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Article

The Effect of Slow-Release Fertilizer on the Growth of Garlic Sprouts and the Soil Environment

by
Chunxiao Han
1,2,
Zhizhi Zhang
1,
Renlong Liu
1,*,
Changyuan Tao
1 and
Xing Fan
1
1
School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China
2
SDIC Xinjiang Luobupo Postash Co., Ltd., Hami 839099, China
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(15), 8216; https://doi.org/10.3390/app15158216
Submission received: 4 June 2025 / Revised: 15 July 2025 / Accepted: 22 July 2025 / Published: 24 July 2025
(This article belongs to the Section Ecology Science and Engineering)
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Versions Notes

Abstract

To address the issue of excessive chemical fertilizer use in agricultural production, this study conducted a pot experiment with four treatments: CK (no fertilization), T1 (the application of potassium magnesium sulfate fertilizer), T2 (the application of slow-release fertilizer equal to T1), and T3 (the application of slow-release fertilizer with the same fertility as T1). The effects of these treatments on garlic seedling yield, growth quality, chlorophyll content, photosynthetic characteristics, and the soil environment were investigated to evaluate the feasibility of replacing conventional fertilizers with slow-release formulations. The results showed that compared with CK, all three fertilized treatments (T1, T2, and T3) significantly increased the plant heights and stem diameters of the garlic sprouts (p < 0.05). Plant height increased by 14.85%, 17.81%, and 27.75%, while stem diameter increased by 9.36%, 8.83%, and 13.96%, respectively. Additionally, the chlorophyll content increased by 4.34%, 7.22%, and 8.05% across T1, T2, and T3, respectively. Among the treatments, T3 exhibited the best overall growth performance. Compared with those in the CK group, the contents of soluble sugars, soluble proteins, free amino acids, vitamin C, and allicin increased by 64.74%, 112.17%, 126.82%, 36.15%, and 45.43%, respectively. Furthermore, soil organic matter, available potassium, magnesium, and phosphorus increased by 109.02%, 886.25%, 91.65%, and 103.14%, respectively. The principal component analysis indicated that soil pH and exchangeable magnesium were representative indicators reflecting the differences in the soil’s chemical properties under different fertilization treatments. Compared with the CK group, the metal contents in the T1 group slightly increased, while those in T2 and T3 generally decreased, suggesting that the application of slow-release fertilizer exerts a certain remediation effect on soils contaminated with heavy metals. This may be attributed to the chemical precipitation and ion exchange capacities of phosphogypsum, as well as the high adsorption and cation exchange capacity of bentonite, which help reduce the leaching of soil metal ions. In summary, slow-release fertilizers not only promote garlic sprout growth but also enhance soil quality by regulating its chemical properties.

