Soil bulk density is a basic but important physic soil property related to soil porosity, soil moisture and hydraulic conductivity, which is crucial to soil quality assessment and land use management.  In this study, we evaluated the spatial variability of soil bulk density in the 0–20, 20–40, 40–60 and 60–100 cm layers as well as its affecting factors in Southwest China’s agricultural intensive area.  Results indicated the mean value of surface soil bulk density (0–20 cm) was 1.26 g cm–3, significantly lower than that of subsoil (20–100 cm).  No statistical difference existed among the subsoil with a mean soil bulk density of 1.54 g cm–3.  Spatially, soil bulk density played a similar spatial pattern in soil profile, whereas obvious differences were found in details.  The nugget effects for soil bulk density in the 0–20 and 20–40 cm layers were 27.22 and 27.02% while 12.06 and 3.46% in the 40–60 and 60–100 cm layers, respectively, gradually decreasing in the soil profile, indicating that the spatial variability of soil bulk density above 40 cm was affected by structural and random factors while dominated by structural factors under 40 cm.  Soil organic matter was the controlling factor on the spatial variability of soil bulk density in each layer.  Land use and elevation were another two dominated factor controlling the spatial variability of soil bulk density in the 0–20 and 40–60 cm layers, respectively.  Soil genus was one of the dominated factors controlling the spatial variability of soil bulk below 40 cm. 

Understanding soil nutrient distributions and the factors affecting them are crucial for fertilizer management and environmental protection in vulnerable ecological regions.  Based on 555 soil samples collected in 2012 in Renshou County, located in the purple soil hilly area of Sichuan Basin, China,  the spatial variability of soil total nitrogen (TN), total phosphorus (TP) and total potassium (TK) was studied with geostatistical analysis and the relative roles of the affecting factors were quantified using regression analysis.  The means of TN, TP and TK contents were 1.12, 0.82 and 9.64 g kg–1, respectively.  The coefficients of variation ranged from 30.56 to 38.75% and the nugget/sill ratios ranged from 0.45 to 0.61, indicating that the three soil nutrients had moderate variability and spatial dependence.  Two distribution patterns were observed.  TP and TK were associated with patterns of obvious spatial distribution trends while the spatial distribution of TN was characterized by higher variability.  Soil group, land use type and topographic factors explained 26.5, 35.6 and 8.4% of TN variability, respectively, with land use being the dominant factor.  Parent material, soil group, land use type and topographic factors explained 17.5, 10.7, 12.0 and 5.0% of TP variability, respectively, and both parent material and land use type played important roles.  Only parent material and soil type contributed to TK variability and could explain 25.1 and 13.7% of TK variability, respectively.  More attention should focus on adopting reasonable land use types for the purposes of fertilizer management and consider the different roles of the affecting factors at the landscape scale in this purple soil hilly area.