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Article

Effects of Compound Fertilizer Containing Polyhalite on Soil and Maize Growth Under Different Nitrogen Levels

by
Xiaohan Li
1,
Ruixue Jing
2,
Jimin Guo
3,
Shun Li
4,
Liyong Bai
1,* and
Jiulan Dai
1,*
1
Environment Research Institute, Shandong University, Qingdao 266237, China
2
Liangshan County Agricultural Technology Extension Center, Liangshan 272600, China
3
Weifang Agricultural Technology Extension Center, Weifang 261061, China
4
State Key Laboratory of Microbial Technology, Shandong University, Qingdao 266237, China
*
Authors to whom correspondence should be addressed.
Sustainability 2025, 17(19), 8827; https://doi.org/10.3390/su17198827
Submission received: 27 August 2025 / Revised: 20 September 2025 / Accepted: 29 September 2025 / Published: 2 October 2025
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Abstract

The growing potassium (K) demand and supply–demand imbalance in intensive agriculture require the development of multi-nutrient K fertilizers. Polyhalite (POLY), a multi-nutrient natural mineral rich in K, calcium, magnesium, and sulfur, can enhance soil nutrient diversity and fertility. However, research on its synergistic application with nitrogen (N) fertilizer remains limited. Therefore, this study was designed to apply three different fertilizer composites at four N concentration gradients through field plot experiments to evaluate crop productivity and nutrient use efficiency. Results revealed that the application of both compound fertilizers with N fertilizer increased maize yield, ranging from 1.03% to 11.53%, compared with the PK control. Moreover, 25-7-8 (MOP)(POLY26%) achieved a maximum yield of 9499.88 kg/ha at the N1 (170 kg/ha) level. This represents a significant increase of 11.53% compared with the PK control. Moreover, the application of compound fertilizer containing POLY could significantly increase the N fertilizer utilization rate; improve the quality of maize; and exert a significant effect on soil pH, EC, and nutrient content. This study paves the way for broader application of POLY by establishing its novel role as a sustainable nutrient source. It provides critical strategic guidance for advancing global resource-efficient agriculture.

1. Introduction

In intensive agriculture, potassium (K) fertilization is essential to guarantee an adequate nutrient supply for crops, playing a critical role in enhancing both yield and quality [1,2]. Maize is a crucial global food crop and a rich source of nutrients such as starch, protein, and fat [3,4]. It has a high demand for K nutrients, and its growth and yield are significantly affected by the level of K fertilizer application [5]. K not only promotes the uptake of other nutrients such as nitrogen (N) and phosphorus (P) in maize but also plays an irreplaceable role in many basic physiological and metabolic processes, thereby enhancing the crop’s resistance to drought, disease, and failure [6,7]. These benefits contribute to increased agricultural productivity. In recent years, expanding cultivation areas and growing demand for K have led to excessive fertilizer application. Overuse can reduce fertilizer efficiency, degrade soil environment, and increase the risk of umbilical rot in fruits and vegetables, yellowing of leaves, fruit chaffing, and other deficiencies, thereby affecting the yield and quality of vegetables [8]. Notably, China possesses a mere 5% of the world’s total K resources [9,10], resulting in a sharp disparity between limited resource availability and increasing agricultural consumption. Therefore, studies on the efficient use of K resources in China, optimization of fertilizer management, improvement in fertilizer utilization efficiency, enhancement of soil fertility, and sustainable agricultural development are critical.
The search for alternative K sources is important to promote sustainable agriculture worldwide. Polyhalite (POLY), a multi-nutrient mineral fertilizer, contains four macronutrients: approximately 14% potassium oxide (K2O), 19% sulfur (S), 6% magnesium oxide, and 17% calcium oxide. With its growing application in modern agriculture, POLY is recognized as a superior sulfate-based slow-release K fertilizer, historically utilized in the USSR and currently adopted in countries such as Germany and the United States [11,12]. POLY is not only enriched with key nutrients to provide comprehensive nutrient support for crops but it also effectively improves the efficiency of nutrient uptake and utilization by crops and reduces the use of chemical fertilizers [13,14]. Several studies have shown that POLY significantly increases crop yields and nutritional quality. POLY increases the proportion of oil and protein in mustard seeds and significantly increases K uptake in plants [15,16]; moreover, it significantly increases the yields of mustard seeds, maize, wheat kernels, and peanut [15,17,18]. POLY has significant environmental advantages. It improves soil structure through supplementation of Ca and Mg and enhances moisture and nutrient retention [19,20,21]. Furthermore, the mining process of POLY has a lower impact on the environment compared with other fertilizers [22]. In addition, its low solubility and complex mineral composition give it the property of slow-release K [23,24,25]. The slow-release K fertilizer reduces nutrient loss, prolongs fertilizer availability, and supports sustained crop growth [26,27,28]. In recent years, Tan et al. [19] found that the partial replacement of KCl with POLY improves soil quality and peanut growth more than POLY and KCl alone. Chen et al. [29] found that compound fertilizers containing POLY enhance winter melon yields by 10–17%, significantly improve melon quality, increase nutrient use efficiency, and reduce environmental pollution risks. Nevertheless, current knowledge on the synergistic effects of POLY with nitrogen fertilization remains limited, particularly in maize cropping systems. Thus, research on optimized co-application strategies is crucial to fully exploit the potential of POLY in enhancing maize yield, improving soil health, and promoting sustainability in agriculture.
In modern agricultural production, the selection of fertilization strategies is crucial for optimal crop growth and yield enhancement [30]. Rational combined application of N and K fertilizers has been shown to synergistically improve both yield and quality in rice [31]. Wu [32] showed that the interaction of N and K fertilizers affects crop growth, development, and metabolism. For instance, under suitable moisture conditions, Tang and Huang [33] observed notable effects of N-K interaction on plant height and dry matter accumulation in maize seedlings. Similarly, rational N and K application improved both yield and quality of tobacco leaves [34]. Hou et al. [35] found that the combined application of N and K increased both rice yield and leaf area index over three years. Furthermore, appropriate N and K ratios support robust root development and enhance resistance to abiotic and biotic stresses such as drought, pests, and diseases [36,37,38]. Studies have shown that the increased application of N and K fertilizers enhances K uptake in maize, with significant N and K interactions, and rational N-K combination improved N uptake and translocation in crops, thereby increasing nitrogen use efficiency [33,39]. Nevertheless, despite the established advantages of N–K coordination, studies on the synergism between N and POLY remain limited, especially in maize systems. A significant knowledge gap remains regarding the impact of POLY–N interaction on nutrient use efficiency and soil quality in maize.
The depletion of soil nutrients In Intensive agriculture and excessive use of inorganic fertilizers are becoming widespread in response to growing global food demand [40]. Although long-term application of inorganic fertilizers has enhanced soil fertility and increased crop yields [41,42,43], it has also resulted in several negative impacts, including threats to soil quality, disruption of soil microbial communities, and reduction in the ecological function of water bodies [44,45,46,47]. In modern intensive farming, imbalanced or single-nutrient fertilization poses a serious threat to sustainable productivity [15,48]. Therefore, modern agriculture urgently requires an effective supply of multiple nutrients. POLY offers high nutrient content and slow-release properties, enabling efficient nutrient management and enhanced agricultural sustainability.
N is vital for crop growth, playing a critical role in cell division, protein synthesis, stress resistance, and quality improvement [49,50,51]. Rational N application can significantly improve yields of crops (such as wheat, maize, and rice), enhance N fertilizer utilization efficiency in maize, and reduce resource wastage and N loss [52,53,54]. However, excessive N fertilization is prevalent in high-yielding maize systems [55]. Studies have shown that excessive N application beyond crop demand leads to significant nitrogen losses and environmental pollution [56,57], such as soil acidification and eutrophication of water bodies [58,59,60]. In the wheat–maize rotation system, N leaching occurs mainly during the maize season [61], and NO3−–N leaching increases with increasing N fertilizer application [62]. In response to environmental challenges from excessive nitrogen fertilization, policies such as China’s “Fertilizer Reduction by 2025” [63], the EU’s Farm to Fork Strategy [64], and FAO’s Sustainable Nitrogen Management framework have been implemented globally (https://www.dairysustainabilityframework.org/). Therefore, nitrogen fertilizer application must be optimized, and the N–K ratio should be rationally adjusted. Furthermore, POLY and its compound fertilizers represent a promising strategy to enhance nutrient use efficiency. Consequently, these measures are crucial for advancing sustainable agriculture, improving crop productivity and quality, and reducing environmental impact.
Although many studies have examined the effects of POLY on crop yields and soil nutrients, research on the synergistic effects of POLY-containing composite fertilizers with N fertilization remains scarce. Therefore, we examined the impact of POLY-containing fertilizers on maize yield, agronomic traits, N use efficiency, and soil nutrients under varying N levels. This study aimed to investigate (1) the effects of POLY-containing compound fertilizer on maize yield, nutrient content in grain and stalk, and growth characteristics under varying N rates, (2) its influence on N use efficiency, and (3) its effects on soil nutrients under different N levels. This study investigated the benefits and advantages of applying compound fertilizer containing POLY under varying N levels. The findings will support rational N fertilization and POLY utilization, contributing to enhanced agricultural sustainability through improved fertilization practices.