1. Introduction

Fertilizer use has a history of nearly 180 years [1]. According to the Food and Agriculture Organization of the United Nations (FAO), fertilizers continue to play a critical role in agricultural production, even in technologically advanced countries such as the United States. Proper fertilization can increase crop yields by at least 50–60% [2,3], underscoring the substantial impact of fertilizers on agricultural productivity. Although conventional fertilizers are relatively inexpensive, they exhibit low nutrient use efficiency and release nutrients at rates that are not well aligned with the nutrient uptake patterns of crops [4]. In contrast, slow-release fertilizers (SRFs) can regulate the nutrient release rate of granules, aligning nutrient availability with crop demand. This synchronization facilitates a dynamic balance between nutrient supply and crops’ physiological requirements, thereby enhancing fertilizer efficiency. As a new generation of fertilizers, SRFs offer extended nutrient release periods and significantly reduce nutrient loss [5,6]. In China, the widespread application of conventional nitrogen, phosphorus, and potassium fertilizers often exceeds the maximum nutrient uptake capacity of crops [7]. Prolonged overuse of such fertilizers can inhibit vegetable growth, lower crop quality, and lead to several adverse effects, including reduced nutrient use efficiency, soil eutrophication, and disruption of the soil’s microbial communities [8,9,10].
Slow-release fertilizer is a novel type of fertilizer designed to release nutrients gradually in a predetermined manner through specific regulatory mechanisms [4]. Its core feature lies in extending the nutrient release period and enhancing fertilizer use efficiency. In recent years, various classification approaches have been proposed, resulting in inconsistency and the absence of a unified, systematic classification method or comprehensive framework for slow-release fertilizers. Fu et al. [11] proposed a classification method based on the release mechanism, integrating physical and chemical classification principles and, for the first time, introducing the concept of composite slow-release fertilizers. This approach divides slow-release fertilizers into three main categories to meet the scalability demands of classification. Among them, physical slow-release fertilizers are the earliest developed and most widely used, accounting for over 95% of the total [12]. These fertilizers utilize materials such as sulfur, resin, and bentonite to form physical barriers that regulate nutrient release and are further categorized into physical coating types and mineral types. Physical coating types: These fertilizers encapsulate nutrient granules with materials such as sulfur or resin, forming a coating that slows nutrient release. The release mechanisms include (1) rupture types, represented by sulfur-coated urea, which exhibits an inverted L-shaped release curve, and (2) diffusion types, such as polymer-coated fertilizers, which follow an S-shaped release curve consisting of an initial lag phase, a constant-rate release phase, and a decay phase. Mineral types: These types use inorganic materials such as bentonite to adsorb nutrients, forming a physical barrier that achieves slow release through adsorption and retention mechanisms [13,14].
Garlic is a member of the Liliaceae family and the Allium genus. The cultivation of garlic sprouts has a long history in China spanning nearly 2000 years. Garlic sprouts are rich in nutrients such as vitamin C and protein, and their consumption can not only stimulate appetite but also provide various medicinal and health benefits [15]. At present, China holds the largest planting area and yield of garlic sprouts globally. However, the excessive application of chemical fertilizers in garlic production has severely impacted their quality [16]. Therefore, developing methods to cultivate high-quality garlic sprouts has become a critical research focus. Chemical fertilization remains one of the primary strategies for increasing crop yields [17]. Nevertheless, the prolonged and continuous use of conventional fertilizers often results in adverse effects, including soil compaction and nutrient leaching, which ultimately compromise crop yield and quality [18]. These issues pose significant challenges to the sustainable development of agriculture and environmental protection [19]. The controlled nutrient release characteristics of slow-release fertilizers (SRFs) can effectively mitigate the drawbacks associated with traditional fertilization practices [20,21,22]. Numerous studies have demonstrated that substituting conventional fertilizers with SRFs can enhance the concentrations of soluble sugars, soluble proteins, and vitamin C in vegetables, thereby improving their nutritional quality [23]. Additionally, some research indicates that SRFs possess resistance to microbial decomposition, promote soil carbon sequestration, reduce atmospheric carbon emissions, and contribute to alleviating environmental pollution [24,25,26].
The current research on slow-release fertilizers (SRFs) is primarily focused on improving the yield and nutrient use efficiency of major field crops such as rice, corn, and wheat [27,28,29,30]. Liu et al. [31] conducted a meta-analysis to evaluate the effects of slow-release fertilizers (SCRFs) on rice yield and nitrogen use efficiency (NUE), as well as their environmental dependence. The results indicated that compared with conventional fertilization (CF), SCRF application significantly increased the rice yield and nitrogen uptake (NU) by 6.0% and 11.1%, respectively. Moreover, the NUE increased from 35.7% under CF to 44.9% under the SCRF treatment. A further analysis revealed that the SCRF application rate and rice variety were key factors influencing yield. Specifically, when the application rate was below approximately 80%, the rice yield declined, whereas rates above 90% significantly enhanced yields. Pengfu Hou [32] compared the application effects of two types of controlled-release fertilizers—sulfur-coated urea (SCU) and resin-based fertilizer (RBB)—under direct seeding and transplanting methods for Wuyunjing 23 rice during 2013–2014 field trials. The findings showed that RBB performed significantly better than SCU. Among the treatments, basal application of RBB (RBBBP) resulted in the highest yield under transplanting conditions, while basal application of RBB also achieved the best performance in direct seeding. Both types of controlled-release fertilizers enhanced the nitrogen use efficiency. Li et al. [33], through a two-year field experiment, demonstrated that the application of 360 kg N ha−1 of SRF (SF360) could significantly optimize the spring maize production system. Compared to conventional fertilization at 405 kg N ha−1 (CF405), SF360 reduced the nitrogen input by 11.1% while increasing the yield by 3.2%, the nitrogen use efficiency by 22.2%, and economic benefits by 17.5%. Shoukat et al. [34] investigated the issue of late sowing in wheat production in Punjab Province, Pakistan. Their results indicated that applying sulfur-coated urea (SCU) combined with a high sowing rate of 150 kg ha−1 (S3) significantly improved the performance of late-sown wheat. The SCU+S3 treatment increased the leaf area index by 0.99 cm2 and plant height by 8.24% and significantly enhanced both spikelet number and spike length.
Currently, limited data are available on the effects of slow-release fertilizers (SRFs) on garlic sprout growth, quality, and soil fertility, and studies investigating the influence of the soil’s nutrient content on vegetable quality remain scarce. It is hypothesized that the application of SRFs combined with reduced fertilization can promote growth in garlic sprouts, enhance their quality, increase the availability of soil nutrients, improve soil fertility, and ultimately increase yield. Therefore, based on previous research, this study used a prepared slow-release fertilizer for pot experiments, added a slow-release fertilizer reduction group, and tested the impact of the slow-release fertilizer on the soil environment [35]. This study aims to investigate the effects of SRF application and fertilizer reduction on the growth performance, quality, and chemical properties of the soil. This study overcomes the limitations of the traditional research which has primarily focused on either fertilizer efficiency or environmental impact in isolation. It not only investigates the promoting effect of slow-release fertilizers on the growth of garlic seedlings but also evaluates their influence on the soil’s nutrient levels and heavy metal content. By ensuring the sustained release of nutrients, the application of slow-release fertilizers can reduce the risk of environmental pollution caused by nutrient leaching into soil and water bodies. This dual benefit of enhancing the crop yield while protecting the environment offers a promising strategy for advancing green and sustainable agricultural development.

2. Materials and Methods

2.1. The Test Materials

The soil used in the experiment was dried at 60 °C for 24 h and then pretreated by passing it through a 1 mm sieve. Its primary chemical properties were as follows: pH 7.53, organic matter 3.27 g/kg, total potassium 17.8 g/kg, total phosphorus 0.55 g/kg, available sulfur 77.65 mg/kg, available phosphorus 9.8 mg/kg, available potassium 88 mg/kg, and exchangeable magnesium 1.93 mg/kg.
Purple-skinned garlic from Jiangsu Province, China, was used as the experimental crop. The potassium magnesium sulfate fertilizer was supplied by Xinjiang Guotou Lop Nur Potash Co., Ltd. (Hami, China), while the potassium magnesium sulfate slow-release fertilizer was produced by the School of Chemistry and Chemical Engineering, Chongqing University (Chongqing, China). The slow-release fertilizer used in this study was composed of potassium magnesium sulfate, phosphogypsum, and bentonite as the core materials, with paraffin wax serving as the outer coating. The production process for the slow-release fertilizer is illustrated in Figure 1. Each slow-release fertilizer formulation consisted of potassium magnesium sulfate fertilizer, phosphogypsum, bentonite, paraffin wax, and a binder.

