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Article

Effects of Soil Physical and Chemical Properties on the Quality of Nanjing ‘Yuhua’ Tea, a Type of Famous Green Tea

by
Zemao Liu
1,
Deyu Yang
2,
Guangtao Zhang
1,
Lihong Zheng
1,
Chen Chen
3,
Xin Sun
1 and
Fangyuan Yu
1,*
1
Collaborative Innovation Centre of Sustainable Forestry in Southern China, College of Forest Science, Nanjing Forestry University, Nanjing 210000, China
2
Peking University Institute of Advanced Agricultural Sciences, Shandong Laboratory of Advanced Agricultural Sciences, Weifang 261325, China
3
School of Landscape and Horticulture, Yangzhou Polytechnic College, Yangzhou 225009, China
*
Author to whom correspondence should be addressed.
Horticulturae 2023, 9(2), 189; https://doi.org/10.3390/horticulturae9020189
Submission received: 27 November 2022 / Revised: 28 January 2023 / Accepted: 31 January 2023 / Published: 2 February 2023
(This article belongs to the Special Issue Advances in Tea Plant Biology and Tea Quality Regulation)
Browse Figure
Versions Notes

Abstract

:
Tea has become one of the most popular drinks worldwide because of its special taste and health benefits. ‘Yuhua’ tea is a famous specialty of Nanjing, China, a green tea produced from Camelia sinensis. It belongs to the green tea category and is one of the top ten most famous teas in China. The physical and chemical properties of soil are very important to the quality of tea. However, at present, there are few instances of research on the soil of the ‘Yuhua’ tea cultivation area. To provide the theoretical basis for improving the final quality of ‘Yuhua’ tea by soil management measures, the effects of the soil’s physical and chemical properties on the contents of important chemical components (free amino acids, tea polyphenols, and caffeine) of ‘Yuhua’ tea in three different cultivation areas in Nanjing were studied. The quality of ‘Yuhua’ tea is greatly influenced by the physical and chemical properties of the soil in which the tea plant grows. The free amino acid content and tea polyphenol content of tea leaves were significantly and negatively correlated with soil bulk density, pH, and water-soluble calcium content, while a significantly positive correlation was noticed between the two criteria and soil effective nitrogen, phosphorus, and potassium content. Additionally, the growth of the tea tree will be hampered and perhaps stressed if the soil’s helpful mineral concentration is too low. The tea tree may produce more caffeine as a result, which could lower the quality of the tea leaves. Therefore, implementing targeted soil management techniques is the key to promoting tea tree growth and improving tea quality. This study laid the foundation for the adoption of corresponding soil management measures to improve the quality of ‘Yuhua’ tea.

1. Introduction

Tea is one of the most popular beverages in the world, owing to its distinct flavor, taste, and health advantages [1,2]. In the past ten years, the area used to produce tea has expanded by nearly 66%, reaching 5.3 million tons on 3.5 million hectares of land across 50 tea-growing nations [3]. In addition to having a delicious, nuanced flavor and a rich cultural history, tea provides health benefits for preventing and treating several pathological conditions, including cancer, inflammation, cardiovascular disease, obesity, COVID-19, etc. [4,5,6,7,8].
The polyphenols, caffeine, and amino acids in fresh tea leaves are the three key elements that define the quality of tea, even if different processing techniques have an impact on the ultimate quality of commercial tea [9]. This is particularly true for green tea because it is an unfermented tea and its early preparation involves actions such as steaming or roasting to deactivate the oxidase activity, preventing any oxidation from happening during further processing steps [10]. The astringency of green tea is thought to be determined by polyphenols, whereas freshness and sweetness are provided by free amino acids and bitterness by caffeine [11,12]. The combination and harmony of these three chemicals establish the vital taste character, organoleptic quality, and monetary worth of green tea [13]. Additionally, green tea must generally have a phenol-to-ammonia ratio of no more than seven, and a low phenol-to-ammonia ratio can improve the quality of green tea [14].
The physical and chemical properties of the soil are essential factors affecting the yield and quality of the tea [15,16]. On the one hand, the tea tree’s ability to absorb water and nutrients from the soil is directly influenced by the physical and chemical characteristics of the soil, and on the other hand, the essential plant elements in the soil are also closely related to the tea tree’s secondary metabolism, which includes the biosynthesis of chemical substances that determine the tea’s quality [17]. In addition, changes in soil properties resulting from severe erosion and declining fertility have disturbed many tea plantations, leading to falling output and quality [18]. And regrettably, the increase in the frequency of extreme weather events, such as heavy precipitation, that have been brought on by human activity and global climate change will give soil erosion a stronger push, and this tendency is anticipated to become more prominent in the future [19,20]. Unquestionably, this has raised the bar for managing tea gardens. Therefore, it is critical to investigate how soil characteristics affect tea quality and implement focused soil management strategies to preserve and enhance tea quality. In the current study, we investigated the relationship between soil characteristics and fresh tea quality in three ‘Yuhua’tea production regions in Nanjing to establish a foundation for understanding this relationship and implementing appropriate soil management practices to enhance tea quality.

