Geology Reference
In-Depth Information
Worked example 5.1 - soil water and air contents, and bulk density
A soil core of 5 cm diameter and 8 cm height contains 257 g of fresh
soil. After drying the soil at 1101C, the soil weighed 196 g. Calculate
(i) the gravimetric soil water content, (ii) the volumetric soil water
content, (iii) the soil bulk density, (iv) the % pore space, (v) the %
water-filled pore space, and (vi) the % air-filled pore space.
Density of water
¼
0.997 g cm
3
Volume of soil core
¼
p.r
2
.h
¼
3.14
(2.5)
2
8
¼
157 cm
3
(i) Gravimetric soil water content
¼
[(257-196 g)/196 g]
100
¼
31%
(ii) Volumetric soil water content
¼
[(257-196 g)/(0.997 g cm
3
157
cm
3
)]
100
¼
39%
(iii) Soil bulk density
¼
196 g/157 cm
3
¼
1.25 g cm
3
Assuming a soil particle density of 2.6 g cm
3
, the volume of particles
in the soil core
¼
196 g/2.6 g cm
3
¼
75.4 cm
3
(iv) % pore space
¼
[(157 cm
3
75.4 cm
3
)/157 cm
3
]
100
¼
52%
(v) % water-filled pore space
¼
volumetric water content
¼
39%
(vi) % air-filled pore space
¼
52%
39%
¼
13%
What is the weight of this soil in 1 ha to a depth of 20 cm?
1ha
¼
10,000 m
2
, therefore volume of soil to 20 cm
¼
10,000 m
2
0.2
m
¼
2000 m
3
Bulk density of soil
¼
1.25 g cm
3
R1.25 t m
3
2000 m
3
1.25 t m
3
¼
2500 t of soil.
Table 2 Determination of soil water and air contents and bulk density
Gravimetric soil water content
(%)
(Mass of water/mass of oven dry soil)
100
Volumetric soil water content
(%)
(Volume of water/volume of soil core)
100
Soil bulk density (g cm
3
)
Mass of oven dry soil/volume of soil core
Mass of oven dry soil solids/volume of soil solids
Soil particle-specific gravity (g
cm
3
)
Pore space (%)
((Soil core volume-volume of soil particles in
core)/soil core volume)
100
Solid material (%)
(Volume of soil particles in core/total core
volume)
100
Water-filled pore space (%)
Volumetric soil water content
Air-filled pore space (%)
% pore space-% water-filled pore space
