Comparison of methods for determining bulk densities of rocky forest soils

Measurement of forest soil bulk density is often hampered by coarse Fragmen ts. In this study, five methods to determine total and fine bulk density an d coarse-fragment content of a rocky forest soil in western Montana were ev aluated. Two methods of core sampling (small and large diameter cylinders), two methods of soil excavation and volume determination (water and polyure thane foam), and a nuclear source moisture gauge were tested at two depths (0-10 cm and 10-20 cm) on a soil with a 35% slope and 45% rock content. In the surface 10 cm, total and fine soil bulk density values were greatest fr om the nuclear gauge. The two excavation techniques gave similar results. V olumetric rock-fragment content calculations using the small diameter cylin der were significantly lower than those using the other methods. At the 10- to 20-cm depth, all methods except the large diameter cylinder gave compar able results for total soil bulk density. The small diameter core method ga ve the highest estimate of fine bulk density at this depth. All methods are easy to use. Soil excavation using the polyurethane foam for volume determ inations is the simplest method and has low standard errors.