Soil fertility evaluation and spatial variation of apple orchards in Zhaotong

【Objective】Soil nutrient management holds considerable significance for apple orchard productivity. However, the research on the spatial distribution characteristics of soil nutrients, along with the impacts of altitude and cultivation duration on nutrient dynamics and overall fertility, remains limited. This study assesses soil nutrient contents and fertility in apple orchards. It further analyzes the spatial distribution of these nutrients. The findings are anticipated to forge a theoretical foundation for town specific orchard management, facilitate the development of targeted fertilization strategies, and ultimately enhance soil nutrient management practices. Concurrently, this work seeks to advance ecologically sustainable and economically efficient apple production systems.【Methods】70 apple orchard sampling plots were established across Zhaotong region, with the quantity determined based on the planting area and distribution within each town. Soil properties, including pH value, organic matter (OM), hydrolyzable nitrogen (HN), available phosphorus (AP), and available potassium (AK), were measured. Soil fertility was comprehensively evaluated via integrated statistical analysis and the fuzzy mathematics meth- od. Moreover, geostatistical analyses were performed using software GS + 9.0 to fit variogram models and select optimal parameters. Kriging method was applied for spatial optimal unbiased interpolation, and spatial distribution maps were constructed using software ArcGIS 10.8.1.【Results】Soil pH in Zhaotong’s apple orchards ranged from 4.41 to 7.38, exhibiting acidic to neutral soil characteristics. Acidic and weakly acidic soils dominated, accounting for 81.43% of the total. The average contents of OM, HN, AP, and AK were 23.59 g · kg-1 , 105.38 mg · kg-1 , 53.19 mg · kg-1 , and 318.80 mg · kg-1 , respectively. Among them, OM, AP, and AK were present in abundant quantities, while HN exceeded the medium level. The descriptive statistical analysis showed significantly different maximum and minimum values of fertility indexes of pH, OM, HN, AP, and AK, indicating significant spatial variability and inhomogeneity among orchards. Besides, the variation coefficients of these five indexes were 11.59%, 25.69%, 41.62%, 74.22%, and 62.89%, respectively, belonging to the moderate variation level. Soil pH exhibited the smallest coefficient of variation, reflecting the lowest overall spatial variability and relatively stable characteristics of soil pH across Zhaotong’s apple orchards. The coefficient of variation of AP was the highest (74.22%), indicating that AP was most affected by environmental or human factors in the study area. Moreover, the semi- variance function analysis of soil fertility indicators using software GS + 9.0 showed that pH, OM, HN, AP, and AK were most suitable for Gaussian model. In the orchards of the study area, the nugget coefficients of soil OM, AP, and AK were greater than 0.5 and less than 0.75, and the spatial variability showed moderate auto-correlation. This indicated that the variation was affected by both structural factors and random factors, and the random factors were the main factors. The nugget coefficient of pH and HN exceeded 0.75, showing weak auto-correlation in space, indicating the dominant role of random factors in its spatial variation. Soil pH exhibited the largest range (9.144 km), indicating relatively uniform distribution of pH in a large spatial range. In contrast, the range of OM in the study area was the smallest (0.811 km), reflecting a narrow spatial correlation scope. This suggested that OM was strongly affected by random factors, and that the spatial distribution characteristics were more complex. The spatial distribution of fertility indexes was analyzed using software ArcGIS 10.8.1 ordinary kriging interpolation. Results revealed significant patchy characteristics of the spatial distribution of pH and HN, and the spatial distribution of OM, AP, and AK presented strip and patchy distribution. Meanwhile, there were significant differences in the soil fertility distribution of apple orchards in different towns. Integrated soil fertility analysis of the study area’s apple orchards revealed that the distribution range of soil integrated fertility index (IFI) was from 0.21 to 1.0, corresponding to a moderate intensity variation level. The fertile apple orchards accounted for 31.43%, which was distributed along the northwest to southeast of the study area. Moreover, analysis of soil comprehensive fertility indicated that 31.43% of the apple orchards achieved a fertile or higher fertility level, while medium fertility and lower fertility orchards each accounted for 34.29%. Significant spatial variability of soil fertility was observed in each town. Zhaoyang District concentrated 90.90% of high fertility apple orchards, and Ludian County was dominated by medium and low fertility levels. Orchards with comprehensive fertility reaching Grade I were primarily distributed in Leju in Zhaoyang District, followed by Yongfeng. Correlation analysis showed that IFI was significantly and negatively correlated with soil pH, while it showed significant and positive correlations with OM, HN, AP, and AK. Altitude was significantly and negatively correlated with OM, AP, and IFI. Tree age exhibited significant positive correlations with AP, AK, and IFI. As indicated by the path analysis, OM, HN, AP, and AK had significant effects on IFI, while AP exhibited the greatest direct and indirect effects on IFI. The effects of pH, altitude, and tree age on IFI were not significant.【Conclusion】Soil pH in this area was suitable for apple growth, and the overall fertility level of apple orchards was above medium. Soil spatial variation was affected by both random and structural factors, with random factors playing a dominant role. For soil management, it was necessarily important to strengthen orchard management awareness and practices in some towns, increase the application of organic matter and phosphorus fertilizers, supplement potassium fertilizers in a timely manner, and control nitrogen fertilizers.