Physical degradation characteristics and mechanism of orchard soil in Weibei Region

【Objective】 Apple production contributes to the economy in Shaanxi province greatly. But soil quality of the orchard land has degradated greatly after years of fruit production. That may has led to many issues such as trees weakening, fast aging and reduced resilience. Specifically the apple tree valsa canker and leaf defoliation diseases occur more frequently, and fruit yield and quality decreases greatly. The objective of this study was to investigate the characteristics, mechanism and degree of soil physical degradation in different aged orchards, to provide scientific basis for orchard soil management and eventually to improve the yield and quality of apple production. 【Methods】 Four replicates of 10-, 10-20-and 20-year old orchards were selected for the study. Soil samples within two-thirds of the radius of the tree canopy projection to the trunk were taken. Soil samples were used to measure physical properties such as soil bulk density, compaction, porosity, saturated hydraulic conductivity, and clay content. Comparison was conducted between the orchard and adjacent similar farmland soils. 【Results】 Soil bulk density and compaction increased with orchard age and soil depth. Especially at 20 cm soil layer, soil bulk density reached 1.45-1.61 g/cm3, compaction reached 933-2433 kPa. Porosity of the soil profile in 0-20 cm soil layer remained 50%, and the soil structure was in good condition. However, soil porosity reached 40%-46% in the 20-60 cm soil layer, which was in a state of compaction and severe compaction. Soil saturated hydraulic conductivity decreased even in the surface layer as the orchard aged. In the 10-20 and 20-year-old orchards, soil saturated hydraulic conductivity in the subsurface declined to 46.88 and 20.89 cm/d, reducing the infiltration of the rainfall and the capacity of soil water storage. Soil clay content increased with the depth of soil profile. Clay content at the 0-30 cm depth decreased with increasing orchard age but increased below the 30 cm layer. Further analysis found that the clay content was significantly correlated with soil bulk density, compactness, and porosity. Using packing density as index to evaluate the degree of compaction in orchard soil, the result showed that the soil packing density of orchards was above 1.40 g/cm3 underneath 20 cm depth in Weibei region. The orchard soils in this area have reached the moderate degree of compaction.【Conclusions】 The main characteristics of soil physical degradation of orchards in Weibei Region were reduced soil porosity, increased soil bulk density and compaction, and decreased soil saturated hydraulic conductivity. The main processes and mechanism of orchard soil degradation are clay translocation and accumulation of clay at deep soil. Reduced plow and aeration on soils is the main external cause to anabatic dominanted soil degradation. This is reflected with less soil aggregates.