Objective Soil management in ancient tea gardens is crucial for the quality of tea leaves. However, research on the fertility status of the soil in the ancient tea gardens of Nanhua County and its impact on tea quality has not commenced.

Methods This study measured 8 soil nutrient indicators and 14 tea quality components for 136 ancient tea trees from two representative ancient tea gardens in Nanhua County, and calculated the Soil Fertility Comprehensive Index (IFI) using fuzzy mathematics. Subsequently, the relationship between soil nutrients and tea quality was studied using correlation and principal component analysis.

Results The results showed that the pH values of the ancient tea garden soils were mostly between 4.20 and 5.80, with abundant of total nitrogen (TN), total potassium (TK) and available potassium (AK), and higher levels of available nitrogen (AN), available phosphorus (AP) and soil organic matter (SOM). However, TP was relatively low, and some AP levels were found to be low. The calculation of IFI indicated that the values ranged from 0.201 to 0.990, reflecting that the soil fertility quality of the local ancient tea gardens has reached at least level Ⅱ, and except for pH and TK, other indicators showed a strong correlation with IFI. Tea quality components such as water-soluble substances, tea polyphenols, and free amino acids were mainly influenced by AP and TK, while theaflavins, including CG, GC and GCG, were mainly affected by AP, AN, and SOM.

Conclusion The soil nutrient level in the ancient tea gardens of Nanhua County is moderately high and evenly distributed spatially. Based on the above analysis, it is recommended to timely supplement the soil with phosphorus fertilizers, nitrogen fertilizers, and organic fertilizers to improve the nutritional components in the soil, thereby enhancing tea quality and promoting sustainable production in the tea gardens.