## 4 Discussion
### Assessment of Soil Fertility Quality Status  
In this study, five soil indicators were screened by principal component analysis, and finally, organic matter, total nitrogen, available potassium and available phosphorus and nitrogen were identified as MDS. The SQI model is constructed according to the determined MDS. The cumulative contribution rate of the MDS soil index constructed in this study was 69.63%. Correlation analysis showed that all soil indexes were significantly correlated with MDS soil index construction. Soil organic matter, available potassium, available phosphorus, nitrate nitrogen and total nitrogen play an important role in the construction of soil nutrient evaluation MDS [58]. This study also found that soil total nitrogen, hydrolyzable nitrogen and total potassium accounted for a larger weight in MDS, and their contribution to SQI was also greater, indicating that these indicators were important factors for evaluating soil fertility and productivity. Different factors were selected to participate in the evaluation of soil quality, resulting in slightly different key factors for the final evaluation. Soil quality assessment is sufficient to study specific land functions and land use types.  
In addition, the purpose of soil quality assessment is to determine the soil quality status by analyzing soil indicators, and then evaluate the yield potential of local wheat. Soil quality plays an important role in improving the environment and crop productivity. Although previous studies have used many methods to calculate soil quality, the relationship between SQI and crop biomass is less mentioned [59]. Therefore, their research may be lack of biological significance. In this study, there was a significant (\(p<0.01\)) positive correlation between wheat yield and soil quality index. Wheat yield increased with the increase in soil quality, indicating that the soil properties selected from the full data set had biological significance and could effectively evaluate the status of soil as a wheat production medium. The results of [PERSON] et al. [60] showed that the main reason for the difference in plant yield was the difference in soil quality, and plant yield was the embodiment of soil quality. [PERSON] [61] found that soil biological properties can better respond to soil quality changes, which is an important part of soil quality evaluation.  
According to the distribution of the SQI index, referring to the results of previous studies [62; 63], most of the soil nutrients in this study area were at a high level (0.54-0.74) or a low level (0.14-0.42). The results of [PERSON]'s index analysis showed that the nutrient contents were significantly different in space. The high-value areas of soil organic matter, total nitrogen, alkali-hydrolyzable nitrogen and available phosphorus were distributed in the central and northeastern parts of the study area, which was consistent with the spatial distribution of soil SQI index. The reason for the large fluctuation in soil fertility may be caused by uneven fertilization. The author's investigation found that the central part of the study area has abundant water sources, concentrated population density, and frequent human farming activities. However, the population density in the northwest and southeast is low, and there is a lack of standardized agricultural fertilization management, that is, the input of organic fertilizer and nitrogen fertilizer is insufficient, and the application of potassium fertilizer is less. Fertilization measures should be formulated to increase the input of organic matter, nitrogen fertilizer and potassium fertilizer in the area for a long time. In land use, it is necessary to strengthen the combination of use and nutrition, standardize the return of straw to the field, improve the soil nutrient content in the southeast and northwest of the study area, and prevent serious soil degradation caused by planting.