2.2. The Experimental Design

The pot experiment was conducted at Chongqing University (Chongqing, China) from 11 December 2024 to 19 January 2025.
Four fertilization treatments were designed, as outlined in Table 1: no fertilization (CK), the application of potassium magnesium sulfate fertilizer (T1), the application of an equal amount of slow-release fertilizer (T2), and the application of a slow-release fertilizer with an equivalent nutrient content (T3). For each treatment, five garlic bulbs of a uniform size and with a healthy appearance were selected and planted into plastic pots, with five bulbs per pot (21 cm in height and 23 cm in diameter). A total of 12 pots were prepared, each containing 2 kg of homogenized soil, and the pots were grouped into sets of five.
All treatments were conducted using a one-time fertilization approach. A predetermined amount of fertilizer was uniformly applied to the flowerpots by placing it at a depth of approximately 6 cm, followed by coverage with a layer of soil. A cross-shaped mark was drawn on the soil’s surface to divide each pot into four equal sections. The garlic sprouts were then planted at the intersection point of the cross and at the midpoints of each quadrant, ensuring uniform spacing among all plants. This planting layout was designed to maintain an appropriate distance between each crop and the center of the pot, thereby facilitating consistent sampling and ease in the growth observations. The initial soil moisture content was controlled at 60%, and water was added every three days to maintain soil moisture without causing water accumulation. Approximately 200 mL of water was applied each time.
At harvest, the plant height, stem diameter, and yield were recorded for each treatment. One garlic sprout per pot was reserved and refrigerated for a subsequent quality analysis. Plant height was measured using a ruler as the distance from the soil surface to the tip of the tallest leaf when fully extended. The diameter of the pseudostem was determined using a vernier caliper, corresponding to the maximum basal diameter of the plant. The relative chlorophyll content (SPAD) was measured using a TYS-A chlorophyll meter. Garlic sprout yield was determined by weighing them with an electronic balance. The contents of soluble sugars, soluble proteins, free amino acids, vitamin C, and allicin in the garlic sprouts were determined using the anthrone colorimetric method [36], the Coomassie Brilliant Blue G-250 colorimetric method [37], the indene trione colorimetric method [38], UV spectrophotometry [39], and the phenylhydrazone method [40], respectively. Each parameter was measured in triplicate, and the average values were reported.

2.3. Testing of the Garlic Sprout Growth Indicators

Plant height was measured using a tape measure (Deli, Ningbo, China; precision: 1 mm), while stem thickness was determined using a vernier caliper (Deli, China; precision: 0.01 mm). The fresh weight of the garlic seedlings was accurately measured using an electronic balance (Leqi, Jiaxing, China; precision: 0.01 g), and the relative chlorophyll content of the leaves was assessed using a portable chlorophyll meter (Jinke Keda, BeiJing, China; precision: ±0.1 SPAD).

2.4. Testing of the Physiological Indicators of the Garlic Sprouts

2.4.1. Determination of the Soluble Sugar Content Using the Anthrone Colorimetric Method

This study quantitatively analyzed the soluble sugar content of the garlic sprout leaves using the anthrone colorimetric method [41]. The specific operation process is as follows: fresh leaf samples are selected, surface-cleaned, cut, and mixed, and an appropriate amount of the sample is accurately weighed and placed into a graduated test tube. A total of 5–10 mL of distilled water is added for boiling water bath extraction (30 min) to sufficiently extract sugar substances. All of the extract is transferred into a 50 mL volumetric flask up to the mark. Then, 0.5 mL of the test solution is accurately transferred into a 20 mL test tube, 0.5 mL of anthrone ethyl acetate mixed reagent and 5 mL of concentrated sulfuric acid are added in sequence, shaken quickly, and mixed well; then, it is placed in a boiling water bath for precise reaction for 60 s. After natural cooling of the reaction system, the absorbance value was measured at a wavelength of 630 nm using a reagent blank as a reference. Finally, based on the linear equation fitted by the standard curve, the precise content of soluble sugars in the sample is calculated.

2.4.2. The Coomassie Brilliant Blue Colorimetric Method for the Soluble Protein Content

The soluble protein content in the garlic seedling leaves was determined using the Coomassie Brilliant Blue G-250 colorimetric method [36]. Specifically, fresh leaf samples were homogenized with 5 mL of distilled water, and the resulting mixture was centrifuged at 8000 r·min−1 for 10 min. The supernatant was collected as the test solution. Subsequently, 1.0 mL of the supernatant was mixed with 5.0 mL of Coomassie Brilliant Blue G-250 dye solution in a test tube and allowed to stand at room temperature for 5 min to ensure complete binding between the dye and the protein. The absorbance of the solution was then measured at 595 nm using a spectrophotometer, with the untreated dye solution serving as the blank control. The soluble protein content was calculated by comparing the absorbance value to a pre-established standard curve.

2.4.3. Determination of the Free Amino Acid Content Using the Indene Ketone Colorimetric Method

The content of free amino acids in the garlic sprout leaves was determined using the indene ketone colorimetric method [37]. Fresh leaf samples were first homogenized with 10% acetic acid solution. The homogenate was centrifuged at 8000 r·min−1, and the supernatant was collected and diluted with an acetic acid–sodium acetate buffer solution (pH: 5.4) in a 50 mL volumetric flask. Then, 1.0 mL of the diluted sample was transferred into a 20 mL graduated test tube, followed by the sequential addition of 1.0 mL of buffer solution, 3.0 mL of hydrated indene ketone color reagent, and 0.1 mL of ascorbic acid solution. The mixture was thoroughly mixed and heated in a boiling water bath for 15 min to allow for the color reaction. Upon completion, the reaction was immediately terminated in an ice water bath, and the solution was diluted to 20 mL using a 60% ethanol solution. The absorbance was measured at 570 nm using a spectrophotometer, with the reagent blank serving as a reference. The free amino acid content was calculated based on a pre-established standard curve regression equation.

2.4.4. Determination of the Vitamin C (Vc) Content Using UV Spectrophotometry

The vitamin C (Vc) content in the garlic sprout leaves was determined using ultraviolet (UV) spectrophotometry [38]. Fresh leaf samples were homogenized with a known volume of oxalic acid solution and transferred into a 100 mL volumetric flask, where the volume was adjusted with oxalic acid solution to prepare the extract for analysis. Subsequently, a dual-system measurement was performed. For the acidic system, 0.2 mL of the extract was placed into a 10 mL graduated test tube, followed by the addition of 0.8 mL of 2% oxalic acid solution, and the volume was adjusted accordingly. The absorbance (A_acid) was measured at 243 nm, using oxalic acid solution as the blank. For the alkaline system, another 0.2 mL of the extract was added to a separate 10 mL graduated test tube, mixed with 0.8 mL of 1 mol/L NaOH solution, and allowed to react for 15 min. The solution was then diluted with distilled water, and its absorbance (A_alkali) was measured at 243 nm, using NaOH solution as the blank. The vitamin C content was calculated based on the difference in the absorbance between the alkaline and acidic systems (ΔA = A_alkali − A_acid), using a pre-established standard curve.

