1. Introduction
Tea,
Camellia sinensis (L.) O. Kuntze (Theaceae), is a unique and evergreen plant that is mainly dispersed in tropical and subtropical areas. The growth of a 60–100-cm-high natural shrub is maintained on tea plantations for collecting tender leaves [
1]. Well-drained, deep, and well-aerated soils and sandy loam to clay loam textures, and even extremely coarse and gravelly soils, are the optimal soil conditions suggested for tea growth [
2]. In addition, tea trees grow well in soils with 2–52% of the clay content, possibly containing 1.7–2.3% of soil organic matter (SOM) and 20.8–28.2% of water-holding capacity (WHC) [
3]. Conditions that are conducive to tea plantations include a soil pH of 4.0–6.0, a temperature of 20.2–25.4 °C, an annual precipitation of 1500–2000 mm, and a humidity of 80–90% [
4]. From the sea level up to ~2200 m above the sea level, the elevation of a plantation drastically affects the quality and quantity of tea leaves [
5]. When the soil pH drops to less than 4.0, the growth of tea trees is adversely affected [
6,
7].
Soil conditions are important in that its biochemical properties might be changed by tea cultivation due to the specific natural characteristics of tea plants and agronomy management practices that supplement tea plants [
8]. In addition, soil properties determine the availability of essential nutrients and thus, the nutrient uptake by plants [
9,
10]. Correspondingly, 11.0% nitrogen (N), 1.65% phosphorous (P), 3.7% potassium (K), 3.1% calcium (Ca), 1.07% magnesium (Mg), and 24 mg/kg copper (Cu) are reported to be necessary nutrients for tea [
11]. Thus, tea trees grow well in several acidic soils, extending far down from the surface soil and penetrating well into the lowest available soil depth [
5]. The application of calcium oxide decreases the soil acidity, releases exchangeable cations, and also improves the soil fertility [
12]. However, most of the acid soils in India exhibit supplementary problems, with a low base saturation percentage (<75%) and low concentrations of exchangeable cations [
2]. Negative effects of soil acidity on vegetable growth are closely correlated with the availability of essential nutrients, which might increase with the reduction in pH, such as for Cu and zinc (Zn). The SOM might retain heavy metals in the solid segments of the soil, while dissolved SOM can increase the adsorption of heavy metals through acidic soils [
13,
14].
The management of soil N has been extensively conducted for tea farming and has been described in several reviews [
15,
16]. N plays an important role in establishing proteins, and it plays an important role in chlorophyll synthesis. Hence, acidic soils not only affect the status of soil nutrients but also the tea ingestion security [
15,
16]. Moreover, the nitrification rate affects the soil pH and inorganic N content at different soil depths [
17]. Soil aggregates are principally formed by physical, chemical, and biological processes, which are essentially responsible for their stabilization. Moreover, types of clay, the calcium carbonate content, and SOM constitute soil properties that exhibit a high correlation with soil aggregate stability [
18]. N, P, and K are essential macronutrients for the growth of tea trees [
19]. However, the availability of P is always low in soils due to its low natural content as well as the high P fixation capability of the soil [
20]. Exploration demonstrations revealed that more than 70% of tea soils are deficient in P [
21]. Therefore, a large amount of P fertilizers, as well as frequent application, is needed for tea plantations. Furthermore, the type and content of these essential elements and tea quality are closely related. N, P, and K are reported to support the contents of flavone, total polyphenols, free amino acids (FAAs), catechins, and theanine [
22]. In addition, the application of K and Mg can increase the contents of FAAs and caffeine of different tea tree species [
23]. Some efforts are devoted to the analysis of the effect of the changing chemical material expended to fertilize soil on quality parameters of produced tea [
24].
Tea quality is mainly determined by its flavor, and it is affected by the quality of fresh, tender leaves as well as the fermentation processes of producers. The flavor of prepared tea leaves is mainly attributed to the abundant chemical compounds, including 20–40% of total polyphenols, 10–30% catechins, 1–2% flavones, 2–5% caffeine, and 14–17% protein. These compounds are either secondary metabolites accumulated in tender shoots during the growth and harvest periods or substrates participated the fermentation processing [
25].
Overall, the quality of tea leaves can be affected by the tea species, management, plantation, and climatic factors [
3]. Sustenance is unique among the major influences, and numerous tests are conducted to analyze the effects of soil fertility management on the quality of different types of tea [
17]. A number of previous studies focused on the soil properties of tea plantations; however, less information is available with regards to the comparison of different soil physical and chemical properties and the effect of tea cultivation on organic components in tea infusions with respect to soil properties [
26]. Most of the studies on tea quality are specialized studies, such as soil quality and absorption in leaves [
17,
26]. To observe effects of soil properties and organic components on one of the important types of tea in central Taiwan, soil samples and corresponding tender leaves were collected from the same plantation and analyzed with respect to good (G) and bad (B) growth exhibitions as distinguished by aerial photographs. This study aimed to identify the major soil properties via the growth determination of tea plants and further develop an assessment model based on soil quality (SQ) to distinguish between G and B and correlate it with the contents of organic compounds in the infusions of tender leaves. The results of this study can provide useful information regarding alternative fertilization and soil management practices for sustainable tea cultivation.