2. Materials and Methods

2.1. Plot Characteristics and Soil Analysis

The experimental farmland is situated in Fangxi Village, Quanpu Town, with a total area of 2470 m2 with center coordinates of 116.111231° E, 35.660292° N. The region experiences a continental monsoon climate. Maize and wheat are the main perennial crops in the experimental plots, and their stalks consistently returned to the field. The soil consisted of silty loam, classified as fluvo-aquic soil. Table S1 presents detailed temperature and weather information. The maximum temperature during planting was 33 °C, and the minimum temperature was 12 °C. The average summer temperature of Liangshan County is 28.0 °C. The concentration of rainfall during the planting period is in July and August, and the total summer precipitation is 148.8 mm. Temperature and precipitation data are sourced from the Central Meteorological Observatory (https://www.nmc.cn/). Prior to experimental planting, the test field was uniformly divided into three columns, and soil from each column was taken and mixed separately via serpentine sampling and used as background values for this test field. The basic physical and chemical properties of the plots are shown in Table 1.

2.2. Experimental Design

In July 2023, the plots were divided according to the experimental design scheme. The soil samples of each plot were collected before the experiment, and the fertilizers were applied and mixed in each plot according to the dosage in the experimental design scheme. The maize was grown on each plot. In August 2023, the topdressing fertilizer was applied according to the experimental design scheme. In September 2023, the agronomic traits of maize were measured at the filling stage. In October 2023, the maize in each experimental plot was harvested, and the soil and maize samples were collected. The agronomic traits of maize were measured at the mature stage.
In this study, the maize variety Quanke 789 was selected, which is the primary promoted variety in the Huang-Huai-Hai summer maize planting area, approved by the state in 2020. Its growth period is about 102 days, exhibiting characteristics of high yields, good quality, and good stress resistance.
The experiment involved a completely randomized block design with 13 treatments, and each treatment was repeated four times in the experiment. Each plot had a planting area of 20 m2, with a 1.5 m spacing between adjacent plots. Maize was planted at a density of 150 plants per plot, with row spacing of 0.5 m and plant spacing of 0.2 m. POLY is a naturally occurring, evaporated mineral formed from the dried-up bed of an ancient sea or ocean. Chemically, it is a hydrated K, Ca, and magnesium sulfate salt with the chemical formula K2SO4·MgSO4·2CaSO4·2H2O [17]. Three potassium-based fertilizers were evaluated in this study. The first was a conventional N, P, and K compound fertilizer, designated as 15-15-15 (MOP), with a nutrient content of 15% N, 15% phosphorus pentoxide (P2O5), and 15% K2O. The second fertilizer, 15-15-15 (MOP)(POLY25%), was derived from the 15-15-15 (MOP) compound and maintained the same nutrient composition: 15% N, 15% P2O5, and 15% K2O. However, in this formulation, 25% of the K2O was sourced from POLY, while the remaining 75% was supplied by MOP. The third fertilizer, 25-7-8 (MOP)(POLY26%), was based on a 25-7-8 (MOP) N, P, and K compound fertilizer and contained 25% N, 7% P2O5, and 8% K2O. In this case, 26% of the K2O was provided by POLY and 74% by MOP.
All treatments received a uniform application of P and K. A basal fertilizer application was applied at sowing. This basal dose consisted of 60% of the total N, all P, and all K fertilizers. It was applied using a layered application method. The remaining 40% of N was applied as topdressing at the jointing stage. In this study, three fertilizers were evaluated at four N levels for maize cultivation. Detailed application rates for each treatment are provided in Table 2.

2.3. Sample Collection

Soil sampling shall be conducted in accordance with the Chinese agricultural industry standard NY/T 395-2012 [65]. Soil at a depth of 0–20 cm was collected from each plot before and after planting, including soil shaken down using the root-shaking method [66]. Soil samples from each plot were combined with mixed soil samples from five sampling points. During the collection, extraneous materials in the soil were removed and sealed with polyethylene plastic bags to prevent cross-contamination of the samples. The gathered soil specimens were transferred into plastic sample containers to eliminate stones, plant residues, and other foreign materials present in the soil. These samples were subsequently evenly spread into thin layers and allowed to air-dry at room temperature. Each soil specimen underwent manual processing through hand-crushing with a mortar and pestle, followed by filtration through a 2 mm particle-size nylon mesh sieve for standardization. Soil pieces larger than 2 mm were repeatedly ground and sieved until they all passed, and the sieved samples were mixed well and sampled in quadrature for the analysis of soil pH, soil conductivity (EC), particle size distribution, alkali-hydrolyzable nitrogen, available P, available K, exchangeable Ca and Mg, and available S.
Field measurements of agronomic traits such as maize plant height (MPH), cob leaf area (CLA), maize stem diameter (near the root) (MSD), green leaves number (GLN), and yellow leaves number (YLN) were carried out by selecting six maize plants of similar growth from each plot at the filling and maturity stages of maize. MPH is the length of the maize from the ground to the top (including the ear). CLA is mathematically derived from the product of the leaf shape coefficient, length, and width [67]. MSD, GLN, and YLN were all measured and counted manually. At maize harvest, five maize plants were selected from each plot using the five-point sampling method and transported to the laboratory. The plant samples were divided into grain and stalk parts, washed and dried separately, placed into an oven at 105 °C for 30 min, dried at 65 °C until the respective weights were constant, and ground into powder by using a grinder (AQ-180E-Y, Nail, Cixi, China) for nutrient analysis. These five maize plants were used not only for nutrient determination and analysis but also for the assessment of yield, hundred-grain weight (HGW), grain weight per ear (GWPE), kernel number per cob (KNPC), and stalk weight. Yield, HGW, and GWPE were determined from the weight of the dried grain. KNPC was counted manually. Stalk weight was measured in the field.

2.4. Sample Analysis Methods

2.4.1. Analysis of Soil Sample Indices

In this study, the basic physicochemical properties of soil samples were analyzed following the methods described in Bao’s “Soil and Agricultural Chemistry Analysis” [68] and national standards. Among these, soil pH, EC, available K, and alkali-hydrolyzable nitrogen were determined according to the methods described by Bao Shidan [68]. Available P, exchangeable Ca and Mg, and available S were measured following the Chinese agricultural industry standards NY/T 1121.7-2014 [69], NY/T 1615-2008 [70], and NY/T 1121.14-2023 [71], respectively. Soil pH was measured using a precision pH meter (PHS-3C, Shanghai Leici Scientific Instrument Factory, Shanghai, China) after extraction with CO2-free distilled water at a soil-to-water ratio of 1:2.5. Soil EC was measured using a precision conductivity meter (DDS-307, Shanghai Leici Scientific Instrument Factory) after extraction with distilled water at a soil-to-water ratio of 1:5. Soil particle size distribution was determined by the micro-pipette method [72]. Alkali-hydrolyzable nitrogen was determined using an alkali diffusion method. Available P was extracted with sodium bicarbonate and quantified colorimetrically using a molybdenum-antimony method (L5S, Shanghai Inesa Analytical Instrument Company Limited, Shanghai, China). Available K was extracted with ammonium acetate and measured using AAS (iCE3500, Thermo Fisher Scientific, Waltham, MA, USA). Exchangeable Ca and Mg in soil samples were determined according to the Chinese national standard (NY/T 1615-2008). Available S was extracted with calcium chloride extractant and measured using UV-Vis (L5S, Shanghai Inesa Analytical Instrument Company Limited). Soil standard reference material NSA-2 (Institute of Geophysical and Geochemical Exploration, Chinese Academy of Geological Sciences, Langfang, China) and blank samples were included in the experimental analyses for calibration, validation, and quality control. In addition, each soil sample from the test field was measured in the same batch to ensure that the matrix effects of each sample from the test field remained consistent.

