commercially available small-particle slow-release fertilizers exhibited a 99% cumulative nutrient release within one min [35,38]. In contrast, the large-particle slow-release fertilizer developed in this study achieved an 80% cumulative nitrogen release over 26 min, significantly prolonging the nutrient release duration. The attenuated nutrient release rate observed in large-particle slow-release fertilizers can be attributed to their reduced specific surface area compared to small-particle fertilizers (Figure 1). This reduction diminishes the contact area with soil water, consequently decelerating nutrient dissolution and overall release. To address this, we incorporated auxiliary materials such as non-metallic minerals (specifically, attapulgite and bentonite) and algal extracts into the formulation. The integration of these supplementary substances enhances the fertilizer’s nutrient retention capacity and moderates the nutrient release rate during the later stages of the large-particle slow-release fertilizer’s lifecycle. A previous study synthesized a spherical fertilizer by extruding a mixture of fertilizer and adhesive, followed by water extraction at 25 °C [39]. This fertilizer exhibited an initial nutrient release rate of 46.34%, compared to 59.02% for the control group. Furthermore, the large-particle slow-release fertilizer required 15 days to release 70% of its nutrients into the soil, whereas conventional fertilizers achieved the same release in only 3 days, indicating a significant reduction in nutrient release rate [40]. Tang et al. utilized polymeric chemical synthetic materials for slow-release fertilizers, combining them with crushed conventional fertilizers using a grinder [41]. The resultant mixture was then fortified with activated weathered coal humic acid and extruded to produce large-particle slow-release fertilizers. Their findings discovered no significant difference in leaching time between the large-particle ordinary compound fertilizer and the standard compound fertilizer in soil column experiments [42]. However, the leaching time for the large-particle activated humic acid slow-release fertilizers was approximately five times longer than that of the conventional compound fertilizers, significantly reducing the nutrient release rate and extending the nutrient release duration [40,43]. The large-particle slow-release fertilizer developed in this study demonstrated superior slow-release efficacy compared to fertilizers produced by direct extrusion. This superior performance is attributed to the enrichment of mineral crystals in the outer layer of fertilizer granules, which facilitates intra-particle diffusion processes and imposes boundary layer limitations on nutrient release (e.g., N, P, and K) (Figure 2). These findings have been corroborated by microbeam characterization (e.g., FTIR) and kinetic modeling. Additionally, certain studies have synthesized granular fertilizers enriched with mineral crystals, which support the prolonged action of carbamide granule cores and the accumulation of soil moisture, thereby enhancing plant stress tolerance [44,45].

4.3. Influence of Large-Particle Slow-Release Fertilizer on Cassava Growth

Cassava is a crucial dietary staple for approximately 800 million people globally, with cultivation practices significantly influencing both yield and economic viability [46,47]. The cassava crop lacks a specific maturity phase, as starch deposition and reallocation continue throughout its growth cycle [47]. Judicious fertilizer application is instrumental for enhancing cassava yields, whereas imprudent fertilization can lead to excessive above-ground biomass growth, potentially impeding nutrient accumulation in the roots [48,49]. Given cassava’s extended growth cycle and the challenges associated with post-planting row management, including fertilization, the crop necessitates long-term, slow-release fertilizers to meet its nutritional requirements throughout the growth period [19,50,51]. This study selected cassava as the model crop for evaluating the efficacy of large-particle slow-release fertilizers (LPFs) (Figure 1). The experimental results indicated that LPF application significantly bolstered cassava yield by 33.6% compared to small-particle slow-release fertilizers (SPFs) under equivalent nutrient conditions (Figure 2). When the nitrogen, phosphorus, and potassium content of LPF was reduced by one-third (designated as the LPFR treatment), the yield increment over SPF was 7.1%, although this difference was not statistically significant. These findings suggest that LPF can effectively synchronize nutrient release with cassava growth phases, substantially augmenting yield, and that a one-third reduction in nutrient content does not adversely affect yield performance. Parallel studies on corn have demonstrated that LPF application at equal nutrient levels resulted in higher yields [16]. Similarly, in apple cultivation, LPF significantly elevated yield under equivalent nutrient conditions. Notably, when the nutrient content of LPF was halved, the impact on apple yield was negligible, indicating that a 50% reduction in nutrient application did not result in a significant yield decrease [41]. In summary, the synthetic LPFs developed in this study, formulated using non-metallic minerals and seaweed extract, present great potential for cassava cultivation, exhibiting remarkable slow-release capacity, cost-effectiveness, and environmentally friendly characteristics.

