Tobacco (Nicotiana tabacum L.), as a crucial economic crop, has generous benefits for farmers and the Chinese government, as its planting area and tobacco production play a vital role in the world (Ti et al., 2024). In 2022, the tobacco yield per unit area in China reached 2078.8 kg ha⁻¹, and the flue-cured tobacco planting area measured 1.0 million hectares, representing nearly half of global tobacco production (WNBS. PRC, 2022). However, improvements in tobacco yield and quality in China have been significantly limited in recent years due to potassium (K) deficiency in tobacco-planted soil and a comparatively high demand for K in tobacco plants (Hu et al., 2019, 2021; Lu et al., 2022). As a result, K₂SO₄ fertilizer, the main supplement for enhancing the soil K content and tobacco quality, is being overused by farmers in China, and which has caused great environmental harm (Arienzo et al., 2009; Zhang and Kong, 2014), affecting the soil mineral balance (Farrokh et al., 2012) and bacterial community (Song et al., 2019). Therefore, there is an urgent need to determine the optimal application amount of K₂SO₄ fertilizer to improve tobacco yield and quality, prevent soil health deterioration, and then achieve the sustainable and healthy development of the tobacco industry in China.

Due to its advantages of higher K content, good solubility, and low cost, K₂SO₄ fertilizer has become a vital component of tobacco planting recommended for farmers in China (Li et al., 2019). In general, a leaf K content above 2% is considered crucial for enhancing the processing properties of tobacco leaves and improving the burnability of cigarettes (Karbachsch, 1978). A higher K content can improve the photosynthetic performance of tobacco leaves, thus promoting dry matter accumulation and translocation (Andrés et al., 2014; Abbasi et al., 2016). K is also involved in osmoregulation, enzyme activation, and carbohydrate and protein formation (Fageria et al., 2001; Hu et al., 2019). However, there is no linear relationship between the amount of K₂SO₄ fertilizer applied and its effects on the K content of soil or tobacco leaves. By contrast, an excessive K content in soil resulting from K₂SO₄ fertilizer application often has an antagonistic effect on Ca²⁺, NH₄⁺, and Mg²⁺ absorption (Nieder et al., 2011; Lu et al., 2022). Meanwhile, excessive soil K content has also been confirmed to induce plant salt stress, and which lead to the destruction of plant photosynthesis and subsequent yield loss (Zhu et al., 2025). Delayed tobacco maturation and limited nicotine production have also been demonstrated under excessive K₂SO₄ fertilizer application (Vann et al., 2012).

Sulfur (S) is also an essential nutrient for tobacco growth, and it is mainly absorbed from the soil. It has been suggested that crop nutrient uptake generally increases with increasing soil availability. The S content in tobacco leaves is recommended to be below 0.67%, which is crucial for the combustion of cigarettes (Hu et al., 2020). Soil acidification has been confirmed after long-term K₂SO₄ fertilizer application when an excessive amount of SO₄²⁻ is applied to tobacco-planted soil (Xu and Ji, 2001). The Ca²⁺ and Mg²⁺ contents are also affected by soil residual SO₄²⁻ through the formation of CaSO₄ and MgSO₄ compounds (Jiang et al., 2024). Moreover, the bacterial community and functional pathways of tobacco-planted soil are destroyed when the K₂SO₄ fertilizer application amount is above the recommended level (Wang et al., 2021). Maintaining S levels within the optimal range in tobacco-planted soil is essential for tobacco yield and quality.

The balance between different soil mineral composition is crucial for improving plant growth. It is generally believed that antagonism and synergism are common in ion absorption (Bakker et al., 2013). Besides, the ratio of ion content, such carbon-to-nitrogen ratio, is also essential for enhancing plant yield and quality (Cheng et al., 2025). A higher ammonium (NH₄⁺) to K ratio can lead to soil acidification due to more H⁺ ions were released during the conversion of NH₄⁺ to nitrate (NO₃^–) in agricultural production (Jones et al., 2019). Meanwhile, the imbalance in plant nutrient uptake also leads to more severe soil acidification (Dong et al., 2021). An optimal nitrogen to potassium ratio has also been shown to promote the maturation of tobacco leaf and enhance its quality (Schwamberger and Sims, 1991; Hu et al., 2021).

