Soil Quality Evaluation and Limiting Factor Analysis of Trichosanthes kirilowii Maxim. Planting Areas Based on the Minimum Data Set: A Case Study of Hubei Province, China

Soil quality constitutes a fundamental foundation for maintaining soil productivity, ecological function and crop health. However, existing research on soil quality assessment and its limiting factors in Trichosanthes kirilowii Maxim. planting areas remains insufficient. To construct a soil quality assessment system suitable for T. kirilowii Maxim. planting areas and identify key limiting factors, this study took 46 sampling sites from three typical T. kirilowii production areas in northwestern Hubei, the Jianghan Plain and southeastern Hubei Province as research objects. Thirteen physicochemical soil indicators were measured. Principal component analysis (PCA) combined with the Norm value was used to derive the minimum data set (MDS). Three scoring functions were compared to evaluate their applicability for soil quality assessment. The obstacle degree model was applied to identify the main limiting factors of soil quality, and partial least squares structural equation modeling (PLS-SEM) was used to explore the potential relationships associated with soil quality variation. The results show that an MDS composed of soil organic matter (SOM), alkali-hydrolyzable nitrogen (AN), bulk density (BD), electrical conductivity (EC) and total phosphorus (TP) can effectively characterize soil quality. Compared with linear standardization and membership function standardization, the non-linear function (SNL) standardization method performs the best (Ef = 0.78, ER = 0.05), and a soil quality index (SQI) calculated based on the MDS and total data set (TDS) shows a significant positive correlation (R2 = 0.851, p < 0.001). The average SQI of T. kirilowii planting areas in Hubei Province is 0.49, which is a medium level, and shows obvious spatial differences among different regions, in the order of northwestern Hubei (0.59) > southeastern Hubei (0.49) > Jianghan Plain (0.39). Obstacle degree analysis shows that organic matter and alkali-hydrolyzable nitrogen are the main limiting factors affecting soil quality, but there are obvious differences among regions. PLS-SEM analysis indicated that soil fertility-related factors were positively associated with SQI (path coefficient = 0.720, p < 0.001), whereas the soil’s physical and salinity-related factors showed negative associations with SQI (path coefficient = −0.259, p < 0.05). Altitude showed indirect associations with SQI through its relationships with soil properties, with a total effect of 0.480, and the model explained 85.7% of the variation in SQI. This study confirms that the constructed MDS performs comparably to the TDS in the present dataset for soil quality assessment in T. kirilowii planting areas in Hubei Province. Insufficient organic matter and alkali-hydrolyzable nitrogen were identified as the main factors limiting soil quality, suggesting that improving organic matter accumulation and nitrogen availability should be prioritized in soil management in T. kirilowii planting areas. This study can provide a theoretical basis for soil quality assessment, precise fertilization and sustainable soil development in T. kirilowii planting areas.