2.4.5. Determination of the Allicin Content Using the Phenylhydrazone Method

The allicin content in the garlic seedling leaves was determined through the phenylhydrazone method [39]. Fresh leaf samples were thoroughly ground in a mortar to prepare a homogenate, which was then transferred into a 100 mL volumetric flask. The volume was adjusted with distilled water, and the mixture was maintained at a constant temperature of 25–35 °C for 1 h. Subsequently, 2 mL of the extraction solution was mixed with 8 mL of trichloroacetic acid solution. After centrifugation, 2 mL of the supernatant was collected and reacted with an equal volume of 0.2% 2,4-dinitrophenylhydrazine solution. The resulting precipitate was separated through centrifugation and dissolved in 7 mL of ethyl acetate. The solution was then extracted with 10 mL of ammonia water, and the ammonia water phase was retained as the test solution. Using ammonia water as the blank, the absorbance of the test solution was measured at 420 nm. The allicin content was calculated according to a pre-established standard curve.

2.5. Testing of the Soil’s Chemical Properties

The specific testing items and methods for the soil’s chemical properties are shown in Table 2:

2.6. Data Analysis

Microsoft Excel was used for preliminary data processing. Statistical data analyses of the release data were performed using an ANOVA and a means comparison through Tukey’s test at p < 0.05 using SPSS 27.0 software. Graphical representations of the results were generated using Origin 2021.

3. Results and Analysis

3.1. The Effects of Different Treatment Groups on the Yield per Unit Area of Garlic Sprouts

During vegetable cultivation, excessive fertilization not only fails to increase crop yield but may also reduce it. Therefore, replacing conventional fertilizers with new types of fertilizer is considered an effective approach to improving crop yield. As shown in Figure 2, the T3 treatment group, which received slow-release fertilizer with an equivalent nutrient input, exhibited the highest garlic seedling yield. The yields of garlic sprouts in the CK, T1, T2, and T3 groups were 1.08, 1.30, 1.29, and 1.38 kg/m2, respectively. All of the fertilizer-treated groups had significantly increased garlic sprout yields compared to that in the CK group (p < 0.05). Among them, the T3 treatment yielded the highest increase, improving by 6.15% relative to T1 and exhibiting the highest input–output ratio. This result indicates that replacing conventional fertilizers with slow-release fertilizers can significantly enhance garlic seedling yields and economic efficiency. The improved yield is attributed to the controlled and prolonged nutrient release from slow-release fertilizers, which aligns more effectively with crop nutrient uptake patterns during various growth stages. These findings are consistent with those of Yanmei Li et al. [49], who reported an increase of 4600 kg/ha in the tomato yield with the application of slow-release fertilizers. Similarly, a study by Zareabyaneh et al. [50] demonstrated that the use of sulfur-coated urea increased the potato yield by 49.76%, outperforming traditional urea fertilization. These results collectively support the conclusion that substituting conventional fertilizers with slow-release formulations can enhance crop productivity.

3.2. The Effects of Different Treatment Groups on Garlic Sprout Growth

As shown in Figure 3, the application of fertilizer in the T1, T2, and T3 treatment groups significantly increased plant height and stem thickness compared with those in the control group (p < 0.05). The plant heights in the T1, T2, and T3 groups increased by 14.85%, 17.81%, and 27.75%, respectively. No significant difference in plant height was observed between the T1 and T2 groups (p > 0.05). Similarly, stem thickness increased by 9.36%, 8.83%, and 13.96% in the T1, T2, and T3 groups, respectively, relative to that in the control. A significant difference (p < 0.05) in stem thickness was detected between the T3 group and both the T1 and T2 groups, whereas no significant difference was found between the T1 and T2 groups. These results indicate that the application of slow-release fertilizer can promote garlic sprout growth while reducing the overall fertilizer input. Under conditions of equal fertility, the slow-release potassium magnesium sulfate fertilizer demonstrated a more pronounced effect on garlic sprout growth. Although the fertility level in the T2 group was lower than that in the T1 group, there was no significant difference in the plant height and stem diameter of the garlic seedlings between the T2 and T1 groups (p > 0.05) under the application of slow-release fertilizer. However, both parameters in the T2 group were significantly different compared to those in the T3 group (p < 0.05). This is likely due to the rapid nutrient loss in the T1 group during crop growth, which limits timely nutrient uptake by the plants. In contrast, the slow-release fertilizers applied in the T2 and T3 groups provided a more sustained and balanced nutrient supply, thereby promoting the growth in the garlic seedlings’ height and stem diameter. These results are consistent with the experimental findings reported by Dutra [51] and Lang [52].

3.3. The Effects of Different Treatment Groups on the Chlorophyll Content of the Garlic Seedlings

As shown in Figure 4, the SPAD values of the garlic seedling leaves in the four different treatment groups exhibited a consistent pattern throughout the growth stages. In the early growth stage, the SPAD values were relatively low due to insufficient chlorophyll synthesis. As the garlic seedlings developed rapidly, the SPAD values increased [53], attributed to leaf maturation and enhanced photosynthetic activity. In the middle and late stages, the SPAD values fluctuated within a certain range, likely due to peak chlorophyll accumulation during the vigorous growth period and environmental influences such as alternating sunshine and shade [54]. The relative chlorophyll contents in the T1, T2, and T3 treatment groups were significantly higher than those in the CK group, with increases of 4.47%, 7.69%, and 8.23%, respectively. These results suggest that the application of slow-release fertilizers in place of conventional chemical fertilizers can significantly enhance the relative chlorophyll content in garlic sprouts. This is because magnesium is a key component of chlorophyll, playing a vital role in its formation and in facilitating photosynthesis. Compared with the T1 group, the application of the slow-release fertilizers in the T2 and T3 groups could continuously provide a balanced nutrient supply to the crops, thereby increasing the relative chlorophyll content in the garlic seedlings.
Chlorophyll, the primary pigment involved in photosynthesis, directly reflects the intensity of crops’ photosynthetic activity and serves as an accurate indicator of the photosynthetic rate. Measurement of the SPAD values is based on the absorption characteristics of chlorophyll, particularly its peak absorbance in the blue and red light regions. By comparing the absorption and transmittance of specific light wavelengths, the SPAD values can be calculated, which indirectly reflect the relative chlorophyll content in the leaves.
The results of this study demonstrate that substituting conventional fertilizers with slow-release fertilizers significantly increases the chlorophyll content in garlic seedling leaves, thereby enhancing the efficiency of the conversion of light energy into chemical energy. These findings are consistent with those of Iqra Ghafoor [55], who reported that applying 130 kg N ha−1 of biologically active sulfur-coated urea significantly increased chlorophyll content, and Joshi et al. [56], who observed elevated SPAD values in maize following the application of slow-release fertilizer.

