Abstract: 【Objective】The gravel mulched field is a unique method of tillage in arid and semi-arid areas of northwest under the natural condition with severe weather, poor soil and terrain condition, through the long-term production practice, continuous summarization and innovation. Preservation and improvement of soil fertility quality is a foundation for maintaining land productivity, preventing land degradation in the gravel mulched field. 【Method】A total of 110 soil samples (0-20 cm) were collected in a grid of 10 m×10 m. Soil indices were selected as evaluating factors including soil organic carbon, total N, total P, total K, available P, available K, pH and electrical conductivity. Correlation coefficient method was used to determine the weight value of soil fertility index. The curve type and turning point value of membership function were defined by soil characteristics and crop varieties. Then membership degree and weight value were combined to calculate the soil integrated fertility index (IFI). The geostatistics method was used for building the best semi variance function models, and analyzing spatial variation and distribution pattern of soil fertility. The evaluation of classification of soil fertility could be studied by the soil IFI and its spatial distributions. 【Result】The classical statistics indicated that the mean value of pH was 9.15 showed alkalinity, and the coefficient of variation was 0.01 showed medium variation. The coefficient of variations for other fertility indexes ranged from 0.10 to 0.72 showed moderate variation. The range of soil characteristics showed an obvious difference at field scale. The soil IFI in the study area was 0.18-0.59 with the average value of 0.34. The coefficients of nugget for fertility indexes were less than 25%, which demonstrated strong spatial dependence. The ordinary Kriging interpolation map indicated that the soil organic carbon distributed evenly with the mean value of 1.5-2.5 g·kg^-1, which had island high levels in the southern study area. The electrical conductivity also distributed evenly with the mean value of 100-300 μs·cm^-1, which had high levels in the northern study area. Total N, total P and total K had low levels with plaque distribution. The available P and available K were higher in the northern study area. The pH value also distributed patchily. The land of Grade Ⅲ indicated medium fertility condition and distributed most widely in the study area, covered 56% of the total area. The land of Grade Ⅱ indicated fecund fertility condition and represented the main type in the south of the land, covered 25% of the total area. The land of Grade Ⅳ accounted for 18% of the total area, which indicated poor fertility condition. The soil property indexes and IFI value demonstrated that soil fertility level was relatively low in the gravel mulched field. 【Conclusion】The spatial distribution patterns of soil index and IFI were complicated and had no regularity with patch distribution at field scale. According to the soil index and IFI, the levels of soil fertility were low in the gravel mulched field. The soil organic carbon and total N were the main limiting factors restraining soil fertility. The methods of increasing organic fertilizers, crop rotation and fallow were practicable and necessary for fertilizing soil, increasing yields and improving crop quality.

Key words: soil fertility, spatial variability, membership function, gravel mulched field, evaluate