2.4.2. Analysis of Plant Sample Indices

The nutrient elements in maize grains and straw were determined using the following standard methods: total nitrogen was analyzed according to the method described by Bao Shidan [68]; total sulfur and total phosphorus were determined using the methods established by Lu Rukun [73]; total Zn, Fe, K, Ca, Mg, and Mn were measured in accordance with Chinese National Standards GB 5009.14-2017 [74], GB 5009.90-2016 [75], GB 5009.91-2017 [76], GB 5009.92-2016 [77], GB 5009.241-2017 [78], and GB 5009.242-2017 [79], respectively. Total N in grain and stalk was quantified using a semi Kjeldahl analyzer (Haineng K9840, Hanon Future Technology Group Company Limited, Jinan, China). The grain and stalk of maize plants were digested using the nitric-perchloric acid digestion method. About 0.5000 g of grain and 0.2000 g of stalk were placed in the digestion tube and added with 10 mL of HNO3 (GR) and 3 mL of HclO4 (GR), respectively. The tank was covered and left for 1 night before being digested using a fully automatic digester (DigestLinc-ST60D, Beijing Puli Taike Instrument Company Limited, Beijing, China). The following day, digestion and acid removal were carried out according to the setting procedure. After cooling, ash was dissolved in 1% nitric acid solution, and the sample digest was transferred into a 25 mL volumetric flask. The volume was fixed to the scale and mixed for the determination of K, Ca, Mg, iron (Fe), manganese (Mn), zinc (Zn), P, and S. The grain and stalk were quantified colorimetrically using the molybdenum-antimony method (L5S, Shanghai Inesa Analytical Instrument Company Limited). The S in grain and stalk was determined colorimetrically using a turbidimetric method (L5S, Shanghai Inesa Analytical Instrument Company Limited). The K, Ca, Mg, Fe, Mn, and Zn contents were measured using AAS (iCE3500, Thermo Fisher Scientific). For calibration, validation, and quality control of maize samples, the maize standard reference material GBW10012 (GSB-3, Institute of Geophysical and Geochemical Exploration, Chinese Academy of Geological Sciences) and blank samples were added to each nutrient determination.

2.5. Data Analysis

In this study, data in figures or tables are presented as the mean ± standard deviation (M ± SD) of four replicates. Data obtained from experiments were analyzed using Microsoft Excel 2016 (Microsoft, Redmond, DC, USA) and SPSS 26.0 (IBM, Chicago, IL, USA). Before conducting the analysis of variance, the normality and homogeneity of the data were assessed. One-way ANOVA was used to evaluate differences between treatments, and the least significant difference (LSD) was applied to test the significance between means. Duncan’s test was used to test for significant differences (p < 0.05). Graphs were plotted using Origin 2021 (Origin Lab, Northampton, MA, USA).
Alternatively, Nitrogen Agronomic Efficiency (NAE), Partial Factor Productivity of Nitrogen (PFPN), and Agronomic Use Efficiency (AUE) were calculated as suggested by Carciochi et al. [80], Nie et al. [81], and Lu et al. [59]:
N A E k g k g = g r a i n   y i e l d + N − g r a i n   y i e l d − N N   f e r t i l i z e r
P F P N k g k g = g r a i n   y i e l d + N N   f e r t i l i z e r
A U E k g k g = g r a i n   y i e l d + N g r a i n   y i e l d − N

3. Results

3.1. Effect of Compound K at Different N Levels on Maize Yield

The combination of N fertilizer and compound fertilizer promoted the increase in maize yield. The yield of T1 was the smallest at 8518.13 kg/ha, whereas the yield of T4 was the largest at 9499.88 kg/ha. The application of different treatments of N content and compound K fertilizer increased the yield by 1.03–11.53% relative to T1. As shown in Figure 1a, the yield of T4 was the highest, followed by that of T6, and the maize yield of T1 was the lowest. Compared with T1, the yields in T4 and T6 increased significantly (p < 0.05, Duncan test). Furthermore, T4 had the highest increase in yield, increasing by 11.53% compared with T1, whereas T13 demonstrated the least yield increase, with a rise of 1.03% compared with T1.
In the present study, the N fertilizer content could significantly affect yield at the same level of K application. The between-group analysis (Figure 1b) revealed that the yield of N1 was the highest, followed by that of N2, and the yield of N0 was the lowest. Compared with N0, the yield in N1 increased significantly (p < 0.05, Duncan test). The increase in yield content in N1 was the highest, increasing by 5.83% compared with N0, and the increase in yield content in N4 was the lowest, increasing by 1.75% compared with N0. Thus, at the same level of applied K, the highest yield was obtained at N1, and the amount of N fertilizer applied at the N1–N2 level could adequately satisfy the growth requirements of maize. The amount of N fertilizer applied at the N3–N4 level did not lead to a significant difference in yield, although an increasing tendency was observed (p > 0.05, Duncan test).
Figure 1a showed that the yield of market 15-15-15 (MOP) and 15-15-15 (MOP)(POLY25%) initially increased and then decreased with the increase in N application, whereas the yield of 25-7-8 (MOP)(POLY26%) decreased with the increase in the amount of N applied. Li et al. [82] found that the effects of different applications of N fertilizer on maize kernel yield are quadratic; the application of moderate amounts of N fertilizer helped increase yield after returning to the field, but excess application of N fertilizer caused a decrease in maize kernel yield. Market 15-15-15 (MOP), 15-15-15 (MOP)(POLY25%), and 25-7-8 (MOP)(POLY26%) demonstrated the highest yields of 8809.13, 9095.75, and 9499.88 kg/ha at the N3, N2, and N1 levels, respectively. Consistent with the principles of resource conservation and efficient agriculture, 25-7-8 (MOP)(POLY 26%) is recommended to meet the growth requirements of maize when applied at the N1 level.

3.2. Effect of Compound K at Different N Levels on Agronomic Traits

To investigate the influence mechanism of different fertilizer distributions on the agronomic traits of maize, this study further elaborated the influence of morphological indicators such as MPH, MSD, CLA, and yield components (such as HGW and GWPE). It formed the theoretical basis for precision fertilization aimed at high-yield, high-quality cultivation.
As shown in Table S2, the HGW of maize ranged from 19.43 to 25.18 g, and the GWPE ranged from 0.19 to 0.25 g. Moreover, T6 showed the highest HGW and GWPE, which were significantly higher than those of T8 (p < 0.05, Duncan test). In addition, the treatment with POLY achieved relatively high HGW and GWPE at all four N levels. At maturity, the MSD of T10 was the smallest at 67.13 mm, and the MSD of T6 was the largest at 69.83 mm. Furthermore, POLY application resulted in a relatively large MSD at maturity under four N levels. The combination of N fertilizer and a complex fertilizer containing POLY increased stalk weight and MSD during the filling period, albeit not significantly (p > 0.05, Duncan test). Compared with T13, the CLA in T2, T6, T9, and T10 increased significantly at the filling stage. In addition, POLY-containing composite fertilizers enhanced the GLN at filling, and increased MPH and MSD at filling under N1–N3 levels. They also significantly improved CLA at maturity under N2 and N4 levels. At maturity, the CLA of T10 was the lowest, whereas that of T4 was the highest. In addition, treatments with POLY composite fertilizer produced relatively large CLA at maturity under N1, N2, and N4 levels. The KNPC ranged from 476.80 to 618.40, with the lowest in T13 and the highest in T4. In the case of 15-15-15 (MOP)(POLY 25%), the KNPC first increased and then decreased with the increase in the amount of N fertilizer.
In brief, the findings for MPH and GLN at maturity showed opposite trends. POLY application increased YLN and promoted MSD at maturity. In summary, N fertilizer application did not significantly affect GLN, MPH, or MSD at the filling stage, nor CLA, YLN, GLN, MPH, or MSD at the maturity stage.

3.3. Effect of Compound K at Different N Levels on N Fertilizer Utilization

Fertilizer utilization efficiency is determined by the crop’s nutrient uptake capacity and the nutrient supply from both soil and fertilizer [83]. As shown in Table 3, market 15-15-15 (MOP) achieved relatively high NAE, NFPF, and AUE at the N1, N1, and N3 levels with values of 1.32, 51.43, and 1.03, respectively. 15-15-15 (MOP)(POLY25%) achieved relatively high NAE, NFPF, and AUE of 2.89, 51.78, and 1.07 at the N2, N1, and N2 levels, respectively. Notably, 25-7-8 (MOP)(POLY26%) at the N1 level attained the highest values across all indicators, with NAE, PFPN, and AUE reaching 5.78, 55.88, and 1.12, respectively, showing significant differences compared with other treatments (p < 0.05, Duncan test). These results showed that N application in this growing season could improve economic, environmental, and agronomic efficiency. The use of reduced N fertilizer not only increases yield but also helps lower production costs and reduces environmental impacts [29,58]. The findings indicated that low N rates combined with compound fertilizers can adequately support maize growth, likely due to balanced nutrient supply and controlled release mechanisms [58].
Specifically, applying 25-7-8 (MOP)(POLY 26%) at the N1 level significantly enhanced NAE, PFPN, and AUE. Under the same conditions of 25-7-8 (MOP)(POLY 26%) fertilizer application, the NAE of the N1 gradient significantly increased by 3.86, 7.26, and 16.18 times compared with N2, N3, and N4, respectively. Similarly, PFPN increased by 27.64%, 48.09%, and 68.84%, and AUE increased by 8.50%, 9.46%, and 10.39%. Moreover, at the N1 rate, 25-7-8 (MOP)(POLY26%) treatment increased NAE by 337% and 246% compared with market 15-15-15 (MOP) and 15-15-15 (MOP)(POLY25%), respectively. PFPN and AUE also improved by 8.66% and 7.93%, respectively, relative to the two control treatments. This study provides valuable insights for advancing precision nutrient management, mitigating agricultural pollution, and promoting resource-efficient and environmentally sustainable farming practices.

