Porosity
Its the ratio of total volume of pores over the total volume of soil.
The volume of pores is studied through the following summation, the sumation is the volume of gas pores and liquid pores
The bottom figure gives a descriptive image showing the three volume regions in a studied soil sample, these are the following, these are the soild volume, the liquid volume and finally and the gas volume. The brown volume represnts the solid material while the dark blue volume represnt the liquid then the gas volume is a light blue.
The strength of the porosity parameter comes from that it can give an indication on how the structure of the soil sample is acessiable to the fluid side, the bulk volume is always constant, while what changes is the gas and liquid constituants.
Good Read
This paper gives you the required numbers to understand particle size distribution in soils:
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