The optimal soil pH for tobacco growth is between 5.5 and 6.5 (Dai et al., 2021; Zhang et al., 2016), and the deficiency of soil phosphorus (P) and K has been demonstrated when soil is acidified (Martins et al., 2014). Tang et al. (2020) further confirmed that total sugar, reducing sugar, and K content in tobacco were negatively correlated with soil pH. Wan et al. (2020) showed that bacterial community functions, including S cycling-related enzymes and proteins, were all downregulated in acidic soils. Moreover, pH is widely considered a dominant predictor of soil microbial activity, which not only affects tobacco growth but also influences disease occurrence (She et al., 2017; Wang et al., 2019). Improving pH in tobacco-planted soil has become an important prerequisite for optimizing soil and tobacco mineral composition.

K₂SO₄ fertilizer application is a common choice for farmers to address K deficiency in tobacco-planted soil. Although many advantages of applying K₂SO₄ fertilizer on soil structure, mineral composition, and tobacco growth have been verified (Vann et al., 2013; Zhang and Kong, 2014), the optimal application amount and matched-use technologies are still unclear in Shandong, especially in long-term fertilization experiments. Optimizing the K₂SO₄ fertilizer application amount is necessary to achieve healthy soil and high-quality tobacco leaves, and the application amount may vary among tobacco-growing regions. Farmers need an optimal recommended amount when applying K₂SO₄ fertilizer in the field.

We hypothesized that changing the K₂SO₄ fertilizer application amount would affect soil physicochemical properties and then impact tobacco tissue K content and dry matter accumulation. Therefore, the objectives of this study were (a) to quantify the effects of various K₂SO₄ fertilizer application amounts on soil pH, mineral composition, and soil-related enzyme activity and to compare the above values with the initial soil physicochemical properties at 10 years ago; (b) to investigate the relationship between tobacco total matter accumulation, tissue K content and soil physicochemical characteristics; and (c) to determine the optimal K₂SO₄ fertilizer application amount for both tobacco quality enhancement and soil health.

The physicochemical properties and microbial communities in soil which serve as the foundation for crop growth are affected by crop species, fertilizer application amount, and the local ecological environment (Eslaminejad et al., 2020; Shao et al., 2020; Zhang et al., 2023a). Shandong is located in northern China, and has become a representative tobacco-growing region in the country (Figure 1). The soil types and climate characteristics shows regional features. Besides, fertilizer application is widely recommended for tobacco production due to its many advantages. As a result, the long-term application of K₂SO₄ fertilizer in tobacco cultivation may lead to the unique soil physicochemical properties and diverse performance of dry matter accumulation in Shandong. Previous studies have confirmed the decline in tobacco-planted soil quality, especially soil acidification, is becoming increasingly urgent due to K₂SO₄ fertilizer application (Dai et al., 2021; Lu et al., 2022). Soil pH is recognized as a pivotal predictor of soil health, which not only affects soil microbial activity and composition but also regulates soil enzyme activity (Wan et al., 2020). In this study, we further confirmed that K₂SO₄ fertilizer application significantly reduced the soil pH, and found which was decreased by ~10% with the long-term K₂SO₄ fertilizer addition over the past 10 years (Figure 4). The possible reasons for this performance were mainly attributed to K₂SO₄ fertilizer application increasing the introduction of not only K⁺ but also SO₄²⁻ into the soil (Li et al., 2019). However, tobacco absorbs much more K⁺ than SO₄²⁻, resulting in the accumulation of residual SO₄²⁻ in soil. Therefore, in order to maintain charge balance, the tobacco roots secrete H⁺ to replace K⁺ (Wallace, 1994). Meanwhile, the residual SO₄²⁻ in soil forms precipitate with Ca²⁺ and Mg²⁺ and is leached from the soil, leading to a further reduction in the content of base ions. Besides, the significantly inhibited soil acid buffer capacity due to excessive K₂SO₄ fertilizer use may serve as another key reason (Dong et al., 2022). In the long term, excessive application of K₂SO₄ fertilizer to achieve favorable tobacco yield and quality is unsustainable from the perspective of soil health.

