Since entering a new era, the promotion and application of new tea plant varieties and cultivation technologies have led to a new period of rapid development in China's tea industry. However, since China's tea plants are mainly cultivated in mountainous and hilly areas with poor Soil conditions, coupled with excessive long-term use of chemical fertilizers by some tea farmers and neglect of ecological environmental protection, the ecological environment of tea gardens has been deteriorating. Soil, as the foundation for tea plant growth, is also facing issues of quality degradation, which has become an important factor limiting the development of China's tea industry. This article reviews recent research on the improvement of tea garden soils in China, analyzes and summarizes the current status of soil quality degradation and the effectiveness of improvement measures, and offers projections.
I. Current Status of Degradation of Tea Garden Soil Quality
1. Acidification of Tea Garden Soils
The pH range generally suitable for tea plant growth is 4.0–6.5, with the most optimal pH being 4.5–5.5. All major tea-producing regions in China exhibit varying degrees of soil acidification, with a trend of increasing severity. Studies show that the average pH of China's tea garden soils is 4.68, with approximately 44% of tea gardens having a pH below 4.5. Acidification of tea garden soils is an ecological environmental issue.
(1) Exogenous Factors: In the management of tea gardens, there is a habit of applying large amounts of physiologically acidic nitrogen fertilizers, which increases the amount of NH4+ in the soil, forming cation accumulation zones. Additionally, under the action of rainwater runoff, base cations are continuously leached out, destroying the soil aggregate structure and leading to soil acidification. Furthermore, acid dust particles and acidic gases produced from the combustion of mineral materials such as oil and coal will directly enter the soil via rainfall, causing soil acidification. (2) Endogenous Factors: Tea plant roots have a pronounced “Calcium-averse aluminum-accumulating” characteristic. As the planting duration increases, the mineralization of silicate compounds and iron-containing compounds accelerates, resulting in the loss of potassium and calcium and the accumulation of aluminum and silicon, which also accelerates the acidification of tea garden soils. After tea plant leaves naturally fall, decompose, and return to the soil, the roots release organic acids and H+ secretions, which affect the soil's acidity. After tea plants absorb ammonium nitrogen, their roots secrete large amounts of H+, and the nitrification of ammonia also produces significant amounts of H+, accelerating the acidification process of tea garden soils. The roots of other plants in the tea garden may also secrete acidic substances under specific conditions.
2. Heavy Metal Pollution of Tea Garden Soils
Soil acidification reduces the soil's adsorption capacity for heavy metal ions, activating them and increasing their toxicity to tea plants. Human activities are generally considered the primary source of heavy metal pollution in tea garden soils. The degree of heavy metal pollution in tea garden soils is closely related to factors such as fertilizers, pesticides, cultivation methods, and environmental pollution caused by nearby industries and transportation “three wastes.”
Excessive levels of heavy metals in tea garden soils can exert stress on tea plant growth. Cadmium, chromium, and lead can damage chloroplast structures, reduce the accumulation of photosynthetic pigments, and inhibit photosynthesis in tea plants. High concentrations of cadmium and chromium can disrupt the Antioxidant enzyme system in tea plants, affecting cellular membrane structure and function. Lead concentration is inversely proportional to the activity of superoxide dismutase, peroxidase, catalase, and other enzymes.
Impact of Lead Stress on Chlorophyll Content in Tea Leaves
3. Destruction of Physical Structure of Tea Garden Soils
Without external interference, the solid, gaseous, and liquid phases of tea garden soils remain within a reasonable proportion, coordinating the relationship between Water, nutrients, air, and heat in the soil. However, with the long-term application of fertilizers, harvesting, pruning, and other agricultural activities, along with the fact that most tea gardens are located in mountainous and hilly areas, the internal balance of the soil is disrupted due to inadequate infrastructure, neglect of organic fertilizer application, and dense planting, among other external factors. This results in the destruction of the originally loose structure of the soil, making it hard and compacted, reducing its water absorption capacity. Most of the water is lost through surface runoff and evaporation, while the remaining water that enters the soil is transformed into gravitational water due to reduced pore space, decreasing the soil's water retention capacity. Moreover, the decrease in aerated pore space reduces the soil's air content, affecting the respiration of soil microorganisms and roots.
II. Improvement Measures for Tea Garden Soils
1. Application of Organic Fertilizers
The application of organic fertilizers in tea gardens can alleviate soil acidification, with an increase in soil pH corresponding to the increased quantity of organic fertilizers used. The pH after full organic fertilizer treatment is 4.65, whereas the pH after chemical fertilizer treatment is 3.93. Organic fertilizers not only contain rich organic matter but also provide various nutrients required for plant growth. Studies show that continuous and long-term application of organic fertilizers in tea gardens can significantly increase soil organic matter and N, P, K content. Additionally, long-term application of organic fertilizers increases the activity of various enzymes in tea garden soils, such as catalase, invertase, and phosphatase, enriching the population and abundance of soil microorganisms, promoting the formation of ideal microbial community structures, thereby activating soil nutrients, increasing the effective forms of N, P, and K in the soil, and improving the utilization efficiency of fertilizers.
2. Application of Soil Conditioners
Naturally occurring mineral alkaline fertilizers are often used as soil conditioners to improve acidification in tea gardens. Since these conditioners contain abundant Ca, Mg, S, and other mineral elements, they can increase the mineral nutrients in the soil. Additionally, Ca2+, Mg2+, and K+ compete with Al3+ for exchange sites in the soil, neutralizing H+ in the soil solution, raising the soil pH, and thus improving the acidified condition of the soil.
Biomass charcoal, as a healthy bio-soil conditioner, has been widely used to improve acidified soils in tea gardens. Applying biomass charcoal can increase the soil's organic carbon content. Due to its lower bulk density compared to soil, biomass charcoal can reduce soil bulk density and improve soil structure. Additionally, biomass charcoal has a large specific surface area and is rich in various functional groups on its surface, which can increase the soil's cation exchange capacity, especially for acidic soils. Its inherent alkaline characteristics give it the potential to improve acidic soils. Moreover, because of its multilevel porous structure, biomass charcoal has strong adsorption capacity, effectively enhancing the soil's water-holding capacity. It can also increase the adsorption of soil nitrogen, reducing nitrogen leaching, extending the residence time of mineral nitrogen in the soil, and enhancing the soil's nitrogen supply efficiency.
3. Intercropping Green Manures in Tea Gardens
Intercropping green manures in newly cultivated tea gardens can effectively cover the ground surface, maintain soil temperature and Humidity, mitigate surface runoff erosion, and reduce the loss of water and nutrients from tea