3.4. Effect of Compound K at Different N Levels on Elements in Maize Grain and Stalk Nutrients

3.4.1. Effect of Compound K at Different N Levels on Elements in Maize Grain Nutrients

To further investigate the effects of compound fertilizer application on the absorption of nutrients in maize grain, this study systematically analyzed the differences in the contents of macroelements (N, P, and K) and microelements (Ca, Mg, S, Fe, Mn, and Zn) in maize grain under different fertilizer modes. The results will provide theoretical bases for the directional control of maize grain’s nutrient quality and the development of functional food products.
Nutrient uptake in maize grain is shown in Figure 2. The total N content of 13.32 g/kg in grain of T6 was significantly higher than that of 12.77 g/kg in T1 (p < 0.05, Duncan test). Market 15-15-15 (MOP), 15-15-15 (MOP)(POLY25%), and 25-7-8 (MOP)(POLY26%) had the highest content of total N in the grain at 12.96, 13.32, and 13.00 g/kg under N1, N2, and N2, respectively. For compound K fertilizer, application of N fertilizer at the N1 or N2 level could meet the efficient use of N in maize grain (Figure 2a). At the N1, N2, and N3 levels, 25-7-8 (MOP)(POLY26%) resulted in the highest P utilization by the seeds, followed by 15-15-15 (MOP)(POLY25%). Market 15-15-15 (MOP), 15-15-15 (MOP)(POLY25%), and 25-7-8 (MOP)(POLY26%) had the highest total P content of 3.67, 3.58, and 3.50 g/kg in grain under N4, N4, and N3, respectively (Figure 2b). T1 had the lowest total K content of 1.93 g/kg in grain, and the rest of the treatments had significantly higher total K content than T1, indicating that the increased application of compound fertilizer was effective in promoting K uptake in maize grain, which was in agreement with the findings of Tiwari et al. [16]. 25-7-8 (MOP)(POLY 26%) resulted in the highest utilization of total K in maize grain at the N2 and N3 levels. Market 15-15-15 (MOP), 15-15-15 (MOP)(POLY25%), and 25-7-8 (MOP)(POLY26%) had the highest content of total K in the grain at 5.03, 4.81, and 4.88 g/kg under N4, N3, and N3, respectively (Figure 2c).
T1 had the lowest total Ca content of 68.89 mg/kg in grain, and the total Ca content of the remaining treatments was significantly higher than that of T1, indicating that the increased application of compound fertilizer could effectively promote the absorption of Ca in maize grain. Market 15-15-15 (MOP), 15-15-15 (MOP)(POLY25%), and 25-7-8 (MOP)(POLY26%) had the highest content of total Ca in the grain at 226.46, 219.29, and 187.01 mg/kg under N4, N4, and N3, respectively (Figure 2d). T1 had the lowest total Mg content of 0.85 g/kg in the grain and was significantly higher than T1, except for T2, T4, and T5 (p < 0.05, Duncan test). At the N2, N3, and N4 levels, 25-7-8 (MOP)(26% POLY) resulted in the highest utilization of Mg in maize grain. Moreover, market 15-15-15 (MOP), 15-15-15 (MOP)(POLY25%), and 25-7-8 (MOP)(POLY26%) all had the highest total Mg contents in the grain at the N4 level, measuring 1.25, 1.28, and 1.33 g/kg, respectively (Figure 2e). T1 had the lowest content of 2.19 g/kg of total S in grain, and T13 had the highest content of 3.70 g/kg of total S in grain. The total S content was significantly higher than PK control in all treatments, except T2, T4, and T5, which was consistent with the pattern of total Mg content in grain. Market 15-15-15 (MOP), 15-15-15 (MOP)(POLY25%), and 25-7-8 (MOP)(POLY26%) all had the highest total S contents in grain at N4, measuring 3.55, 3.43, and 3.70 g/kg, respectively. All three fertilizers resulted in the highest S content in the grain of maize with the increase in N application. The S content in maize grain increased gradually with the increase in N application of all three fertilizers (Figure 2f).
The Fe content in the grain of the compound fertilizers increased gradually with the increase in N application rate, and the contents of total Fe in the grain were the highest under N4, measuring 36.22, 41.83, and 57.55 mg/kg. At the N1 and N2 levels, 15-15-15 (MOP)(POLY25%) resulted in the highest uptake of Fe from the grain, whereas 25-7-8 (MOP)(POLY26%) resulted in the highest utilization of Fe from the grain at the N3 and N4 levels (Figure 3a). Market 15-15-15 (MOP) showed an increase in Mn content in grain with increasing N application, and the remaining two composite fertilizers containing POLY showed an increasing and then decreasing trend in Mn content in grain with increasing N application. Market 15-15-15 (MOP), 15-15-15 (MOP)(POLY25%), and 25-7-8 (MOP)(POLY26%) had the highest content of total Mn in the grain at N4, N3, and N3, measuring 65.25, 77.16, and 82.82 mg/kg, respectively (Figure 3b). The highest Zn utilization at the N4 level was observed in composite fertilizers containing POLY. Moreover, market 15-15-15 (MOP), 15-15-15 (MOP)(POLY25%), and 25-7-8 (MOP)(POLY26%) had the highest total Zn content of 8.84, 8.00, and 6.72 mg/kg in the grain at N2, N2, and N4, respectively (Figure 3c).

3.4.2. Effect of Compound K at Different N Levels on Elements in Maize Stalk Nutrients

To further investigate the effects of compound fertilizer application on the absorption of nutrients in maize stalk, this experiment systematically elaborated the differences in the absorption of macroelements (N, P, and K) and microelements (Ca, Mg, S, Fe, Mn, and Zn) in maize stalk under different fertilizer application modes. This study can offer theoretical support for the resourceful utilization of stalk and precise fertilizer application.
Maize stalk nutrient uptake is shown in Figure 4. The N content in maize stalk increased gradually with increasing N application and was highest in T13 with 12.44 g/kg stalk (Figure 4a). At the same N gradient, maize stalk under 25-7-8 (MOP)(POLY26%) application had the highest total P content, and the total P content of 1.25 g/kg in T10 was significantly higher than that of 0.91 g/kg in T1, which indicated that 25-7-8 (MOP)(POLY26%) at N3 could significantly increase the P content in stalk (Figure 4b). T1 had the lowest stalk total K content of 2.16 g/kg, and all the treatments, except T13, had significantly elevated total K contents. T2 had the highest K content in maize stalk, indicating that market 15-15-15 (MOP) at N1 promoted K uptake and utilization by maize stalk (Figure 4c).
T1 had the lowest content of 1.50 g/kg of total Ca in maize stalk, and T13 had the highest content of 4.21 g/kg of total Ca in maize stalk. Market 15-15-15 (MOP), 15-15-15 (MOP)(POLY25%), and 25-7-8 (MOP)(POLY26%) were each higher than the other N levels at 2.98, 4.04, and 4.21 g/kg of total Ca, respectively. In addition, Ca uptake and utilization by 25-7-8 (MOP)(POLY26%) was always highest at the N2, N3, and N4 levels (Figure 4d). At the N4 level, market 15-15-15 (MOP), 15-15-15 (MOP)(POLY25%), and 25-7-8 (MOP)(POLY26%) were each higher in total Mg than the other N levels at 0.37, 0.45, and 0.54 g/kg, respectively. The total Mg content of maize stalk at the N4 level was significantly higher in these three K fertilizers than in T1 (p < 0.05, Duncan test). In addition, Mg uptake and utilization by 25-7-8 (MOP)(POLY 26%) was always the highest at all four N levels (Figure 4e). At the N3 level, the total S content of maize stalk under market 15-15-15 (MOP), 15-15-15 (MOP)(POLY25%), and 25-7-8 (MOP)(POLY26%) was significantly higher than that of T1, which measured 2.61, 2.21, and 2.23 g/kg, respectively. Thus, the application of 230 kg/ha N significantly promoted the S utilization of maize stalk (Figure 4f).
Market 15-15-15 (MOP), 15-15-15 (MOP)(POLY25%), and 25-7-8 (MOP)(POLY26%) had the highest concentrations of total Fe in stalk under N3, N2, and N3 at 18.39, 9.32, and 13.99 mg/kg, respectively. 25-7-8 (MOP)(POLY 26%) promoted the highest Fe uptake from maize stalk at the N2 and N4 levels, whereas market 15-15-15 (MOP) promoted the highest Fe uptake from maize stalk at the N1 and N3 levels (Figure 5a). Market 15-15-15 (MOP), 15-15-15 (MOP)(POLY25%), and 25-7-8 (MOP)(POLY26%) had the highest concentrations of total Mn in stalk under N3, N4, and N3 at 15.05, 12.09, and 13.49 mg/kg, respectively. 25-7-8 (MOP)(POLY26%) promoted Mn uptake by maize stalk at the N1, N2, and N4 levels (Figure 5b). The Zn content of 27.89 mg/kg in the stalk of T1 was the highest. Market 15-15-15 (MOP), 15-15-15 (MOP)(POLY25%), and 25-7-8 (MOP)(POLY26%) had the highest levels of total Zn in stalk under N3, N1, and N1, measuring 27.16, 27.59, and 27.50 mg/kg, respectively (Figure 5c).

