Agriculture Reference
In-Depth Information
Box 3.4
Useful Calculations Involving Soil Bulk Density
We may expand equation 3.1 to write
Volume of soil
Volume of soil solids
Porosity
(B3.4.1)
Volume of soil
Mass of o.d. soil
Volume of soil solids
1
Porosity
(B3.4.2)
Volume of soil
Mass of o.d. soil
BD
1
Porosity
(B3.4.3)
p
Using equation B3.4.3, we can show, for example, that a soil of
BD
1.33 Mg/m
3
and particle density 2.65 Mg/m
3
has a porosity of 0.5 m
3
/m
3
or 50%.
Laboratory soil analyses are normally reported on a unit mass basis (e.g.,
“available P” in
g P/g soil). However, such measurements often need to be
related to the field, where units such as kg P/ha (or lb/ac) are more appropriate.
Implicit in this field unit is a volume of soil, usually measured to a depth of 15 cm
(6 in.), which approximates the cultivation depth. Laboratory measurements per
unit mass can be translated to a field basis by calculating the mass of soil in 1 ha
to a depth of 0.15 m, using the soil
BD
.
For example, for a soil of
BD
1.33 Mg/m
3
, the mass
M
of o.d. soil per ha-
0.15 m is
M
(ha-0.15 m)
1995 Mg/ha (B3.4.4)
The use of this relationship in equation B3.4.4 is demonstrated in subsequent
chapters. The conversion to American units is given in appendix 15.
1.33
0.15
10
4
• The
volumetric water content
(
v
or theta v) is given by
Volume of water
v
(3.4)
Volume of soil
Box 3.5 shows the relationship between
g
and
v
.
Air-filled Porosity
Air fills the soil pore space not occupied by water. The air-filled porosity
3.3.3
(ep-
silon) is defined by
Volume of soil air
(3.5)
Volume of soil
from which it follows that
Porosity
(3.6)
When a soil drains, after being thoroughly wet by rain or irrigation, the largest
pores empty quite rapidly, usually within 2 days, after which the drainage rate