2. Material and Methods

2.1. Site Information and Experimental Design

On the 17th and 18th of April, 2021, research was carried out at three locations in Nanjing: Huanglongxian (118°41′46″ E, 31°47′10″ N), Zhongshanling (118°51′35″ E, 32°3′59″ N), and Pingshan (118°50′51″ E, 32°27′27″ N). A sunny area with a slope of 5–10° in the above three locations where tea is grown was selected as the study area. Further, the sampling area at each site was randomly divided into 3 blocks, with about the same amount of tea leaves collected from each block, and 9 soil samples were collected in an “S” pattern from each block, which were immediately mixed to represent the overall soil condition of the block. Additionally, soil samples were taken along the vertical projection of the edge of the tea tree where the tender leaves were harvested, and after appropriate removal of debris from the ground, the original soil was taken with a ring knife at 0–20 cm and 20–40 cm soil layers and brought back to the laboratory. A sufficient number of young leaves were collected according to the standard of one bud and one leaf and brought back to the laboratory in ice boxes.

2.2. Determination of Physical and Chemical Properties of Soil Samples

A portion of the soil sample was then dried at 105 °C, weighed, and its bulk density calculated. Meanwhile, the other soil samples were air-dried, ground, and sieved through a 2 mm sieve to determine the mechanical composition of the soil, pH, ammonium nitrogen, available phosphorus, and available potassium. The soil’s mechanical composition was then determined by a specific gravity meter; the soil’s pH was determined by the potentiometric method. Then the soil ammonium nitrogen was determined by the KCl leaching—Indophenol blue colorimetric method, the effective potassium by NH4OAc leaching—flame photometric method, and the effective phosphorus was determined by ammonium fluoride-hydrochloric acid solution leaching—spectrophotometric method [21]. What is more, the determination of the relative proportions of powder, clay, and sand in the soil was made according to the criteria of Balasubramanian and Hallam (2017) [22] and then the method of Carpenter et al. (2007) [23] was used to determine the classification of the soil at different sites.

2.3. Determination of the Quality Index Content of Tea Leaves

After being brought back to the laboratory, the tender leaves collected were quickly placed in the oven and baked at 105 °C for 15 min, and then at 70 °C to a constant mass. Then, according to the following method, a sufficient quantity of dried tea samples was prepared. The free amino acid content was determined by ninhydrin chromogenic spectrophotometry [24], and the tea polyphenol content was determined by reference to GB/T 8313-2018 [25], while the caffeine content was determined by reference to GB/T 8312-2013 [26].

2.4. Statistics Analysis

Three replicates were used to set up each measurement, and the results were displayed as mean ± standard deviation. Using Excel 2010, the data processing was carried out. One-way analysis of variance (ANOVA) and Duncan’s multiple comparisons were performed using SPSS 26.0, and differences between various treatment groups are indicated by different letters (p < 0.05) where they are significant.