Most K uptake by plants comes from the soil, which is partially determined by the fertilizer application amount. Therefore, farmers often hold the misconception that more fertilizer application amount leads to higher yield and benefits in production. As a result, K₂SO₄ fertilizer has been overused for decades in tobacco cultivation (Lu et al., 2022; Zhang et al., 2025). However, excessive K nutrient beyond what plants can uptake will lead to some disadvantages, such as salt stress and soil nutrient imbalance (Vann et al., 2012; Zhu et al., 2025). Previous research on the regulation of soil physicochemical properties and microbial communities focuses on KCl fertilizer (Grant and Bailey, 1994). In this study, significant increases in soil AK and tobacco tissue K contents were confirmed after the long-term K₂SO₄ fertilizer application, and the initial soil AK when the experiment was designed increased by more than 100% under K₁₆₅+S_95.7 and K_247.5+S_125.4 (Figure 5). However, although soil AK was positively correlated with tobacco tissue K content, there was no linear relationship between the amount of K₂SO₄ fertilizer applied and tobacco leaf K content (Figure 9, Table 2). The K content in tobacco leaves no longer increased when the application amount of K₂SO₄ fertilizer exceeds K₁₆₅+S_95.7 (Table 2). By contrast, a large amount of residual K⁺ and SO₄²⁻ in soil due to excessive use of K₂SO₄ fertilizer becomes the main cause of soil acidification (Jakobsen, 1993). However, the soil AN content in our study is not consistent with that observed in previous studies, which showed a higher AN level when excessive K₂SO₄ fertilizer was applied (Lu et al., 2022; Figure 6). A possible explanation is that tobacco roots are not completely damaged when excessive K₂SO₄ fertilizer is used, enabling tobacco roots to absorb AN from soil. Moreover, the excellent transferability of nitrogen nutrition in soil also weakened the effects of K₂SO₄ fertilizer application amount on soil AN content.

Soil enzyme activity is a key factor affecting soil quality and is involved in the decomposition of organic matter, the cycling of nutrients, and the regulation of plants growth (Liu et al., 2021; Zhang et al., 2023b). Soil urease and acid phosphatase are mainly involved in the metabolism of soil nitrogen and the transformation of phosphorus, respectively (Cantarella et al., 2018). Meanwhile, soil arylsulfatase performs a crucial function in the transformation of soil available sulfur (Tabatabai and Bremner, 1970). Generally, the performance of soil enzyme activity is considered to be regulated by the soil physicochemical properties. In this study, the changes of urease activity were consistent with that of soil AN (Figures 6, 8). However, it is unfortunate that we did not further investigate the variations in soil phosphorus and available carbon in this study, and thus their relationship with activities of acid phosphatase and sucrase was not explored. Excessive use of K₂SO₄ fertilizer led more residual SO₄²⁻ in soil, which may disrupt S cycling-related process (Jiang et al., 2024). In this study, the activity of soil arylsulfatase was significantly decreased with increasing K₂SO₄ fertilizer application amount, and the main reason was attributed to the drop in soil pH caused by residual H⁺ and SO₄²⁻ (Chen et al., 2019). In addition, the differences in soil texture and microbial community caused by long-term K₂SO₄ fertilizer application were also key factors affecting soil arylsulfatase activity (Stott and Hagedorn, 1980).