3.5. Effect of Compound K at Different N Levels on Soil pH, EC, and Nutrients

To further investigate the mechanism by which compound fertilizer application affects soil nutrition, the experiment systematically analyzed the basic physicochemical properties of soil under different fertilizer application modes. The variations in pH, EC, alkaline dissolved nitrogen, and other indicators were examined to establish a theoretical foundation for soil improvement and precise nutrient management.
For soil pH (Figure 6a), all post-test measurements exceeded pre-test values. The pH of T4 was significantly lower than that of T1, before and after testing. For soil EC (Figure 6b), the post-testing soil EC was higher than that of market 15-15-15 (MOP) for all the treatments utilizing complex fertilizers containing POLY, except T3. From Figure 6c, the comparison of the alkali-hydrolyzable nitrogen content in the soil before and after each treatment revealed that T3 exhibited the most significant increase, with a rise of 16.04% compared with pre-experimental measurements. However, with the exception of T1, T3, T9, and T10, alkali-hydrolyzable nitrogen decreased in all treatments. From Figure 6c, compared with pre-experiment levels, the available K content in the soil of each treatment decreased, but the reduction in the available K content in the soil of T13 was the largest, decreasing by 31.37% compared with before the experiment. The reduction in available K content in the soil of T6 was the smallest, decreasing by 16.97% compared with before the experiment. These results indicated that the reduced levels of available K content after the experiment than before the experiment may be caused by a combination of crop uptake and the inter-root environment. Except for T12 and T13, all the treatments applying a compound fertilizer containing POLY resulted in relatively high levels of available K in the soil after testing.
As shown in Figure 6d, the highest P content of 21.22 mg/kg was observed in T4 before the experiment. A comparison of the available P content in the soil before and after the test of each treatment revealed that T11 exhibited the most significant increase, with a rise of 46.44% relative to the pre-experiment levels. However, most of the available P content was reduced because K enhances P transport, which promotes photosynthesis and the production of carbohydrates in the crop, thereby increasing the productivity and quality of the produce [84,85]. From Figure 6d, a comparison of the available S content in the soil before and after the test of each treatment demonstrated that the decrease in available S content in soil in T9 was the highest, with a decrease of 13.89% relative to pre-experiment levels. The available S content of the soil increased in all treatments, except for T8, T9, T12, and T13, where it decreased.
As shown in Figure 6e, the exchangeable Ca content in soil decreased across all treatments when comparing pre- and post-test values. Notably, the reduction in exchangeable Ca content for T1 was the least, with a decline of 15.07% compared with the initial measurements. The soil in T12 had the highest value, which decreased by 58.74% compared with before the experiment. Thus, the application of N fertilizer enhanced the accumulation of Ca by maize, which resulted in reduced exchangeable Ca levels in the soil after testing [86]. This study revealed that all but three treatments under the N2 level of application of complex fertilizers containing POLY resulted in relatively elevated soil exchangeable Ca content after testing. From Figure 6f, a comparison of the exchangeable Mg content in the soil before and after the test of each treatment revealed that only the exchangeable Mg content in the soil of T9 decreased, exhibiting a reduction of 21.83% compared with the pre-experiment levels. The exchangeable Mg content in the soil of other treatments increased. The literature revealed that the uptake of K and Ca by plants inhibits the uptake of Mg [6]. POLY is rich in MgSO4, which can increase the amount of exchangeable Mg in the soil.

4. Discussion

4.1. Maize Yield and Growth

In this study, increased N application generally raised yield compared with the PK control. The highest significant yields occurred at the N1 (170 kg/ha) and N2 (200 kg/ha) levels. Therefore, N1 and N2 were identified as the optimum N application rates in this study. This agrees with reports by Ren et al. [52] and Han et al. [87] that increased N application significantly enhances yield. Furthermore, this study demonstrated that a moderate increase in N elevated GLN at both filling and maturity stages and enhanced MPH at the filling stage. This finding agrees with Zhai et al. [88], who confirmed that moderate N enhances plant height and improves stunting resistance. In contrast, this study found that excessive N application reduced CLA at the filling stage and increased YLN during both the filling and maturity stages. Furthermore, higher N levels did not enhance CLA at maturity and even reduced MPH. They are also consistent with Liu et al. [89] and Gao et al. [90], who demonstrated that excess N delays maturity and inhibits N transfer from nutrient organs to seeds, adversely affecting crop production. In addition to adversely affecting crop growth itself, excessive N application can also trigger serious environmental problems. It promotes eutrophication [91] and increases emissions of nitrous oxide, a potent greenhouse gas [92]. These impacts underscore the importance of optimizing N management to enhance agronomic efficiency and support global environmental sustainability.
The POLY applied in this study matched the composition of soluble POLY from the Upper Yangzi region [11], supporting its potential utilization in Chinese agriculture. The results revealed that the POLY dosage significantly contributed to the increase in yield under identical conditions of N application. Singh et al. [17] and Dal Molin et al. [93] also found that the application of POLY significantly increases the maize yield. The study further revealed that POLY-containing fertilizers increased CLA at maturity and MSD. They also significantly raised HGW and GWPE, and improved GLN during the filling stage. Additionally, POLY enhanced MPH and MSD during filling at N1–N3 levels and increased CLA at maturity under N2 and N4 regimes. At the N3 and N4 levels, POLY fertilizers raised GLN but reduced MPH at maturity. Although the POLY application increased YLN, it promoted MSD growth. Overall, these results underscore the role of POLY in improving maize growth traits. Bejarano Herrera et al. [94] and da Costa Mello et al. [24] indicated that the application of POLY effectively improves the agronomic traits of sugarcane and tomato crops.

4.2. N Fertilizer Utilization

Optimizing soil nutrient management requires a thorough understanding of N fertilizer effects, as NUE varies significantly across soil types and management practices [95]. Reyes-Matamoros et al. [96] reported that there is an interaction between N application rates and maize varieties in Mexico. They also proposed that optimizing N application rates tailored to different maize varieties is crucial for enhancing NUE. This study revealed significant differences in NAE, PFPN, and AUE under different N levels. 25-7-8 (MOP)(POLY26%) at the N1 rate achieved the highest values in these efficiency indices.
Similarly, Ren et al. [52] and Xu et al. [97] highlighted that moderate N application in China improves N utilization and reduces environmental pollution. Violeta Mandić et al. [54] emphasized that appropriate N rates in Europe enhance NUE, mitigate environmental N losses, and promote sustainable agriculture. Mondal et al. [53] reported that in India, the agronomic efficiency of nitrogen (AEN) initially increases with higher N fertilizer application but eventually decreases under excessive N input. Consequently, they proposed that applying optimal N fertilizer rates could enhance AEN. Conversely, excessive N application reduces N fertilizer efficiency and increases the risk of N loss. This aligns with the findings of Ma et al. [98], who reported that excessive N fertilizer application increases alkaline cation levels in the soil, leading to elevated soil salinity. This subsequently inhibits root N uptake and ultimately impairs crop growth and yield. Therefore, improving NUE is essential for promoting sustainable crop production and mitigating the negative impacts of nitrogen losses [99].

4.3. Maize Nutrient Uptake

4.3.1. Grain Nutrient Uptake

The optimal supply of N is crucial for plant growth and significantly enhances nutrient content in maize grain and stalk [100,101]. This study demonstrated that appropriate N application promoted N uptake in maize grains. Increased N fertilizer also enhanced the uptake of total P, K, Ca, Mg, S, Fe, and Mn. Notably, Mn uptake was optimized at the N2 application level. These findings are partially consistent with Huang et al. [100], who reported that N application increases N, Ca, Fe, Cu, Zn, and B mass fractions in maize grain, while Mg and Mn mass fractions are less affected by the amount of N applied. Similarly, Wyszkowski and Brodowska [102] found that N fertilization leads to increased contents of Mn and Fe; increased Fe: Zn and Fe: Mn ratios; and decreased contents of cadmium, lead, nickel, and cobalt in maize. Du et al. [103] demonstrated that rational N application promotes leaf growth and enhances N translocation efficiency in maize, thereby increasing grain N concentration and yield. Jiang [104] found that the contents of N, P, and K in maize grain initially increased and then decreased with increasing N application. Javed et al. [105] found that optimal NH4+ application reduced sodium uptake and enhanced K accumulation in maize. They also proposed that balanced N fertilization could suppress Na and Cl absorption while facilitating the acquisition of K, Ca, and Mg. However, the study also found that excessive N application reduced N and Zn uptake in grain. Han et al. [106] discovered that the Zn and Fe contents in maize grain with excessive N application no longer increased or even showed a decreasing trend.
Furthermore, the study revealed that POLY-containing composite fertilizers significantly improved the uptake and utilization of multiple nutrients in both grain and stalk at the same N level. The highest uptake of P at N1–N3 levels and the maximum Fe and Mn uptake across all N levels were achieved with POLY-based fertilizers. These results align with Pramanick et al. [15], who also reported enhanced macro- and micronutrient uptake in Indian mustard following POLY application.

