Difference between soil Water Holding Capacity and soil pore volume?
As Friddy says, if it's not constrained and relatively intact you're going to have issues with pore volume calculations. Generally the saturate-and-dry method, using a soil core, will give you both the water mass and the mass of sediment. Then you'd use the average density of soil (which will depend somewhat on the minerology, and if it's got lots of organic matter--which can be calculated by burning off OM in a 400 C oven) to get at the particle volume. The known volume of the core minus the estimated particle volume would give you the pore volume.

Googling bulk density and particle density should find lots of info on how exactly to do this calculation.