3.4. The Effects of Different Treatment Groups on Physiological Indicators of Garlic Sprouts

Soluble sugars are the primary energy substances in plant cells, playing a crucial role in photosynthesis by converting light energy into chemical energy and storing it in the form of carbohydrates. As shown in Figure 5a, the soluble sugar contents in the CK, T1, T2, and T3 groups were 1.90%, 2.32%, 2.40%, and 3.13%, respectively. Compared with that in the CK group, the T1, T2, and T3 treatments increased the soluble sugar content by 22.10%, 26.32%, and 64.74%, respectively, with a statistically significant improvement (p < 0.05). Furthermore, the T3 group showed a significantly higher soluble sugar content than that in both T1 and T2 (p < 0.05).
Soluble protein content is an important physiological and biochemical indicator in plants and a key component of various enzymes in garlic sprouts [57]. As shown in Figure 5b, the soluble protein contents of the CK, T1, T2, and T3 groups were 3.45, 5.99, 5.61, and 7.32 mg/g, respectively. The T1, T2, and T3 treatments increased the soluble protein content by 73.62%, 62.61%, and 112.17% compared to that in CK, with all of these increases being statistically significant (p < 0.05). Additionally, the T3 group exhibited significantly higher soluble protein levels than those in T1 and T2 (p < 0.05).
As shown in Figure 5c, the free amino acid contents in the CK, T1, T2, and T3 groups were 2.20, 3.45, 3.49, and 4.99 mg/g, respectively. Compared with that in CK, the free amino acid content was increased by 56.82%, 58.64%, and 126.82% in the T1, T2, and T3 treatments, respectively, with all showing significant increases (p < 0.05).
Vitamin C is an essential nutrient for humans and is closely related to plant metabolism, as many metabolic enzymes are composed of soluble proteins [58]. Measuring the soluble protein content is therefore important for understanding enzyme activity. As shown in Figure 5d, the vitamin C contents in the CK, T1, T2, and T3 groups were 162.68, 191.09, 197.35, and 221.48 μg/g, respectively. Compared with that in CK, the T1, T2, and T3 treatments increased the vitamin C content by 17.46%, 21.31%, and 36.15%, respectively. Significant increases were observed (p < 0.05), except that between T1 and T2, which did not differ significantly (p > 0.05).
Allicin is a unique nutritional component of garlic sprouts responsible for their characteristic pungent flavor [59]. It enhances appetite, exhibits antimicrobial properties, and inhibits bacterial and fungal growth. Allicin is composed of sulfur-containing compounds and is synthesized through the enzymatic conversion of alliin into pyruvic acid. As shown in Figure 5e, the allicin contents in the CK, T1, T2, and T3 groups were 5.95%, 7.72%, 7.21%, and 8.13%, respectively. Compared with that in CK, the T1, T2, and T3 treatments increased the allicin content by 38.10%, 28.98%, and 45.43%, respectively, with all of these differences being statistically significant (p < 0.05).
Although the fertility level in the T2 group was lower than that in the T1 group, the physiological indicators of the garlic sprouts in the T2 group—such as soluble sugars, free amino acids, and vitamin C—were able to reach or even exceed those in the T1 group due to the encapsulation and adsorption effects of the slow-release materials. In contrast, the contents of protein and allicin in the T2 group showed a certain degree of reduction compared to those in the T1 group, which can be primarily attributed to the lower overall fertility in the T2 treatment.
The results of this study indicate that although both the T1 and T2 treatments led to varying degrees of improvements in garlic sprout quality, the T3 treatment showed the most significant enhancement. This suggests that replacing conventional potassium magnesium sulfate fertilizer with slow-release fertilizer can markedly improve the quality of garlic sprouts while appropriately reducing the amount of chemical fertilizer used. Soluble sugars, soluble proteins, free amino acids, vitamin C (VC), and allicin are important indicators reflecting vegetable quality, collectively influencing the commercial value and taste of garlic sprouts. Slow-release fertilizers enhance multiple physiological traits in crops by regulating the dynamic balance of nutrient release. Slow-release fertilizer can improve the quality of garlic sprouts by extending the nutrient release period and enhancing their nutrient uptake.
Cheng Wang et al. [60] found that compared with conventional fertilization, the application of slow-release fertilizer increased the soluble sugar and soluble protein contents in chives by 8.5% and 4.6%, respectively. Guiting Yang et al. [61] investigated the effects of liquid urea formaldehyde slow-release fertilizer on spinach through pot experiments, focusing on yield, quality, root growth, antioxidant enzyme activity, and nutrient absorption. Their results showed that the vitamin C concentration in spinach treated with the slow-release fertilizer increased significantly by 7.34% to 30.07% compared with that treated with the conventional fertilizer. Similarly, Yahya Faqir et al. [62] reported that chitosan-microsphere-based controlled-release nitrogen fertilizer enhanced the stem length, stem diameter, branch number, pod number, total amino acid content, and vitamin C content in rapeseed. These findings are consistent with the results of the present study, confirming that slow-release fertilizers not only enhance crop yields but also significantly improve quality-related physiological indicators, providing essential technical support for the advancement of green agriculture.
Nutrient uptake and its spatial distribution within the plant are critical determinants of both crop yield and quality. Although this study validated the benefits of slow-release fertilizer through physiological assays in garlic sprouts, it did not assess the actual nutrient absorption. Future work should quantify the nutrient concentrations in the root, stem, and leaf tissues to characterize uptake patterns. Additionally, calculating the fertilizer use efficiency for the four treatment groups would elucidate how replacing conventional fertilizers with slow-release formulations—and potentially reducing total fertilizer inputs—affects the nutrient recovery by the crop. Such analyses would further substantiate that slow-release fertilizers significantly enhance nutrient absorption compared to that under standard fertilization practices.