4.3.2. Stalk Nutrient Uptake

Guo et al. [107] demonstrated that N supply could affect ion homeostasis in maize. The study found that increased N application elevated the contents of total N, Mg, and S in maize stalk, all of which were significantly higher than those in the PK control. These results indicated that fertilization at N3 and N4 levels significantly promoted the utilization of N, Mg, and S, thereby improving maize quality. Consistent with these findings, Zhang et al. [108] found that total N uptake increased with increasing N application. Cao et al. [109] also found a highly significant correlation between N and S uptake accumulation in maize plants. Additionally, the study demonstrated that Mg and Ca contents in cob leaves increased with N dosage, which may be a result of Ca and Mg dissolution and improved crop growth [110,111]. Furthermore, the study demonstrated that N uptake promoted Ca uptake by the stalk, showing a synergistic effect. Conversely, insufficient N inhibited Ca uptake, reflecting an antagonistic effect. These observations align with Grzebisz et al. [112], who reported that Ca levels in the stems of seed plants are consistent with increasing N levels, and inadequate N fertilizer leads to reduced Ca and Mg requirements for plants.
What is more, the study revealed that POLY-containing composite fertilizers led to the highest uptake of P and Zn at N1, N2, and N4 levels. They also exhibited the highest Fe uptake at the N2 and N4 levels, and the highest Mn uptake at the N1 and N4 levels. Notably, the treatment with 25-7-8 (MOP)(POLY26%) showed optimal uptake and utilization of Ca, Mg, and S across all N levels. Overall, POLY-based fertilizers enhanced a wide range of mineral elements uptake and utilization in maize, especially in combination with N fertilizer, which was consistent with the findings of Lillywhite et al. [113].

4.4. Soil Quality and Nutrient Content

The study found that compound fertilizers containing POLY significantly increased soil pH and delayed acidification processes. This effect is attributed to the introduction of alkaline ions through POLY application, which effectively mitigated soil acidification [114]. Furthermore, POLY releases substantial amounts of CaSO4, MgSO4, and sulfides [115], while SO42− strongly displaces OH− groups on soil surfaces [116], collectively contributing to the reduction in soil acidity. Moreover, the study indicated that most POLY treatments yielded higher EC after planting compared with 15-15-15 (MOP), further validating their characteristic delayed nutrient release. Except for T12 and T13, all POLY-amended treatments increased soil available K, confirming its strong K release capacity. These observations align with Lewis et al. [14], who reported that the sustained nutrient release capability of POLY, especially its K-releasing property, contributes notably to soil nutrient supply. What is more, the study revealed that POLY-containing fertilizers substantially elevated the content of exchangeable Ca, Mg, as well as available S in the soil. These findings further demonstrate POLY’s role in mitigating fertilizer-induced soil acidification, thereby supporting sustainable agricultural practices through enhanced long-term soil health, reduced reliance on chemical amendments, and minimized environmental impacts.
However, application of POLY-containing compound fertilizers on alkaline soils warrants careful consideration. While the efficacy of POLY may be higher in acidic and neutral soils, this hypothesis necessitates further experimental validation. Beyond agronomic effects, key limitations of POLY must be addressed prior to large-scale use. These include inconsistent solubility, regional scarcity, and higher costs relative to conventional fertilizers. Such constraints may limit practical deployment and economic feasibility, especially in resource-limited farming systems. A full assessment of POLY-based fertilization should incorporate agronomic, economic, and environmental dimensions to accurately determine its potential for sustainable nutrient management.

5. Conclusions

This study revealed that market 15-15-15 (MOP), 15-15-15 (MOP)(POLY25%), and 25-7-8 (MOP)(POLY26%) had the highest yield increase at N3, N2, and N1, respectively, while meeting agronomic efficacy criteria. The application of 170 kg/ha N with 25-7-8 (MOP)(POLY26%) achieved maximum yield and N fertilizer utilization. In addition, moderate N fertilization combined with POLY significantly improved maize growth, agronomic traits, and yield components at maturity. In contrast, excessive N increased YLN and reduced MPH, highlighting the need for optimized N application. Moreover, different fertilization treatments significantly influenced nutrient uptake in maize stalks and grains, with appropriate N supplementation (particularly under low-N conditions) effectively enhancing grain nutritional quality. In addition, the application of POLY-containing fertilizer significantly influenced soil pH, EC, and nutrient content. It slowed down soil acidification and demonstrated potential for sustained nutrient release. Overall, these findings indicate that POLY fertilizers with reduced N input enhance productivity and sustainability by saving fertilizer and improving soil. However, this study is limited to a single growing season and lacks molecular-level mechanistic elucidation. Future work should incorporate multi-year trials, microbial interactions, and lifecycle assessments to better understand POLY’s long-term efficacy and environmental impact.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su17198827/s1, Table S1: Temperature and weather conditions during the experimental period; Table S2: Comparison of maize agronomic traits under different treatments.

Author Contributions

Conceptualization, X.L. and J.D.; methodology, X.L., S.L., L.B. and J.D.; software, X.L. and L.B.; validation, X.L. and J.D.; formal analysis, X.L., S.L. and L.B.; investigation, R.J.; resources, R.J., J.G. and J.D.; data curation, X.L. and J.D.; writing—original draft preparation, X.L.; writing—review and editing, R.J., J.G., S.L., L.B. and J.D.; visualization, X.L.; supervision, X.L., L.B. and J.D.; project administration, J.D.; funding acquisition, J.D. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Anglo American Project of United Kingdom, grant number 149000-SDU-149013-23; the Youth Fund from Natural Science Foundation of Shandong Province, grant number ZR2023QD015; the Intramural Joint Program Fund of State Key Laboratory of Microbial Technology, grant number SKLMTIJP-2024-06; the Rural Revitalisation Project of Shandong Province, grant number SDXCZX202511-02; the Postdoctoral Applied Research Project of Qingdao, grant number QDBSH20250102005.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Acknowledgments