2. Materials and Methods
2.1. Study Area
The study site was in Nantou county, located in central Taiwan, at a latitude of 23°54′56.38′′ N and a longitude of 120°39′49.93′′ E. In addition, it is the only landlocked county with a total area of 4106 km2. This county exhibits an average annual temperature in the range of 20–23 °C on the ground level and mountains, and the climate is diverse, forming sub-tropical and frigid climate zones with increasing altitude. The annual average precipitation on the ground level and on mountains equals 1750 mm and 2800 mm, respectively. As ~83% of the area is covered by hills and mountains, and its terrain, elevation, and climate conditions are variable, Nantou county is a suitable place for growing tea trees. Soil and tea leaf samples were collected from 18 plots in Mingjian township area. For the same plot, and according to aerial photographs, tea with G and B growth appearances were selected, and soil and tea leaf samples were collected and taken to a laboratory for analysis in 2018–2019.
2.2. Collection and Analysis of the Soil and Tea Leaves
Soil samples were collected from 36 subplots with soil depth intervals of 0–20, 20–40, and 40–60 cm. Locations were randomly selected in each subplot and bored to different depths using an auger. Three soil samples with the same depth were then homogenized as a composite sample. All soil samples were stored in plastic containers and transferred to the laboratory. Then, the samples were dried in air, ground, sieved, and passed through 10-, 80-, or 100-mesh stainless-steel sieves according to the analyzed soil property. The measured soil properties included pH (W
soil/V
water = 1/5) [
27], electrical conductivity (EC
w; W
soil/V
water = 1/5) [
28], 2 M KCl extractable N [
29], 1 M NH
4OAc (pH 7.0) extractable cations [
30], available P, analyzed by the Bray No.1 method and UV spectrometry [
31], and soil organic carbon content (SOC; [
32]). The soil samples were digested using aqua regia, and the total concentrations of Cu and Zn in the digestants were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES, PerkinElmer Avio200, Waltham, MA, USA). Soil wet aggregate stability (WAS) was determined using a wet sieving device in accordance with that reported by Murer et al. [
33].
Tender tea leaves (cultivar: Si Ji Chun) with two leaves and a bud were collected at the same time as the soil. All samples were washed with tap water and deionized water (DI water) to remove adhered dust, dried in an oven at 65 °C for 72 h, and ground into powders using a grinder. A total of 1.0 g of the powdered sample was extracted with 100 mL of DI water at 100 °C for 60 min. The tea infusions were stored in a freezer at 4 °C for further analysis.
To analyze the content of the total polyphenols in tea leaf infusions, gallic acid solution concentrations of 0, 0.01, 0.015, 0.02, 0.025, and 0.03 mg/mL were prepared for the calibration curve. One milliliter of the Folin-Ciocalteu’s reagent and 2 mL of the saturated Na
2CO
3 solution were added to a 1-mL infusion sample. After homogeneous mixing, the sample was quiesced for 1 h, and the total polyphenol content was determined at 700 nm using a visible spectrophotometer (Genesys 30, Thermo, Waltham, MA, USA) [
34].
Gallic acid solutions with concentrations of 0, 0.1, 0.15, 0.2, 0.25, and 0.3 mg/mL were prepared for the calibration curve. One milliliter of the infusion sample with 3 mL pH = 7.5, 0.1 N phosphate buffered saline and 1 mL of tartaric acid as the coloring agent, which was prepared using 100 mg ferrous sulfate and 500 mg potassium sodium tartrate, was dissolved in water and quantitative to 100 mL. After homogeneous mixing and standing for 30 min, the catechin content was determined using a visible spectrophotometer at 540 nm (Genesys 30, Thermo, Waltham, MA, USA) [
34].
The FAAs content was utilized to analyze theanine solutions at concentrations of 0, 0.04, 0.08, 0.12, 0.16, and 0.20 mg/mL for the calibration curve, and a 2-mL infusion sample with 1 mL of ninhydrin agent and 1 mL of a stannous chloride solution was prepared and maintained at a temperature of less than 80 °C in a water bath for 20 min. The 5-mL diluent (isopropanol:H
2O = 1:1) was infused, and the sample was quiesced for 20 min after rapid cooling. The FAAs content was determined using a visible spectrophotometer (Genesys 30, Thermo, Waltham, MA, USA) at 570 nm [
34].
