Comparison of Methods for Determining Bulk Densities of Rocky Forest Soils

Measurement of forest soil bulk density is often hampered by coarse fragments. In this study, five methods to determine total and fine bulk density and coarse‐fragment content of a rocky forest soil in western Montana were evaluated. Two methods of core sampling (small and large diameter cylinders), two methods of soil excavation and volume determination (water and polyurethane 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 from the nuclear gauge. The two excavation techniques gave similar results. Volumetric rock‐fragment content calculations using the small diameter cylinder were significantly lower than those using the other methods. At the 10‐ to 20‐cm depth, all methods except the large diameter cylinder gave comparable results for total soil bulk density. The small diameter core method gave the highest estimate of fine bulk density at this depth. All methods are easy to use. Soil excavation using the polyurethane foam for volume determinations is the simplest method and has low standard errors.