Foliar fertilizers can enhance the absorption capacity of Tea plant roots, allowing the plants to fully absorb nutrients from the soil and avoid wastage of resources; when certain soil properties restrict the root absorption of nutrients, foliar fertilizers can be applied to supplement and prevent deficiencies. Although soil fertilization is the most common and effective method of nutrient application, in some cases, foliar fertilization can be more economically efficient. This is mainly due to the buffering effect of various factors in the soil solution (mineral nutrients, pH, minerals, organic matter) affecting soil fertilization in tea plants, which often requires a larger amount of fertilizer. Therefore, it is necessary to find sustainable intensive approaches to increase crop yields while reducing dependence on agricultural chemicals.

Long-term production practices have shown that foliar fertilization not only improves the color, aroma, and taste of tea but also plays a positive role in increasing tea yield, effectively reducing the risk of excessive use of some root-applied fertilizers. Therefore, foliar fertilization will be an important agronomic measure, especially in terms of improving fertilizer utilization efficiency.

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I. Concept and Characteristics of Foliar Fertilizers

1. Concept of Foliar Fertilizer

Foliar fertilizers refer to fertilizers applied to plant leaves and absorbed for use. They are typically used to supplement essential nutrients such as N, P, K, and other macro and microelements and rare earth elements. In China, the use of foliar fertilizers dates back 200 years to their application by farmers in Zhejiang province on rice crops; abroad, the application of foliar fertilizers began in 1884 when French botanist Grisb applied FeSO4 to grape leaves. Since the 20th century, the United States has conducted field studies on the whole-plant application of foliar fertilizers, gradually applying them in agriculture, primarily on fruit trees like grapes and citrus.

2. Characteristics of Foliar Fertilizers

Foliar fertilizers are applied to the surface of plant leaves, where the nutrients penetrate the cuticle or stomata before entering the cells. They come in many types, including nutritional, regulatory, and biological varieties. Foliar fertilizers have many characteristics, such as high efficiency, low pollution, strong targeting, simple application methods, and high utilization rates. Foliar fertilization is an important agricultural management measure for maximizing crop yields, capable of supplementing soil fertility but not replacing soil fertilization. As a supplement to soil fertilization, foliar fertilization is characterized by its efficiency and rapidity, with total nitrogen recovery rates exceeding 80% in some reports. It can also promote the absorption of nutrients by plant roots. Additionally, foliar fertilizers can enhance photosynthesis and the absorption of nutrients by different organs.

II. Effects of Foliar Fertilizers on Tea Quality Components

1. Effects on Catechin Content

Different types of foliar fertilizers have varying effects on catechin content in tea, and the selection of appropriate foliar fertilizers should be based on the desired quality attributes, particularly for summer and autumn teas. Excessively high catechin content in tea can make the infusion taste bitter, while too low a content can result in a bland flavor, both of which lower tea quality. The proper application of most types of foliar fertilizers can increase catechin content in tea. However, there are some types of foliar fertilizers that can reduce catechin content. Applying high-potassium Nuicui green humic acid organic liquid fertilizer can effectively reduce the catechin content in fresh leaves, resulting in higher sensory evaluation scores for the produced Green Tea compared to using high-nitrogen Nuicui green humic acid organic liquid fertilizer and Medile humic acid water-soluble fertilizer. Similarly, applying Sifu, Bao De, tea-specific foliar fertilizers, and Amptide can also reduce catechin content in fresh leaves. During summer and autumn, due to high temperatures and faster carbon cycle metabolism in tea plants, the catechin content in fresh leaves increases, leading to a bitter taste and reduced quality in the produced tea. Therefore, different foliar fertilizers can be selected according to production needs, providing a theoretical basis for improving the quality of summer and autumn teas through foliar fertilization.

Moreover, the concentration and type of foliar fertilizer applied can affect the catechin content in tea. Applying a 500-fold dilution of neem bio-foliar fertilizer can increase catechin content in tea, whereas 700-fold and 1000-fold dilutions can decrease catechin content in fresh leaves. Different ratios of biogas liquid to water have varying effects on catechin content; when the ratio of biogas liquid to water is 2:1, the catechin content in fresh leaves decreases; when pure biogas liquid is applied, the catechin content in fresh leaves is relatively highest.

2. Effects on Amino Acid Content

Amino acid foliar fertilizers can increase amino acid content in tea without decreasing catechin content, thereby lowering the phenolic-to-amino acid ratio and improving tea quality. Tea plants can directly absorb amino acid foliar fertilizers, enabling their participation in amino acid metabolism within the plant. There are over 20 free amino acids in tea, accounting for approximately 4% of the dry matter in fresh tea leaves, positively influencing tea flavor and aroma. Amino acid content in tea mainly contributes to the fresh and refreshing taste of the infusion and can undergo Maillard reactions with sugars in the tea, imparting a caramel aroma to the dried tea. Li Jie et al. applied amino acid foliar fertilizers at different concentrations to two tea cultivars, Tieguanyin and Benshan, finding that amino acid content increased to varying degrees in both. Tan Zhengchu et al. applied three amino acid foliar fertilizers, tea-specific amino acid foliar fertilizer, Fengnong, and Shengliye, to Baimaozao, observing increases in amino acid content in spring and autumn teas. When Fengnong was applied, the phenolic-to-amino acid ratio was lowest, achieving the best results. Amino acid foliar fertilizers can maintain catechin content in fresh leaves without significant changes while increasing amino acid content, balancing tea flavor and Health benefits. Guofei, Spray Treasure, seaweed fertilizer, and biogas liquid can all increase amino acid content and tea yield simultaneously, enhancing both quality and productivity.

3. Effects on Caffeine Content

Gong Ziming et al. used three foliar fertilizers, Tianyuan organic liquid fertilizer, Wanxing plant amino acid liquid fertilizer, and Bao De microbial foliar enhancer, on Fuyun No. 6, and found that, compared to urea application, the caffeine content in tea increased to varying degrees under the application of all fertilizers except Wanxing. Wu Quancun et al. applied six commonly used tea foliar fertilizers to Longjing 43 in the spring, discovering that Liguo Mei, amino oligosaccharide, and Adosol could all reduce caffeine content. Thus, applying suitable foliar fertilizers can effectively reduce caffeine content in tea, alleviating bitterness and providing a theoretical foundation for improving green tea quality.

4. Effects on Chlorophyll Content

Foliar fertilizers can regulate chlorophyll content in fresh leaves and may influence chloroplast structure. Research has found that amino acid foliar fertilizers, urea, and life essence can all increase chlorophyll content. Furthermore, amino acid foliar fertilizers can promote the growth of leaf area, thickness, and stomata, thus enhancing photosynthesis. Huang et al. sprayed glycine betaine on Baiye No. 1 and observed the ultrastructure of chloroplasts in fresh leaves during different stages of bleaching. They found that 3.2 g/L glycine betaine could promote the conversion of yellow bodies to chloroplasts during the bleaching period. Starch grains were also larger in fresh leaves during the regreening period, capable of storing more substances. Additionally, studies have combined glycine betaine with zinc and found that compared to single zinc applications, mixing the two substances in a specific ratio not only enhances zinc content in Baiye No. 1 more effectively but also significantly increases Theanine content, improving tea quality.

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III. Effects of Foliar Fertilizers on Tea Yield

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