For soil analysis purposes, gravel (or coarse fraction) is defined as any mineral particle that is larger than 2 mm in diameter. As these particles are recognised as having little to no effect on the physical, chemical and biological functioning of the soil, they are removed prior to analysis (i.e. % organic carbon and nutrients are typically assessed on a less than 2 mm component). Therefore, if your soil contains a significant amount of gravel, this must be accounted for before multiplying up using bulk density, otherwise you will overestimate quantities on an area basis. Adding the gravel fraction back in after soil analysis will ?dilute? certain results per unit area (e.g. total organic carbon, nitrogen and phosphorus).