Soil bulk density is the ratio of the mass of dry solids to the bulk volume of the soil. The bulk volume includes the volume of the solids and of the pore space. The determination of bulk density consists of drying (105°C) and weighing a soil sample, the volume of which is known (core method) or must be determined.
Blake, G. R. and Hartge, K. H. 1986. Bulk density. p. 363-375. In: A. Klute et al. (ed.) Methods of soil analysis: Part 1: Physical and Mineralogical Methods. Monograph Number 9 (Second Edition). ASA, Madison, WI.