3. Results

3.1. Soil Properties in Different Sites

As shown in Table 1, the three sites all have a high proportion of silty soil within the range of 0–20 and 20–40 cm. In terms of soil classification, the soils in the three study sites belong to silty soil. The soils at Pingshan had a significantly lower bulk density than at the other two sites for both 0–20 and 20–40 cm. The chemical properties of the soil at the three sites are shown in Table 2. The soil pH at all three sites was within the range (of four to six) suitable for tea tree growth [15]. Although the pH of the soil at Zhongshanling was significantly higher than that at the other two sites for both 0–20 and 20–40 cm. Moreover, for the effectiveness of macronutrients in the soil, ammonium N, available potassium, and available phosphorus were significantly higher in Pingshan soil than in the other two sites. Additionally, the water-soluble calcium content of the soil at Zhongshanling was significantly higher than that at the other sites.

3.2. The Content of Chemical Compositions in Tea Leaves

As shown in Figure 1, the free amino acid content of fresh tea from Pingshan was significantly higher than that of the other sites, 67% and 275% higher than that of Huanglongxian and Zhongshanling, respectively. What’s more, the polyphenol content of fresh tea from Pingshan is also significantly higher than that of other locations, being 8% and 18% higher than that of Huanglongxian and Zhongshanling, respectively. The phenol-to-ammonia ratio of fresh tea at Zhongshanling is significantly higher than at other sites and far exceeds seven, the standard for green tea production. In terms of caffeine, Huanglongjian’s fresh tea contains significantly more than the other two sites, 5% and 19% more than Zhongshanling and Pingshan, respectively.

3.3. Relationship between Tea Leaves Compositions and Soil Properties

Overall, the correlation between each soil property and each tea-leaf chemical component was consistent at 0–20 cm and 20–40 cm soil depth across the three sites (Table 3). The free amino acid and tea polyphenol content of tea leaves were negatively correlated with soil bulk and pH, while a significantly positive correlation was observed between the criteria and the soil effective nitrogen, phosphorus, and potassium content (Table 3). Further, for the phenol-ammonia ratio and caffeine content of tea leaves, there is a significant positive correlation between soil bulk and the criteria, while ammonium nitrogen, available potassium, and available phosphorus were negatively correlated with the criteria (Table 3).

4. Discussion

4.1. Effect of Soil Physical Properties on the Quality of Tea Leaves

The soils at all three study sites were silty, suggesting good drainage, which may be beneficial to the growth of tea trees (Table 1). The lower bulk weight reflects higher soil porosity, which facilitates the growth and development of plant roots [27]. In general, however, this type of soil has a poor fertility retention capacity, so this hints at the importance of enhancing soil fertility in these sites. In this study, the soil bulk density was inversely proportional to the free amino acid and tea polyphenol contents in tea leaves and directly proportional to the phenol-ammonia ratio, which indicated that proper reduction of bulk density could improve the tea quality (Table 3). There are many outstanding studies that show that the addition of biochar can reduce the soil’s bulk density [28,29,30,31]. Therefore, we can consider adding biochar to tea plant cultivation, especially when the soil bulk density is high. Previous studies have also shown that adding biochar can enhance the environmental friendliness of tea plantations by reducing greenhouse gas emissions from acidic soils in tea plantations, in addition to improving soil properties [32]. However, the amount of biochar added should be carefully considered because biochar is commonly alkaline [33], and too much of it is bound to be detrimental to the growth of tea trees.
Soil pH is considered to be the “master variable” of soil chemistry, which directly or indirectly influences the solubility of elements in the soil and determines their mobility and bioavailability [34]. Ma et al. (2017) [35] showed that soil pH was negatively correlated with amino acids, tea polyphenols, and theanine in tea. In this study, the soil pH was inversely proportional to the free amino acid and tea polyphenol contents in tea leaves and directly proportional to the phenol-ammonia ratio, which indicated that proper reduction of the pH value could improve the tea quality. However, it is inadvisable to lower the soil pH below four, as that would be outside the fitness range of the tea tree [15]. Although the study by Ruan et al. (2007) [36] presented different results, it showed that when the soil pH is less than four, the growth of young tea tree shoots is inhibited to a greater extent than nitrogen uptake, leading to an eventual increase in the total amount of free amino acids in tea leaves at the expense of tea yield due to the accumulation effect.
The effect of pH on tea quality may be achieved by influencing the availability of nutrients in the soil. So we correlated pH with the content of macronutrients in the soil (Table 4). Soil pH was inversely proportional to the AN, AP, and AK in tea leaves and directly proportional to the WSC. The same result was also obtained by Khadka et al. (2016) [37]. To further explore the relationship between pH and nutrient elements, we conducted regression analyses of pH and each nutrient element. Overall, the results obtained by linear regression are consistent with those obtained by Pearson correlation analysis (Supplementary Figures S1–S4). In addition, this suggested that pH accounted for about 45%, 40%, 46%, and 53% of the total variability in AN, AP, AK, and WSC, respectively. The research results of Wen et al. (2020) [38] also showed that soil pH value was significantly negatively correlated with available nitrogen (ammonium nitrogen plus nitrate nitrogen), which could provide some evidence for the results of this study. In fact, tea plants, which are suitable for acidic soils, prefer to absorb ammonium nitrogen when absorbing nitrogen from the soil [39]. Furthermore, one possible explanation for the negative correlation between pH and AP is that when the content of soluble calcium in the soil is high, as the pH value increases, Ca precipitates P from the solution, especially when the pH value is less than seven [34]. This is consistent with the fact that pH was positively correlated with water-soluble calcium content and negatively correlated with effective phosphorus content in this study.

