Assessment of Soil Fertility Quality and Driving Factors in Oasis Croplands of the Zhangye Basin, Northwest China

Soil fertility quality is a key indicator of soil productivity and sustainable agricultural development. The Zhangye Oasis, a major Maize seed production base in northwestern China, has experienced significant impacts from intensive human activities on soil conditions. In this study, fuzzy mathematics and structural equation modeling (SEM) were employed to evaluate the fertility characteristics of farmland soils (0–20 cm) in this typical oasis region. Our results showed that the mean values of soil organic matter (SOM), available nitrogen (AN), available phosphorus (AP), available potassium (AK), and pH in Zhangye Oasis farmland were 17.98 g·kg−1, 75.04 mg·kg−1, 22.22 mg·kg−1, 206.81 mg·kg−1, and 8.25, respectively. SOM and AN exhibited pronounced spatial heterogeneity, particularly in farmlands near the oasis margins, where nutrient contents were relatively low. The mean soil fertility quality index (SFI) was 0.64, corresponding to Grade VI farmland quality at the national level. Soil fertility showed moderate spatial autocorrelation, decreasing from southeast to northwest. Significant differences (p < 0.05) were observed among soil types, with wind-sand soil (SFI = 0.48) exhibiting the lowest fertility. The SFI followed the following order: no obvious obstacle (0.67) > saline–alkali (0.59) > irrigated improved (0.57) > sandy farmland (0.48). Continuous Maize cropping reduced SFI by 26.32% compared with the Other—Oilseed crops/Grazing cropping system. Overall, the oasis farmland exhibited relatively low soil fertility, dominated by Grade VI–VII levels. Improving the fertility of continuously Maize-cropped and irrigation-improved farmlands within the constraints of local water resources is crucial for ensuring sustainable agricultural production in the Zhangye Oasis.