As an essential nutrient for tobacco yield and quality, a higher K content not only favors tobacco dry matter accumulation but also improves the processing properties of tobacco leaves. Tobacco leaves are the main location of photosynthesis and metabolic activities and have the highest K content. By contrast, tobacco roots transport most of the absorbed K to the aboveground parts, resulting in the lowest K content in the entire plant (Zheng et al., 2020). Studies have also confirmed that excessive soil AK is not utilized by the plant but reduces yield and economic benefits (Daoud et al., 2020). The K₂SO₄ fertilizer application amount in tobacco cultivation should be within a reasonable range. In this study, we found that the K content in tobacco leaves was significantly higher than that in roots and stems, while the total K content in the tobacco plant, including roots, stems, and leaves, no longer increased significantly when K₂SO₄ fertilizer was applied at rates above those in K₁₆₅+S_95.7 (Table 2). The application amount in K₁₆₅+S_95.7 can meet the K demands of tobacco growth while avoiding soil damage. Due to the introduction of SO₄²⁻ and other elements under excessive K₂SO₄ fertilizer application, soil acidification was further aggravated (Figure 4), which may cause tobacco root damage or ion toxicity (Xu and Ji, 2001). Tobacco plants uptake nutrients from the soil are affected by the soil’s mineral composition. Soil Ca and Mg have been demonstrated to significantly impact tobacco dry matter accumulation (Jiang et al., 2024). In addition, soil pH has significant effects on the mineral composition of tobacco-planted soil as well as soil microbial activity and diversity, which further influence tobacco plant growth (Zeng et al., 2019). Our findings are in accordance with previous studies showing excessive K₂SO₄ fertilizer use may lead to the damage of tobacco root absorption capacity and ion stress, thus reducing tobacco dry matter accumulation (Table 3). Admittedly, the deeper mechanisms of matter accumulation during the tobacco field growth period need to be further investigated in the future from the perspectives of leaf photosynthetic characteristics and matter transformation.

Over the past few decades, fertilizers have played an irreplaceable role in improving crops yield and quality, and in addressing human hunger. However, this benefit is often a single direction that humans demand from the soil. Farmers always pursue higher yield simply by applying excessive amounts of fertilizer to the soil without paying attention to the effects of residual fertilizers on soil texture, microbial activity, and soil carbon pools. In recent years, a large area of soil is facing the risk of severe degradation in China. Therefore, the conduct of long-term fertilization experiment is of great significance for optimizing fertilizer application amount and investigating the diverse responses of soil to fertilizers, and the data is valuable and reliable. In the present study, the core role of pH in regulating soil AK and tobacco tissue K was highlighted (Figure 9). Moreover, we also found that the total dry matter of tobacco was significantly positively correlated with the K content in stem and soil NO₃^–_—N (Figure 9). The above findings provide representative indicators for directly assessing the effects of long-term fertilization application on soil and crops. Unfortunately, the interaction mechanism within soil physicochemical properties was not thoroughly investigated in this study, and the further research on the crosstalk between soil ions, enzyme activity, and microbial communities is urgent in our future research.

Applying K₂SO₄ fertilizer increased the soil NH4⁺–N and NO₃⁻–N contents and improved the AK level in tobacco-planted soil, further promoting the K content in tobacco roots, stems, and leaves and enhancing total dry matter accumulation. However, excessive K₂SO₄ fertilizer application decreased soil pH and arylsulfatase activity and led to soil acidification. In the long term, the initial soil AK and pH exhibited converse responses to the application of K₂SO₄ fertilizer, and soil pH played a dominant factor in affecting total dry matter and the K content in tobacco plants and tobacco-planted soil, showing a significant negative correlation. The feasible solution in Shandong was the mixtures of 550 kg ha⁻¹ compound fertilizer with 15% N, 15% P₂O₅, 15% K₂O, and 12% S and 165 kg ha⁻¹ K₂SO₄ fertilizer with 50% K₂O and 18% S. Our results propose the optimal application amount of K₂SO₄ fertilizer for maximizing soil buffering capacity and improving tobacco leaf quality through a long-term experiment, providing a reference for other tobacco-growing countries where soil AK is deficient.