Caffeine solutions at concentrations of 0, 1.2, 3, 6, 9, and 12 mg/L were prepared for the calibration curve. The caffeine content was directly determined at 276 nm using a PG Instruments Limited T60 UV–Vis spectrophotometer [
35].
The flavone content was analyzed from a 1-mL infusion sample with 9 mL of aluminum(III) chloride (1%, w/w), which was determined by a visible spectrophotometer (Genesys 30, Thermo, Waltham, MA, USA) at 420 nm. The absorbance (A
420) was input into the following formula: flavones (mg/mL) = (A
420 × 320)/10
3 [
36].
2.3. Soil Quality Assessment
The total minimum data set (MDS) was calculated by utilizing 11 soil properties that were selected for SQ assessment, which were recommended by several authors as beneficial soil quality indicators. The SQ score was computed by using a scoring function analysis framework reported previously [
37,
38,
39,
40].
The soil properties fulfilled all of the measured functions, and under the proposed framework, they were given a score of 1.0. These numerical weights were allocated to each soil function according to their significance in fulfilling the overall goals of sustaining SQ under the specific conditions of this study. The consistent numerical weights for individual SQ indicators were multiplied by indicator scores considered by the use of standardized scoring functions (SSFs) that standardize indicator measurements to a value between 0 and 1.0 [
41]. Three SSFs are typically used for the SQ assessment [
42,
43], which are more is better (SSF3), less is better (SSF9), and optimum (SSF5), respectively. The form of the curves created by the scoring curve equation was determined by critical values, as described in
Table 1. The critical values contained threshold and baseline values, which are based on published values. The thresholds of the soil property values were that the scoring function equaled one when the measured soil property was at an optimal level or equaled zero when the soil property was at an unacceptable level. The baselines were the soil property values where the scoring function equaled 0.5 and where the midpoints were between threshold soil property values [
44].
To determine the performance of different indices, the classification accuracy of SQ was assessed for each level by numerical score analysis. The numerical score in the indices was calculated by the direct comparison and Fleiss’ kappa statistic assessment. A direct evaluation of the quality level of each site was performed by comparing the number of points where the combination of the index and indicator exhibited the same soil quality level [
45]. The Fleiss’ kappa statistic assessment was performed by using five soil levels to indicate the level of agreement at which different substances may be valued by different individuals [
46]. The kappa statistic assessment evaluated the difference between the observed agreement between two tables and the agreement resulting from the degree to which the observed amount of agreement between raters exceeded what would be expected if all raters had random ratings. Furthermore, soil fertility specialists were consulted to compare their opinions with the proposed system’s output and to evaluate the agreement by the Fleiss’ kappa statistic assessment. The following limits of agreement were used: (1) poor: <0, (2) slight: 0–0.20, (3) fair: 0.21–0.40, (4) moderate: 0.41–0.60, (5) substantial: 0.61–0.80, and (6) almost perfect: 0.81–1 [
47]. Similarly, the correlation between indices and regression for indicator methods was calculated.
2.4. Statistical Analysis
Statistical analysis of the SQ indicator was performed using Statistical Package for Social Sciences (SPSS, Armonk, NY, USA) software. Variance was examined to determine the consequential relationship between soil properties, SQ scores, and concentrations of organic compounds in the infusion of tea leaves. A one-way analysis of variance and a t-test were performed for the statistical analysis of data (Fleiss’ kappa, regression equations, and scoring functions). Microsoft Excel was used to detect differences in the soil and tea leaves. The least significant difference test was employed to identify significant differences between means. SQ indicators and SQ indices were evaluated for their level of significance at a p-value of less than 0.01, and 0.05 denoted a statistical significance.
5. Conclusions
Based on the 11 soil properties analyzed in this study, the mean values for exchangeable K, available N, and ECw of G were 1.12-, 1.29-, and 1.46-fold greater than those of B, respectively. Approximately 67% of the soil at a depth of 0–60 cm for G exhibited a better SQ score than that of B. On the other hand, based on the Fleiss’ kappa statistic assessment result, WAS, pH, and the exchangeable K content drastically affected the growth exhibition of tea. The mean values for the total phenols, catechins, FAAs, and caffeine of G was 1.02- to 1.08-fold greater than those of B. Moreover, the soil properties that affected the contents of FAAs in the infusions were pH, ECw, SOC, available P, exchangeable Mg, and Cu. As the flavor of the infusion is based on the combination of several organic compounds, it is also difficult to find the most suitable proportion of these five compounds. The survey results could allow for the concentration determination of ranges of these five organic compounds as well as their relationships with different soil properties.