3.5. The Effects of Different Treatment Groups on Soil Nutrients

As shown in Table 3, compared with those under the CK treatment, the T1, T2, and T3 treatments significantly improved the chemical properties of the soil, including increased levels of organic matter, total potassium, available potassium, and available phosphorus. The levels of available nutrients in all of the fertilized treatments were significantly higher than those in the non-fertilized control, aligning with the findings of previous studies which have confirmed that fertilization improves soil fertility. Among the treatments, T3 exhibited the greatest improvement, which can be attributed to the controlled nutrient release of the slow-release fertilizer. The combined effects of the physical coating and adsorption mechanisms enhanced the contents of available potassium, available sulfur, and exchangeable magnesium while simultaneously lowering the pH and increasing the organic matter content. These improvements are mainly due to the gradual nutrient release of the slow-release fertilizer, which minimizes nutrient loss; the calcium displacement effect of phosphogypsum, which alleviates soil salinization; and the adsorption capacity of bentonite, which enhances nutrient retention. As a result, the T3 treatment comprehensively optimized the soil’s chemical properties, ensuring a continuous and balanced nutrient supply.
Qing Teng et al. [63] demonstrated that the application of slow-release nano-fertilizers (with contents of nitrogen, phosphorus, potassium, magnesium, calcium, and humic acid of 26.0%, 17.0%, 13.0%, 0.2%, 0.9%, and 3.0%, respectively) could enhance the soil’s nutrient content, increase the soil’s enzyme activity, and improve the microbial environment, thereby reducing the nutrient loss and promoting a healthier soil ecological system. These findings highlight the high application value of such fertilizers. Replacing conventional chemical fertilizers with organic or slow-release alternatives has proven to be an effective strategy for improving both crop yield and soil quality. Xin Jin [64] reported that the substitution of chemical fertilizers with slow-release fertilizers led to notable improvements in soil quality and crop productivity. The study revealed that compared with conventional chemical fertilizer treatments, the use of slow-release fertilizers significantly increased the soil’s organic matter content. Similarly, Zeli Li et al. [65] found that the application of slow-release potassium fertilizers significantly enhanced the concentration of available potassium in the soil. Compared with traditional potassium chloride treatments, this method satisfied the nutrient demands of maize during its later growth stages better. These findings are consistent with the results of the present study. Therefore, replacing conventional fertilizers with slow-release fertilizers could promote the sustained absorption of nutrients and enhance soil fertility while also reducing the overall usage of chemical fertilizers. This approach contributes to continuous crop yield improvements and maximizes the net economic benefits.
Although this study demonstrates, through testing changes in the soil’s chemical properties after fertilization, that the application of slow-release fertilizers can enhance the soil’s fertility, it lacks research and analysis on the temporal dynamics of the soil’s nutrient availability. Therefore, future work should investigate further how slow-release fertilizers, compared to conventional fertilizers, release nutrients gradually in the soil, improve fertilizer use efficiency, and support nutrient uptake by garlic seedlings better throughout their growth cycle. The ability of crops to absorb potassium and magnesium can also be evaluated by analyzing the soil’s enzyme activity. Key soil enzymes involved in the uptake of potassium, magnesium, and sulfur include urease and phosphatase, which indirectly influence potassium availability by regulating the soil’s pH or competing for adsorption sites. Dehydrogenases and cellulases contribute to the decomposition of organic matter, thereby facilitating magnesium release. Arylsulfatase plays a direct role in the mineralization of organic sulfur, converting it into sulfate ions that are readily available to plants. The activity of these enzymes is strongly influenced by the soil’s microbial communities, pH levels, and organic matter content. Enhancing enzyme activity—through the application of organic fertilizers or the optimization of microbial populations—can improve the availability of potassium, magnesium, and sulfur, ultimately promoting nutrient uptake by crops.
To comprehensively evaluate the effects of different fertilization treatments on the soil’s chemical properties, a principal component analysis (PCA) was performed using seven soil nutrient parameters and pH values from the four treatment groups. As illustrated in the two-dimensional PCA plot (Figure 6), the first and second principal components together accounted for the majority of the variation among the treatments. The cumulative contribution of the first two components reached 79.3%, with PC1 explaining 56.1% of the total variance. This component is primarily associated with exchangeable magnesium, total potassium, available sulfur, available potassium, total phosphorus, available phosphorus, and organic matter. PC2 explains 23.2% of the variance and is mainly associated with pH. Exchangeable magnesium is essential for chlorophyll synthesis, and magnesium deficiency can cause yellowing in older leaves and reduced photosynthetic efficiency. Fast-acting potassium regulates osmotic pressure and sugar transport, thereby influencing the stress resistance and bulb enlargement of garlic seedlings. Potassium deficiency results in scorched leaf margins and yield reduction. Available sulfur is involved in the synthesis of allicin and proteins; its deficiency may cause yellowing in new leaves and a weakened flavor. Total and available phosphorus contribute to root development and energy metabolism, while phosphorus deficiency leads to stunted growth and reduced tillering. Organic matter enhances the soil structure, improves water and nutrient retention capacity, and gradually releases nutrients through microbial mineralization, thereby strengthening the disease resistance in garlic seedlings. Moreover, the loading plot indicates that pH and exchangeable magnesium exhibit strong loadings on the second and first principal components, respectively, suggesting that these variables are representative indicators of the differences in the soil’s chemical properties under various fertilization treatments.

