Agriculture Reference
In-Depth Information
Between and within these aggregates exists a network of spaces called pores. Total
soil porosity is defined by the ratio
Volume of pores
Volume of soil
=
Porosity
Asoil'sAhorizon,containingorganicmatter,typicallyhasaporositybetween
0.5and0.6cubicmeterpercubicmeter(m 3 /m 3 )—alsoexpressedas50%to60%.
In subsoils, where there is little organic matter and usually more clay, the poros-
ityistypically40%to50%.Box4.1describesasimplewayofestimatingasoil's
porosity.
Box 4.1 Calculating Soil Porosity
Asimpleequationforcalculatingporosityis
ρ
ρ
Porosity =−
b
(B4.1.1)
1
p
Inthisequation, ρ p (rho p )istheaveragedensityofthesoilparticles,assumed
tobe2.65Mg(megagram)perm 3 . he term ρ b (rho b )isthesoil'sbulkdensity,
which ranges from less than 1 Mg/m 3 for soils rich in organic matter to 1.0 to
1.4 Mg/m 3 for well-aggregated loamy soils, and 1.2 to 1.8 Mg/m 3 for sands and
compactedsubsoils.husfortheAhorizonofaloamysoilwitha ρ b value of 1.33
Mg/m 3 , we have
133
265
.
.
(B4.1.2)
Porosity =− =
1
05
.
that is, the porosity is 0.5 m 3 /m 3 or50%.
To measure a soil's bulk density, take five to six intact cores with steel
cylinders, preferably at least 5 cm in diameter and 6 to 10 cm deep. Trim the soil
so that the dimensions of the soil core are the same as the cylinder and the soil
volume is easily calculated. Dry the cores in an oven at 105ºC and weigh each
coretoobtaintheweightofoven-dry(o.d.)soil.hebulkdensity ρ b of each core is
calculatedfromtheequation
ρ b = Weight of o.d. soil in the core
Volume of soil core
(B4.1.3)
Calculate the average value of ρ b for the several samples taken and substitute
thisintoequationB4.1.2.Notethatthesoilbulkdensityinthevinerowsand
mid-rows is likely to be different because of compaction by wheeled traffic in the
latter.
 
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