Sincere thanks to Xiaohui Fan for his guidance and Anglo American Woodsmith Limited for providing the fertilizer.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Histogram of yield of different treatments. Note: T1: PK Control (N0), T2: Market 15-15-15 (MOP) at N1 rate, T3: 15-15-15 (MOP)(POLY25%) at N1 rate, T4: 25-7-8 (MOP)(POLY26%) at N1 rate, T5: Market 15-15-15 (MOP) at N2 rate, T6: 15-15-15 (MOP)(POLY25%) at N2 rate, T7: 25-7-8 (MOP)(POLY26%) at N2 rate, T8: Market 15-15-15 (MOP) at N3 rate, T9: 15-15-15 (MOP)(POLY25%) at N3 rate, T10: 25-7-8 (MOP)(POLY26%) at N3 rate, T11: Market 15-15-15 (MOP) at N4 rate, T12: 15-15-15 (MOP)(POLY25%) at N4 rate, T13: 25-7-8 (MOP)(POLY26%) at N4 rate (p < 0.05, Duncan test) in (a). N0 is T1, N1 includes T2–T4, N2 includes T5–T7, N3 includes T8–T10, and N4 includes T11–T13 in (b). The lowercase letters in (a) represent significant differences between the 13 treatments, the red line in (a) represents the variation trend in yield, and the uppercase letters in (b) represent significant differences between the 4 groups.
Figure 1. Histogram of yield of different treatments. Note: T1: PK Control (N0), T2: Market 15-15-15 (MOP) at N1 rate, T3: 15-15-15 (MOP)(POLY25%) at N1 rate, T4: 25-7-8 (MOP)(POLY26%) at N1 rate, T5: Market 15-15-15 (MOP) at N2 rate, T6: 15-15-15 (MOP)(POLY25%) at N2 rate, T7: 25-7-8 (MOP)(POLY26%) at N2 rate, T8: Market 15-15-15 (MOP) at N3 rate, T9: 15-15-15 (MOP)(POLY25%) at N3 rate, T10: 25-7-8 (MOP)(POLY26%) at N3 rate, T11: Market 15-15-15 (MOP) at N4 rate, T12: 15-15-15 (MOP)(POLY25%) at N4 rate, T13: 25-7-8 (MOP)(POLY26%) at N4 rate (p < 0.05, Duncan test) in (a). N0 is T1, N1 includes T2–T4, N2 includes T5–T7, N3 includes T8–T10, and N4 includes T11–T13 in (b). The lowercase letters in (a) represent significant differences between the 13 treatments, the red line in (a) represents the variation trend in yield, and the uppercase letters in (b) represent significant differences between the 4 groups.
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Figure 2. Stacked plots of macronutrient content of grain from different treatments. Note: T1: PK Control (N0), T2: Market 15-15-15 (MOP) at N1 rate, T3: 15-15-15 (MOP)(POLY25%) at N1 rate, T4: 25-7-8 (MOP)(POLY26%) at N1 rate, T5: Market 15-15-15 (MOP) at N2 rate, T6: 15-15-15 (MOP)(POLY25%) at N2 rate, T7: 25-7-8 (MOP)(POLY26%) at N2 rate, T8: Market 15-15-15 (MOP) at N3 rate, T9: 15-15-15 (MOP)(POLY25%) at N3 rate, T10: 25-7-8 (MOP)(POLY26%) at N3 rate, T11: Market 15-15-15 (MOP) at N4 rate, T12: 15-15-15 (MOP)(POLY25%) at N4 rate, T13: 25-7-8 (MOP)(POLY26%) at N4 rate (p < 0.05, Duncan test) in (a–f). Among them, (a) represents total N, (b) represents total P, (c) represents total K, (d) represents total Ca, (e) represents total Mg, and (f) represents total S. All lowercase letters represent significant differences between the 13 treatments.
Figure 2. Stacked plots of macronutrient content of grain from different treatments. Note: T1: PK Control (N0), T2: Market 15-15-15 (MOP) at N1 rate, T3: 15-15-15 (MOP)(POLY25%) at N1 rate, T4: 25-7-8 (MOP)(POLY26%) at N1 rate, T5: Market 15-15-15 (MOP) at N2 rate, T6: 15-15-15 (MOP)(POLY25%) at N2 rate, T7: 25-7-8 (MOP)(POLY26%) at N2 rate, T8: Market 15-15-15 (MOP) at N3 rate, T9: 15-15-15 (MOP)(POLY25%) at N3 rate, T10: 25-7-8 (MOP)(POLY26%) at N3 rate, T11: Market 15-15-15 (MOP) at N4 rate, T12: 15-15-15 (MOP)(POLY25%) at N4 rate, T13: 25-7-8 (MOP)(POLY26%) at N4 rate (p < 0.05, Duncan test) in (a–f). Among them, (a) represents total N, (b) represents total P, (c) represents total K, (d) represents total Ca, (e) represents total Mg, and (f) represents total S. All lowercase letters represent significant differences between the 13 treatments.
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Figure 3. Stacked plots of micronutrient content of grain from different treatments. Note: T1: PK Control (N0), T2: Market 15-15-15 (MOP) at N1 rate, T3: 15-15-15 (MOP)(POLY25%) at N1 rate, T4: 25-7-8 (MOP)(POLY26%) at N1 rate, T5: Market 15-15-15 (MOP) at N2 rate, T6: 15-15-15 (MOP)(POLY25%) at N2 rate, T7: 25-7-8 (MOP)(POLY26%) at N2 rate, T8: Market 15-15-15 (MOP) at N3 rate, T9: 15-15-15 (MOP)(POLY25%) at N3 rate, T10: 25-7-8 (MOP)(POLY26%) at N3 rate, T11: Market 15-15-15 (MOP) at N4 rate, T12: 15-15-15 (MOP)(POLY25%) at N4 rate, T13: 25-7-8 (MOP)(POLY26%) at N4 rate (p < 0.05, Duncan test) in (a–c). Among them, (a) represents total Fe, (b) represents total Mn, and (c) represents total Zn. All lowercase letters represent significant differences between the 13 treatments.
Figure 3. Stacked plots of micronutrient content of grain from different treatments. Note: T1: PK Control (N0), T2: Market 15-15-15 (MOP) at N1 rate, T3: 15-15-15 (MOP)(POLY25%) at N1 rate, T4: 25-7-8 (MOP)(POLY26%) at N1 rate, T5: Market 15-15-15 (MOP) at N2 rate, T6: 15-15-15 (MOP)(POLY25%) at N2 rate, T7: 25-7-8 (MOP)(POLY26%) at N2 rate, T8: Market 15-15-15 (MOP) at N3 rate, T9: 15-15-15 (MOP)(POLY25%) at N3 rate, T10: 25-7-8 (MOP)(POLY26%) at N3 rate, T11: Market 15-15-15 (MOP) at N4 rate, T12: 15-15-15 (MOP)(POLY25%) at N4 rate, T13: 25-7-8 (MOP)(POLY26%) at N4 rate (p < 0.05, Duncan test) in (a–c). Among them, (a) represents total Fe, (b) represents total Mn, and (c) represents total Zn. All lowercase letters represent significant differences between the 13 treatments.
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Figure 4. Stacked plots of macronutrient content of stalk from different treatments. Note: T1: PK Control (N0), T2: Market 15-15-15 (MOP) at N1 rate, T3: 15-15-15 (MOP)(POLY25%) at N1 rate, T4: 25-7-8 (MOP)(POLY26%) at N1 rate, T5: Market 15-15-15 (MOP) at N2 rate, T6: 15-15-15 (MOP)(POLY25%) at N2 rate, T7: 25-7-8 (MOP)(POLY26%) at N2 rate, T8: Market 15-15-15 (MOP) at N3 rate, T9: 15-15-15 (MOP)(POLY25%) at N3 rate, T10: 25-7-8 (MOP)(POLY26%) at N3 rate, T11: Market 15-15-15 (MOP) at N4 rate, T12: 15-15-15 (MOP)(POLY25%) at N4 rate, T13: 25-7-8 (MOP)(POLY26%) at N4 rate (p < 0.05, Duncan test) in Figure (a–f). Among them, (a) represents total N, (b) represents total P, (c) represents total K, (d) represents total Ca, (e) represents total Mg, and (f) represents total S. All lowercase letters represent significant differences between the 13 treatments.
Figure 4. Stacked plots of macronutrient content of stalk from different treatments. Note: T1: PK Control (N0), T2: Market 15-15-15 (MOP) at N1 rate, T3: 15-15-15 (MOP)(POLY25%) at N1 rate, T4: 25-7-8 (MOP)(POLY26%) at N1 rate, T5: Market 15-15-15 (MOP) at N2 rate, T6: 15-15-15 (MOP)(POLY25%) at N2 rate, T7: 25-7-8 (MOP)(POLY26%) at N2 rate, T8: Market 15-15-15 (MOP) at N3 rate, T9: 15-15-15 (MOP)(POLY25%) at N3 rate, T10: 25-7-8 (MOP)(POLY26%) at N3 rate, T11: Market 15-15-15 (MOP) at N4 rate, T12: 15-15-15 (MOP)(POLY25%) at N4 rate, T13: 25-7-8 (MOP)(POLY26%) at N4 rate (p < 0.05, Duncan test) in Figure (a–f). Among them, (a) represents total N, (b) represents total P, (c) represents total K, (d) represents total Ca, (e) represents total Mg, and (f) represents total S. All lowercase letters represent significant differences between the 13 treatments.