5. Conclusions

In this study, we developed a novel large-particle slow-release fertilizer (LPF) based on non-metallic minerals and seaweed extract, which exhibited superior slow-release performance compared to conventional fertilizers. The optimal synthesis conditions were determined as 80 °C drying temperature, 40 t total extrusion force, −0.40 bar drying air pressure, 50% auxiliary mineral content, and 15% water content. Microbeam characterization (e.g., FTIR) and kinetic modeling revealed that the LPF’s superior performance was primarily attributed to the enrichment of mineral crystals in the outer layer of fertilizer granules, facilitating the intra-particle diffusion processes and imposed boundary layer limitations on nutrient release (e.g., N, P, and K). Field experiments further confirmed that these slow-release performances led to nutrient retentions of LPF at the fertilization point within 0–20 cm in both the horizontal and vertical directions. As a result, the excellent nutrient retentions enhanced cassava growth by improving fertilizer partial factor productivity and agronomic efficiency, even with two-thirds of the nutrient contents in the LPFs compared to conventional fertilizers. Conclusively, the developed LPF demonstrates substantial potential for cassava cultivation, which features remarkable slow-release capacity, cost-effectiveness, and environmentally friendly characteristics.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/agronomy15020261/s1, Table S1: Fertilization information for the four treatments in this study; Table S2: Factors and levels for optimizing the fertilizer production process; Table S3: The results of the inter-agent effect test; Table S4: Effects of different slow-release fertilizers on soil basic properties.

Author Contributions

Conceptualization, Investigation, Visualization, Writing—original draft, C.H.; Investigation, Methodology, Data curation, H.W.; Investigation, G.L.; Methodology, J.H., D.W. (Dengfeng Wang), and X.Q.; Investigation, Project administration, Visualization, Writing—review and editing, W.Z.; Investigation, Methodology, Data curation, Visualization, Writing—review and editing, D.W. (Dongming Wu); Investigation, Y.J.; Visualization, Writing—review and editing, M.Z.; Project administration, Y.X.; Data curation, Q.C.; Resources, R.Z.; Resources, Funding acquisition, M.X. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the China Agriculture Research System (CARS-11), the Chinese Academy of Tropical Agricultural Sciences for Science and Technology Innovation Team of National Tropical Agricultural Science Center (CATASCXTD202302), the National Key R & D Program of China (2020YFD1000603, 2023YFD1600605), the Hainan Province Science and Technology Special Fund (ZDYF2021XDNY280), the Hainan Provincial Natural Science Foundation of China (421QN0935, 322MS135, 412RC649), the National Natural Science Foundation of China Projects (32101840), the Central Public-interest Scientific Institution Basal Research Fund for Chinese Academy of Tropical Agri-cultural Sciences (1630042024005, 1630042024007, 1630082024008), and the Chinese Academy of Tropical Agricultural Sciences for Science and Technology Innovation Team of National Tropical Agricultural Science Center (CATASCXTD202412).