3.6. Regulation of the Soil’s Heavy Metal Content by the Slow-Release Fertilizer

As shown in Figure 7, compared with that in the CK group, the metal content in the T1 treatment group increased slightly overall, whereas the metal contents in the T2 and T3 groups generally decreased. Specifically, the Cd content in the T2 and T3 groups decreased by 15.56% and 16.25%, respectively, compared with that in the CK group; the Cr content decreased by 1.18% and 6.79%; the Ni content decreased by 13.53% and 19.54%; and the Zn content decreased by 3.11% and 14.14%, respectively. These reductions are primarily attributed to the addition of bentonite, a material widely used in slow-release fertilizers due to its large specific surface area, strong ion exchange capacity, stable effects, and abundant availability [66]. Bentonite exhibits an effective remediation capacity for soils contaminated with heavy metals. Its ion exchange properties enable the fixation of heavy metal ions in the soil, thereby reducing their bioavailability and limiting their uptake by plants [67]. This makes bentonite a suitable material for the remediation of heavy-metal-contaminated soils. The negatively charged surfaces of clay minerals allow them to adsorb cations, and montmorillonite (a 2:1-type clay mineral) facilitates ion exchange due to the absence of interlayer ion bonds [68]. Additionally, the migration of heavy metals in the soil involves rapid exchange and precipitation–dissolution reactions. The sulfate ions present in phosphogypsum can form insoluble sulfate precipitates with heavy metal ions, thus reducing their mobility and bioavailability [69]. Moreover, phosphogypsum is rich in Ca2+, which competes with heavy metals for adsorption sites on soil colloids, promoting the transformation of heavy metals from adsorbed states into precipitated forms [70].
Esawy et al. [71] investigated the effects of phosphogypsum (PG) and its combination with compost (CP) at a wet-to-weight ratio of 1:1 on the fixation of heavy metals in contaminated soil. The results indicated that the application of PG alone had a more pronounced effect on heavy metal immobilization, whereas the combined use of PG and CP exhibited a slightly weaker fixation capacity. Although the addition of CP contributed to improved plant growth, the fixation of heavy metals was most effective with PG alone. P. Kumararaja et al. [72] demonstrated that the application of 2.5% columnar fixed bentonite significantly improved plant growth while reducing the bioavailable heavy metal content in the soil. This finding suggests that columnar fixed bentonite is effective in remediating heavy-metal-contaminated soils by immobilizing metal ions. In a related study, Kumararaja et al. [73] further showed that the biological concentration factor (BCF) of heavy metals was significantly reduced by improving the soil with 2.5% bentonite. Specifically, in the first and second harvests, the BCF of zinc decreased by 74% and 28%, that for copper was reduced by 38% and 36%, and that for nickel was reduced by 44% and 34%, respectively, due to bentonite’s ability to immobilize heavy metals in contaminated soils. Therefore, the application of slow-release fertilizers in this study effectively reduced metal ion leaching and decreased the extent of soil metal contamination. The T3 group demonstrated a superior performance, attributed to the higher fertilizer application rate, which enhanced the synergistic fixation effects of phosphogypsum and bentonite. Although the data on the heavy metal content in the soil in this experiment was based on a single measurement and could not be statistically analyzed, the measurement results are consistent with the relevant literature, indicating that slow-release fertilizers do not increase the soil’s heavy metal load and may even reduce their bioavailability through physical and chemical fixation. Future research could address the issue of metal content as a separate topic through multiple repeated sampling and then conduct a statistical analysis of the metal contents in different parts of garlic sprouts and the soil’s metal content.

4. Conclusions

The results showed that compared with T1, T2 was able to improve the quality of the garlic sprouts while reducing the fertilizer usage, increasing plant height by 2.58% and enhancing the chlorophyll, soluble sugar, free amino acid, and vitamin C contents by 3.88%, 1.16%, and 3.28%, respectively. However, stem diameter decreased by 0.48%, and the contents of soluble protein and allicin declined to some extent. The effect of T3 was more pronounced, with garlic sprout height and stem thickness increasing by 11.23% and 4.17%, respectively. Simultaneously, the contents of soluble sugars, soluble proteins, free amino acids, vitamin C, and allicin increased significantly, with these improvements ranging from 5.31% to 44.64%. These findings indicate that slow-release fertilizers with equal fertility are more conducive to nutrient absorption and quality enhancements in garlic sprouts. In terms of soil improvement, T3 significantly reduced the pH of alkaline soil and increased the availability of potassium, phosphorus, sulfur, exchangeable magnesium, and the organic matter content, indicating that slow-release fertilizers can optimize soil nutrient supply. Additionally, the heavy metal content in the soil in the T2 and T3 groups decreased compared to that in CK and T1, owing to the chemical precipitation and ion exchange capacity of phosphogypsum, as well as the strong adsorption ability of bentonite. This suggests that slow-release fertilizers possess remediation potential for heavy-metal-contaminated soils. In conclusion, slow-release fertilizers—especially those applied with equal fertility—not only promote growth and enhance the quality of garlic sprouts but also improve soil fertility and reduce heavy metal risks, demonstrating considerable value for sustainable agriculture.

Author Contributions

Conceptualization: R.L.; supervision: C.T. and X.F.; writing—original draft: C.H.; writing—review and editing: C.H. and Z.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by SDIC Xinjiang Luobupo Postash Co., Ltd., grant number H20230384.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data are contained within the article.