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Figure 5. Stacked plots of micronutrient content of stalk from different treatments. Note: T1: PK Control (N0), T2: Market 15-15-15 (MOP) at N1 rate, T3: 15-15-15 (MOP)(POLY25%) at N1 rate, T4: 25-7-8 (MOP)(POLY26%) at N1 rate, T5: Market 15-15-15 (MOP) at N2 rate, T6: 15-15-15 (MOP)(POLY25%) at N2 rate, T7: 25-7-8 (MOP)(POLY26%) at N2 rate, T8: Market 15-15-15 (MOP) at N3 rate, T9: 15-15-15 (MOP)(POLY25%) at N3 rate, T10: 25-7-8 (MOP)(POLY26%) at N3 rate, T11: Market 15-15-15 (MOP) at N4 rate, T12: 15-15-15 (MOP)(POLY25%) at N4 rate, T13: 25-7-8 (MOP)(POLY26%) at N4 rate (p < 0.05, Duncan test) in Figure (a–c). Among them, (a) represents total Fe, (b) represents total Mn, and (c) represents total Zn. All lowercase letters represent significant differences between the 13 treatments.
Figure 5. Stacked plots of micronutrient content of stalk from different treatments. Note: T1: PK Control (N0), T2: Market 15-15-15 (MOP) at N1 rate, T3: 15-15-15 (MOP)(POLY25%) at N1 rate, T4: 25-7-8 (MOP)(POLY26%) at N1 rate, T5: Market 15-15-15 (MOP) at N2 rate, T6: 15-15-15 (MOP)(POLY25%) at N2 rate, T7: 25-7-8 (MOP)(POLY26%) at N2 rate, T8: Market 15-15-15 (MOP) at N3 rate, T9: 15-15-15 (MOP)(POLY25%) at N3 rate, T10: 25-7-8 (MOP)(POLY26%) at N3 rate, T11: Market 15-15-15 (MOP) at N4 rate, T12: 15-15-15 (MOP)(POLY25%) at N4 rate, T13: 25-7-8 (MOP)(POLY26%) at N4 rate (p < 0.05, Duncan test) in Figure (a–c). Among them, (a) represents total Fe, (b) represents total Mn, and (c) represents total Zn. All lowercase letters represent significant differences between the 13 treatments.
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Figure 6. Pre- and post-planting soil nutrient content maps for different treatments. Note: T1: PK Control (N0), T2: Market 15-15-15 (MOP) at N1 rate, T3: 15-15-15 (MOP)(POLY25%) at N1 rate, T4: 25-7-8 (MOP)(POLY26%) at N1 rate, T5: Market 15-15-15 (MOP) at N2 rate, T6: 15-15-15 (MOP)(POLY25%) at N2 rate, T7: 25-7-8 (MOP)(POLY26%) at N2 rate, T8: Market 15-15-15 (MOP) at N3 rate, T9: 15-15-15 (MOP)(POLY25%) at N3 rate, T10: 25-7-8 (MOP)(POLY26%) at N3 rate, T11: Market 15-15-15 (MOP) at N4 rate, T12: 15-15-15 (MOP)(POLY25%) at N4 rate, T13: 25-7-8 (MOP)(POLY26%) at N4 rate (p < 0.05, Duncan test) in Figure (a–f). Among them, (a) represents pH, (b) represents EC, (c) represents alkali-hydrolyzable nitrogen and available K, (d) represents available P and available S, (e) represents exchangeable Ca, and (f) represents exchangeable Mg. All lowercase and uppercase letters represent significant differences between the 13 pre-planting and post-planting treatments, respectively.
Figure 6. Pre- and post-planting soil nutrient content maps for different treatments. Note: T1: PK Control (N0), T2: Market 15-15-15 (MOP) at N1 rate, T3: 15-15-15 (MOP)(POLY25%) at N1 rate, T4: 25-7-8 (MOP)(POLY26%) at N1 rate, T5: Market 15-15-15 (MOP) at N2 rate, T6: 15-15-15 (MOP)(POLY25%) at N2 rate, T7: 25-7-8 (MOP)(POLY26%) at N2 rate, T8: Market 15-15-15 (MOP) at N3 rate, T9: 15-15-15 (MOP)(POLY25%) at N3 rate, T10: 25-7-8 (MOP)(POLY26%) at N3 rate, T11: Market 15-15-15 (MOP) at N4 rate, T12: 15-15-15 (MOP)(POLY25%) at N4 rate, T13: 25-7-8 (MOP)(POLY26%) at N4 rate (p < 0.05, Duncan test) in Figure (a–f). Among them, (a) represents pH, (b) represents EC, (c) represents alkali-hydrolyzable nitrogen and available K, (d) represents available P and available S, (e) represents exchangeable Ca, and (f) represents exchangeable Mg. All lowercase and uppercase letters represent significant differences between the 13 pre-planting and post-planting treatments, respectively.
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Table 1. Basic physical and chemical properties of the soils in tested plots (n = 3).
Table 1. Basic physical and chemical properties of the soils in tested plots (n = 3).
ItemsValues
pH8.01 ± 0.04
EC (μs/cm)355.17 ± 24.00
Available N (mg/kg)134.17 ± 12.30
Available P (mg/kg)17.41 ± 0.49
Available K (mg/kg)96.99 ± 12.55
Available Ca (cmol/kg)32.70 ± 5.11
Available Mg (cmol/kg)3.32 ± 0.12
Available S (mg/kg)30.87 ± 2.29
Table 2. Fertilizer application of all treatments (kg/ha).
Table 2. Fertilizer application of all treatments (kg/ha).
TreatmentsComposition and Nutrient Ratios of Experimental FertilizerN
T1PK Control (N0)0
T2Market 15-15-15 (MOP) at N1 rate170
T315-15-15 (MOP)(POLY25%) at N1 rate170
T425-7-8 (MOP)(POLY26%) at N1 rate170
T5Market 15-15-15 (MOP) at N2 rate200
T615-15-15 (MOP)(POLY25%) at N2 rate200
T725-7-8 (MOP)(POLY26%) at N2 rate200
T8Market 15-15-15 (MOP) at N3 rate230
T915-15-15 (MOP)(POLY25%) at N3 rate230
T1025-7-8 (MOP)(POLY26%) at N3 rate230
T11Market 15-15-15 (MOP) at N4 rate260
T1215-15-15 (MOP)(POLY25%) at N4 rate260
T1325-7-8 (MOP)(POLY26%) at N4 rate260
Note: T1: PK Control (N0), T2: Market 15-15-15 (MOP) at N1 rate, T3: 15-15-15 (MOP)(POLY25%) at N1 rate, T4: 25-7-8 (MOP)(POLY26%) at N1 rate, T5: Market 15-15-15 (MOP) at N2 rate, T6: 15-15-15 (MOP)(POLY25%) at N2 rate, T7: 25-7-8 (MOP)(POLY26%) at N2 rate, T8: Market 15-15-15 (MOP) at N3 rate, T9: 15-15-15 (MOP)(POLY25%) at N3 rate, T10: 25-7-8 (MOP)(POLY26%) at N3 rate, T11: Market 15-15-15 (MOP) at N4 rate, T12: 15-15-15 (MOP)(POLY25%) at N4 rate, T13: 25-7-8 (MOP)(POLY26%) at N4 rate.
Table 3. The maize fertilizer use efficiency of the season relative to treatment.
Table 3. The maize fertilizer use efficiency of the season relative to treatment.
TreatmentsNAEPFPNAUE
T1———
T21.32 ± 2.09 b51.43 ± 2.09 b1.03 ± 0.04 b
T31.67 ± 2.97 b51.78 ± 2.97 b1.03 ± 0.06 b
T45.78 ± 1.21 a55.88 ± 1.21 a1.12 ± 0.02 a
T51.21 ± 1.16 b43.80 ± 1.16 c1.03 ± 0.03 b
T62.89 ± 1.00 b45.48 ± 1.00 c1.07 ± 0.02 ab
T71.19 ± 0.86 b43.78 ± 0.86 c1.03 ± 0.02 b
T81.27 ± 1.04 b38.30 ± 1.04 d1.03 ± 0.03 b
T90.91 ± 2.43 b37.95 ± 2.43 d1.02 ± 0.07 b
T100.70 ± 1.90 b37.73 ± 1.90 d1.02 ± 0.05 b
T110.87 ± 1.32 b33.63 ± 1.32 e1.03 ± 0.04 b
T120.52 ± 1.16 b33.28 ± 1.16 e1.02 ± 0.04 b
T130.34 ± 0.66 b33.10 ± 0.66 e1.01 ± 0.02 b
Note: NAE: Nitrogen Agronomic Efficiency, PFPN: Partial Factor Productivity of Nitrogen, AUE: Agronomic Use Efficiency. T1: PK Control (N0), T2: Market 15-15-15 (MOP) at N1 rate, T3: 15-15-15 (MOP)(POLY25%) at N1 rate, T4: 25-7-8 (MOP)(POLY26%) at N1 rate, T5: Market 15-15-15 (MOP) at N2 rate, T6: 15-15-15 (MOP)(POLY25%) at N2 rate, T7: 25-7-8 (MOP)(POLY26%) at N2 rate, T8: Market 15-15-15 (MOP) at N3 rate, T9: 15-15-15 (MOP)(POLY25%) at N3 rate, T10: 25-7-8 (MOP)(POLY26%) at N3 rate, T11: Market 15-15-15 (MOP) at N4 rate, T12: 15-15-15 (MOP)(POLY25%) at N4 rate, T13: 25-7-8 (MOP)(POLY26%) at N4 rate (p < 0.05, Duncan test). Where all lowercase letters represent significant differences between the 12 treatments.
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Li, X.; Jing, R.; Guo, J.; Li, S.; Bai, L.; Dai, J. Effects of Compound Fertilizer Containing Polyhalite on Soil and Maize Growth Under Different Nitrogen Levels. Sustainability 2025, 17, 8827. https://doi.org/10.3390/su17198827

AMA Style

Li X, Jing R, Guo J, Li S, Bai L, Dai J. Effects of Compound Fertilizer Containing Polyhalite on Soil and Maize Growth Under Different Nitrogen Levels. Sustainability. 2025; 17(19):8827. https://doi.org/10.3390/su17198827

Chicago/Turabian Style

Li, Xiaohan, Ruixue Jing, Jimin Guo, Shun Li, Liyong Bai, and Jiulan Dai. 2025. "Effects of Compound Fertilizer Containing Polyhalite on Soil and Maize Growth Under Different Nitrogen Levels" Sustainability 17, no. 19: 8827. https://doi.org/10.3390/su17198827

APA Style

Li, X., Jing, R., Guo, J., Li, S., Bai, L., & Dai, J. (2025). Effects of Compound Fertilizer Containing Polyhalite on Soil and Maize Growth Under Different Nitrogen Levels. Sustainability, 17(19), 8827. https://doi.org/10.3390/su17198827

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