4.2. Effect of Soil Macronutrients on the Quality of Tea Leaves

Nitrogen, phosphorus, and potassium are macronutrients essential for plant growth and development. Many studies have shown that the application of nitrogen, phosphorus, and potassium fertilizers can improve the tea’s quality [40,41,42,43,44,45]. In addition, although the total nitrogen, phosphorus, and potassium content of the soil may be high, the amount of effective nitrogen, phosphorus, and potassium available for direct plant uptake is generally inadequate, especially in some regions (including East Asia) [46,47]. The quality of the tea leaves at all three sites in this study was not high, as evidenced by the low free amino acid content (the generally normal value is 1–3%) and the relatively large phenol-to-ammonia ratio (around 7 for Huanglongjian tea leaves and a much greater 7 for Zhongshanling tea leaves) (Figure 1). This may be due to the low AN, AP, and AK in the soils of all three study sites, with Huanglongjian and Zhongshanling being particularly low (Table 2). Despite differences in climatic conditions and tea plant varieties, it is generally accepted that the minimum AK concentration required for tea plantation soils is 80 mg/kg [48]. It can be seen that the concentration of AK in the soil was significantly lower than this value at all three sites in this study, except for Pingshan. What is more, the results of this study showed that the AN, AP, and AK contents in soil were positively correlated with free amino acids and tea polyphenols in tea leaves, but negatively correlated with the phenol-ammonia ratio (Table 3). This suggests that increasing the AN, AP, and AK content in the soil can improve the tea’s quality. At the same time, due to its acidic soils combined with the relatively adequate rainfall in most tea-producing areas, the risk of loss of nitrogen, phosphorus, and potassium leaching from tea plantations is high [43,49]. Therefore, nitrogen, phosphorus, and potassium fertilizers should be applied in the appropriate periods and doses to improve the tea’s quality. At the same time, attention should be paid to the amount and balance of chemical fertilizers applied or the switch to organic fertilizers should be made in order to avoid excessive soil acidification, environmental pollution, and a decline in tea quality [14,49,50,51,52].