Conflicts of Interest

Author Chunxiao Han was employed by the company SDIC Xinjiang Luobupo Postash Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Preparation process of slow-release fertilizer.
Figure 1. Preparation process of slow-release fertilizer.
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Figure 2. The effect of different treatment groups on the total yield per unit area of garlic sprouts. The different letters represent a significant difference between different treatments (p < 0.05). Error bars represent ±SD.
Figure 2. The effect of different treatment groups on the total yield per unit area of garlic sprouts. The different letters represent a significant difference between different treatments (p < 0.05). Error bars represent ±SD.
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Figure 3. (a) The effect of different treatment groups on the height of garlic sprouts; (b) The effect of different treatment groups on the stem thickness of garlic sprouts. The different superscripted letters represent a significant difference between different treatments (p < 0.05). Error bars represent ±SD.
Figure 3. (a) The effect of different treatment groups on the height of garlic sprouts; (b) The effect of different treatment groups on the stem thickness of garlic sprouts. The different superscripted letters represent a significant difference between different treatments (p < 0.05). Error bars represent ±SD.
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Figure 4. The effect of different treatment groups on the chlorophyll content in garlic sprouts. Error bars represent ±SD.
Figure 4. The effect of different treatment groups on the chlorophyll content in garlic sprouts. Error bars represent ±SD.
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Figure 5. The effect of different treatment groups on physiological indicators of garlic sprouts: (a) soluble sugar; (b) soluble protein; (c) free amino acid; (d) vitamin C; (e) Allicin. The different superscripted letters represent a significant difference between different treatments (p < 0.05). Error bars represent ±SD.
Figure 5. The effect of different treatment groups on physiological indicators of garlic sprouts: (a) soluble sugar; (b) soluble protein; (c) free amino acid; (d) vitamin C; (e) Allicin. The different superscripted letters represent a significant difference between different treatments (p < 0.05). Error bars represent ±SD.
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Figure 6. Principal component analysis of soil’s chemical properties by different treatment groups.
Figure 6. Principal component analysis of soil’s chemical properties by different treatment groups.
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Figure 7. Soil metal content in different treatment groups.
Figure 7. Soil metal content in different treatment groups.
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Table 1. The design of the pot experiment.
Table 1. The design of the pot experiment.
TreatmentFertilizerPotassium Magnesium Sulfate Fertilizer (g/g)Usage (g)
CK---
T1potassium magnesium sulfate fertilizer1.005.00
T2slow-release fertilizer0.615.00
T3slow-release fertilizer0.618.20
Table 2. Methods for determining the soil’s chemical properties.
Table 2. Methods for determining the soil’s chemical properties.
ProjectTesting MethodsInstruments and EquipmentReferences
Total P⟪Determination of Forest Soil Phosphorus⟫
(LY/T 1232-2015)		Agilent Technologies (Santa Clara, CA, USA) 5110 ICP-OES[40]
Total K⟪Determination of Forest Potassium in Soil⟫ (LY/T 1234-2015)Agilent Technologies 5110 ICP-OES[42]
Available K⟪Determination of Forest Potassium in Soil⟫ (LY/T 1234-2015)Agilent Technologies 5110 ICP-OES[42]
Organic matterNY/T 1121.6-2006 Soil Testing Part 6: Determination of Soil Organic Carbon and Organic MatterFully automatic organic matter analyzer JX-S7066
(JX, Shanghai, CHINA)		[43]
pHNY/T 1377-2007 Determination of pH value in soilLEICI PHS-3C PH Meter
(LEICI, Shanghai, CHINA)		[44]
Available PNY/T 1121.7-2014 Soil testing—Part 7: Determination of available phosphorus in soilUV visible spectrophotometer TU-1900
(Persee, Beijing, CHINA)		[45]
Available SNY/T 1121.14-2023 Soil testing—Part 14: Determination of soil available sulfurAgilent Technologies 5110 ICP-OES[46]
Interchangeable MgNY/T 295-1995 Determination of Cation Exchange Capacity and Exchangeable Base in Neutral SoilsAgilent Technologies 5110 ICP-OES[47]
Metal contentICP-OES measurement of soil metal contentAgilent Technologies 5110 ICP-OES[48]
Table 3. The influence of different treatment groups on the soil’s chemical properties.
Table 3. The influence of different treatment groups on the soil’s chemical properties.
CKT1T2T3
pH7.42 ± 0.01 c7.16 ± 0.04 c6.75 ± 0.02 b6.88 ± 0.03 a
Total K (g/kg)17.30 ± 0.10 b18.23 ± 0.18 a17.55 ± 0.12 ab17.62 ± 0.21 ab
Total P (g/kg)0.53 ± 0.07 a0.53 ± 0.01 a0.54 ± 0.02 a0.53 ± 0.02 a
Available K (mg/kg)80.02 ± 3.94 d691.80 ± 5.31 c338.45 ± 7.50 b788.92 ± 4.01 a
Available P (mg/kg)9.06 ± 0.33 b9.57 ± 0.51 b16.58 ± 0.47 a17.43 ± 0.50 a
Organic matter (g/kg)2.66 ± 0.26 c2.65 ± 0.21 c3.30 ± 0.14 b5.56 ± 0.35 a
Available S (mg/kg)63.25 ± 2.57 d532.3 ± 6.56 b323.25 ± 2.91 c548.03 ± 7.80 a
Interchangeable Mg (mg/kg)1.91 ± 0.13 d3.52 ± 0.07 c2.73 ± 0.21 b3.88 ± 0.14 a
The different superscripted letters represent a significant difference between different treatments (p < 0.05).
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Han, C.; Zhang, Z.; Liu, R.; Tao, C.; Fan, X. The Effect of Slow-Release Fertilizer on the Growth of Garlic Sprouts and the Soil Environment. Appl. Sci. 2025, 15, 8216. https://doi.org/10.3390/app15158216

AMA Style

Han C, Zhang Z, Liu R, Tao C, Fan X. The Effect of Slow-Release Fertilizer on the Growth of Garlic Sprouts and the Soil Environment. Applied Sciences. 2025; 15(15):8216. https://doi.org/10.3390/app15158216

Chicago/Turabian Style

Han, Chunxiao, Zhizhi Zhang, Renlong Liu, Changyuan Tao, and Xing Fan. 2025. "The Effect of Slow-Release Fertilizer on the Growth of Garlic Sprouts and the Soil Environment" Applied Sciences 15, no. 15: 8216. https://doi.org/10.3390/app15158216

APA Style

Han, C., Zhang, Z., Liu, R., Tao, C., & Fan, X. (2025). The Effect of Slow-Release Fertilizer on the Growth of Garlic Sprouts and the Soil Environment. Applied Sciences, 15(15), 8216. https://doi.org/10.3390/app15158216

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