Data Availability Statement

The data will be made available on request.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Agronomy 15 00261 g001
Figure 1. Plate (A) is a microstructural diagram of the fertilizer at the 30 mm scale (i) and the surface (ii) and interior (iii) domains at the 1 mm scale. Plate (B) is the spectral signature (i) and principal component analysis (ii, surface; iii, interior) of the FTIR profiles for the fertilizer.
Figure 1. Plate (A) is a microstructural diagram of the fertilizer at the 30 mm scale (i) and the surface (ii) and interior (iii) domains at the 1 mm scale. Plate (B) is the spectral signature (i) and principal component analysis (ii, surface; iii, interior) of the FTIR profiles for the fertilizer.
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Figure 2. Slow-release performance characteristics of N (a), P (b), and K (c) for small- and large-particle slow-release fertilizers. (A) Release rate curves; (B) pseudo-first-order model fit, (C) pseudo-second-order model fit; (D) intra-particle diffusion model fit.
Figure 2. Slow-release performance characteristics of N (a), P (b), and K (c) for small- and large-particle slow-release fertilizers. (A) Release rate curves; (B) pseudo-first-order model fit, (C) pseudo-second-order model fit; (D) intra-particle diffusion model fit.
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Figure 3. Horizontal distribution of soil nutrients in response to slow-release fertilizer application ((A), nitrogen; (B), phosphorus; (C), potassium). Lowercase letters above each column indicate significant differences between treatments (p < 0.05).
Figure 3. Horizontal distribution of soil nutrients in response to slow-release fertilizer application ((A), nitrogen; (B), phosphorus; (C), potassium). Lowercase letters above each column indicate significant differences between treatments (p < 0.05).
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Figure 4. Vertical distribution of soil nutrients in response to slow-release fertilizer application ((A), nitrogen; (B), phosphorus; (C), potassium). Lowercase letters to the right of the column indicate significant differences between treatments (p < 0.05).
Figure 4. Vertical distribution of soil nutrients in response to slow-release fertilizer application ((A), nitrogen; (B), phosphorus; (C), potassium). Lowercase letters to the right of the column indicate significant differences between treatments (p < 0.05).
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Figure 5. Assessment of fertilizer efficiency in a cassava agroecosystem. (a) crop yield; (b) land contribution rate; (c) fertilizer contribution rate; (d) partial factor productivity; (e) agronomic efficiency. Different letters on bars indicate significant differences between different land uses at p < 0.05.
Figure 5. Assessment of fertilizer efficiency in a cassava agroecosystem. (a) crop yield; (b) land contribution rate; (c) fertilizer contribution rate; (d) partial factor productivity; (e) agronomic efficiency. Different letters on bars indicate significant differences between different land uses at p < 0.05.
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Table 1. Experimental results of the L18(3)6 orthogonal test.
Table 1. Experimental results of the L18(3)6 orthogonal test.
Serial NumberA
(cm)		B
(C)		C
(t)		D
(Bar)		E
(%)		F
(%)		Release Time of 80% Nutrient (min)
13901200.4401526
2190800401516
3170120030517
4280400301514.7
5280800.440516.5
6290800.8301015
7370400401022
8370800.4301022
91801200.4501018
10390400.850523
11270400.4501517
122901200501018
13180400.8401015
142701200.840518
1538080050525
16190400.430515
173801200.8301528
18170800.8501515
Note: A, B, C, D, E, and F indicate the fertilizer diameter, drying temperature, total extrusion force, drying air pressure, proportion of slow-release materials, and proportion of added adhesives, respectively.
Table 2. Comparison of kinetic parameters of pseudo-first-order and intra-particle diffusion models for nutrient release (e.g., N, P, and K) from small- and large-particle slow-release fertilizers.
Table 2. Comparison of kinetic parameters of pseudo-first-order and intra-particle diffusion models for nutrient release (e.g., N, P, and K) from small- and large-particle slow-release fertilizers.
NutrientsTreatmentsPseudo-First-Order ModelIntra-Particle Diffusion Model
K11-R2KpCpP-R2
Nsmall5.8090.7025.0775.0740.948
large0.1330.82669.04531.0780.973
Psmall0.1640.6275.9387.2590.977
large0.1010.39127.47736.3490.989
Ksmall4.4600.8251.4261.9250.902
large1.1230.56544.12017.1710.967
Note: The kinetic models were fitted by non-linear regression, considering the release kinetics as a whole process. The parameters for the pseudo-second-order model are not presented due to poor fitting results, with 2-R2 < 0.3.
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He, C.; Wang, H.; Li, G.; Huang, J.; Wang, D.; Qin, X.; Zhang, W.; Wu, D.; Jiu, Y.; Zhao, M.; et al. A Novel Large-Particle Slow-Release Fertilizer Improves Nutrient Use Efficiency and Yield of Cassava by Boundary Layer Limitation. Agronomy 2025, 15, 261. https://doi.org/10.3390/agronomy15020261

AMA Style

He C, Wang H, Li G, Huang J, Wang D, Qin X, Zhang W, Wu D, Jiu Y, Zhao M, et al. A Novel Large-Particle Slow-Release Fertilizer Improves Nutrient Use Efficiency and Yield of Cassava by Boundary Layer Limitation. Agronomy. 2025; 15(2):261. https://doi.org/10.3390/agronomy15020261

Chicago/Turabian Style

He, Cuicui, Hua Wang, Guichun Li, Jie Huang, Dengfeng Wang, Xindao Qin, Wen Zhang, Dongming Wu, Yuanda Jiu, Min Zhao, and et al. 2025. "A Novel Large-Particle Slow-Release Fertilizer Improves Nutrient Use Efficiency and Yield of Cassava by Boundary Layer Limitation" Agronomy 15, no. 2: 261. https://doi.org/10.3390/agronomy15020261

APA Style

He, C., Wang, H., Li, G., Huang, J., Wang, D., Qin, X., Zhang, W., Wu, D., Jiu, Y., Zhao, M., Xie, Y., Chen, Q., Zhou, R., & Xu, M. (2025). A Novel Large-Particle Slow-Release Fertilizer Improves Nutrient Use Efficiency and Yield of Cassava by Boundary Layer Limitation. Agronomy, 15(2), 261. https://doi.org/10.3390/agronomy15020261

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