4.3. The Special Effects of Caffeine

Caffeine, as an alkaloid, has been shown to have the ability to enhance plant resistance to biotic and abiotic stresses such as bacterial, fungal, insect, and chromium stresses by inducing the salicylic acid pathway in plants [53,54,55,56]. In this study, the caffeine content of tea leaves was negatively and significantly correlated with the AN, AP, and AK contents of the soil (Table 3). This may be because the nutrients in the soil, such as AN, AP, and AK, are not able to support the good growth and development of the tea trees, possibly due to poor management in these three tea-growing locations, and may even put the tea trees under stress from adversity. One possible piece of evidence to support the above hypothesis is that caffeine content has a significant positive correlation with soil bulk density (Table 3). This perhaps indicates that the increase in soil bulk density is detrimental to the growth and development of tea trees. Similar studies have previously shown that caffeine levels in tea increase when the quality of the tea leaves is reduced due to nutrient deficiencies in the soil [57].

5. Conclusions

The quality of ‘Yuhua’ tea, a variety of green tea, is greatly influenced by the physicochemical characteristics of the soil in which the tea plant is grown. Properly lowering the soil bulk density and pH value and increasing the effective nitrogen, phosphorus, and potassium content can effectively improve the quality of ‘Yuhua’ tea by increasing the free amino acid content of tea leaves and tea polyphenol content and reducing the phenol-ammonia ratio of tea leaves. And when the amount of useful minerals in the soil is insufficient, the tea tree’s growth is hampered, and it may even become stressed. This can cause the tea tree to produce more caffeine, which lowers the quality of the tea leaves. Therefore, it is crucial to implement targeted soil management techniques to promote the growth of tea plants and improve the quality of tea leaves.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/horticulturae9020189/s1, Figure S1: The relationship between AN and pH in the three sampling sites; Figure S2: The relationship between AP and pH in the three sampling sites; Figure S3: The relationship between AK and pH in the three sampling sites; Figure S4: The relationship between WSC and pH in the three sampling sites.

Author Contributions

Conceptualization, F.Y.; Data curation, Z.L. and C.C.; Formal analysis, G.Z.; Funding acquisition, F.Y.; Investigation, Z.L., D.Y., G.Z., L.Z. and C.C.; Resources, X.S.; Software, Z.L., G.Z. and L.Z.; Supervision, F.Y.; Writing—original draft, Z.L. and D.Y.; Writing—review & editing, Z.L. and D.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (3197140894), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the Postgraduate Research and Practice Innovation Program of Jiangsu Province (SJKY19_0882).

Data Availability Statement

Not applicable.

Acknowledgments

We’d like to thank our good friends Jia Ying Lai, Wen Jie Wu, Ling Yuan Kong and Shu Zhou for their assistance throughout the experiment.

Conflicts of Interest

The authors declare no conflict of interest.

