Volume and mass relationships of soil constituents

Fig: Schematic diagram of the soil as a three phase system

Where, M_w= Mass of water

            M_s= Mass of soild

            M_t= Total mass (Solid+ liquid)

            V_a= Volume of air

            V_w= volume of water

            V_s= Volume of solid

Some Formulas:

a. Density of solid ( True Specific gravity/ Real specific gravity/ particle density):

• It is defined as ratio of mass of solid to its volume.

ƍ = M_s / V_s. ƍ_w [ ƍ_w= density of water ˜˷ 1]

Here, density is the mass per unit volume of the soil particle.

b. Bulk density of soil ƍ_b or D₀

• It is defined as the ratio of the mass of oven dry soil (or dried particles) to the total volume of soil.

i.e. ƍ_b = M_s/ V_t = M_s/ (V_a+V_w+ V_s)

Note: The term dry bulk density and apparent specific gravity are often used synonymously whereas the term apparent specific gravity denotes a dimensionless quantity. Bulk density is expressed in gm/cm³ or mass per unit volume. The two terms have equal numerical values.

c. Porosity:

• It can be defined as the ratio of the volume of pores (voids) to the total soil volume.

i.e. n = V_f/ V_t = (V_a+V_w)/ (V_s+ V_a+V_w)

d. Soil wetness: The relative water content in the soil.

e. Mass wetness: It is commonly called soil moisture content.