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Horticulturae 09 00189 g001 550
Figure 1. Chemical components in tea leaves in the three sampling sites. The significant variations are shown by different letters at the 0.05 level.
Figure 1. Chemical components in tea leaves in the three sampling sites. The significant variations are shown by different letters at the 0.05 level.
Horticulturae 09 00189 g001
Table
Table 1. Soil physical properties in the three sampling sites.
Table 1. Soil physical properties in the three sampling sites.
SitesDepth (cm)Clay (%)Silt (%)Sand (%)Soil ClassificationBulk Density
(g cm–1)
Huanglongxian0–202.70 ± 0.44 a90.04 ± 0.86 ab7.27 ± 1.16 abSilt1.44 ± 0.12 a
20–402.41 ± 0.18 a88.11 ± 1.77 b9.49 ± 1.86 aSilt1.53 ± 0.03 a
Zhongshanling0–202.33 ± 0.66 a87.86 ± 2.07 b9.82 ± 2.35 aSilt1.52 ± 0.14 a
20–403.24 ± 0.1 a89.14 ± 0.88 b7.62 ± 1.63 abSilt1.56 ± 0.07 a
Pingshan0–202.33 ± 0.51 a91.47 ± 1.21 a6.21 ± 1.5 bSilt1.06 ± 0.07 b
20–402.08 ± 0.87 a92.58 ± 1.27 a5.34 ± 0.76 bSilt1 ± 0.14 b
The significant variations are shown by different letters at the 0.05 level.
Table
Table 2. Soil chemical properties in the three sampling sites.
Table 2. Soil chemical properties in the three sampling sites.
SitesDepth (cm)pHANAPAKWSC
Huanglongxian0–204.95 ± 0.56 ab11.17 ± 2.41 b11.67 ± 6.9 b34.67 ± 11.27 b120 ± 26.46 b
20–405.17 ± 0.37 ab17.27 ± 5.23 b22.11 ± 2.22 b39.07 ± 5.2 b336.67 ± 176.16 b
Zhongshanling0–205.54 ± 0.19 a10.47 ± 2.57 b20.37 ± 7.58 b24.33 ± 11.75 b916.67 ± 125.03 a
20–405.65 ± 0.18 a9.67 ± 1.55 b24.58 ± 4.29 b32.4 ± 7.14 b916.67 ± 128.97 a
Pingshan0–204.54 ± 0.09 b72.63 ± 3.76 a96.86 ± 25.42 a123.47 ± 24.91 a190 ± 20 b
20–404.67 ± 0.27 b79.67 ± 18.99 a67.24 ± 4.72 a179.63 ± 25.85 a130 ± 34.64 b
AN: Ammonium nitrogen; AP: Available phosphorus; AK: Available potassium; WSC: Water-soluble calcium. The significant variations are shown by different letters at the 0.05 level.
Table
Table 3. Correlations of soil properties to the tea-leaves indicators in the three sampling sites.
Table 3. Correlations of soil properties to the tea-leaves indicators in the three sampling sites.
Chemical ComponentsSoil Physical and Chemical Properties
Depth (0–20 cm)Depth (20–40 cm)
BDpHANAPAKWSCBDpHANAPAKWSC
Free amino acids−0.838 **−0.792 *0.878 **0.747 *0.882 **−0.76 *−0.899 **−0.874 **0.844 **0.84 **0.874 **−0.853 **
Tea polyphenols−0.788 *−0.736 *0.839 **0.738 *0.84 **−0.84 **−0.814 **−0.785 *0.832 **0.797 *0.839 **−0.933 **
Polyphenols/Amino acids ratio0.675 *0.799 *−0.667 *−0.562−0.669 *0.871 **0.689 *0.81 **−0.685 *−0.606−0.6490.862 **
Caffeine0.826 **0.519−0.941 **−0.921 **−0.878 **0.1310.901 **0.606−0.874 **−0.953 **−0.92 **0.446
Note: BD: Bulk density; AN: Ammonium nitrogen; AP: Available phosphorus; AK: Available potassium; WSC: Water-soluble calcium. Significance correlation levels: * p < 0.05; ** p < 0.01. Same below.
Table
Table 4. Correlations of soil pH value to macronutrients in the three sampling sites.
Table 4. Correlations of soil pH value to macronutrients in the three sampling sites.
Soil Depth (cm) ANAPAKWSC
0–20pH−0.658−0.609−0.681 *0.685 *
20–40−0.734 *−0.728 *−0.797 *0.777 *
Significance correlation levels: * p < 0.05.
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Liu, Z.; Yang, D.; Zhang, G.; Zheng, L.; Chen, C.; Sun, X.; Yu, F. Effects of Soil Physical and Chemical Properties on the Quality of Nanjing ‘Yuhua’ Tea, a Type of Famous Green Tea. Horticulturae 2023, 9, 189. https://doi.org/10.3390/horticulturae9020189

AMA Style

Liu Z, Yang D, Zhang G, Zheng L, Chen C, Sun X, Yu F. Effects of Soil Physical and Chemical Properties on the Quality of Nanjing ‘Yuhua’ Tea, a Type of Famous Green Tea. Horticulturae. 2023; 9(2):189. https://doi.org/10.3390/horticulturae9020189

Chicago/Turabian Style

Liu, Zemao, Deyu Yang, Guangtao Zhang, Lihong Zheng, Chen Chen, Xin Sun, and Fangyuan Yu. 2023. "Effects of Soil Physical and Chemical Properties on the Quality of Nanjing ‘Yuhua’ Tea, a Type of Famous Green Tea" Horticulturae 9, no. 2: 189. https://doi.org/10.3390/horticulturae9020189

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