METER TEROS 12 - Page 21

17
TEROS 12
Saturation extract EC is the EC of pore water removed from a saturated paste. Saturation
extract EC can be measured directly if distilled water is used to wet the soil until the soil
saturates. The soil is then placed on filter paper in a vacuum funnel and suction is applied.
An EC measurement on the water extracted from the saturated sample will give the
saturation extract EC.
Theoretically, the pore water EC and saturation extract EC are related by the degree of
saturation (VWC/total porosity) of the soil.
An example calculation illustrates this relationship:
Example A soil is at 0.1 m
3
/m
3
VWC, has a pore water EC of 0.7 dS/m, and a dry bulk density
of1.5Mg/m
3
The total porosity of the soil can be calculated using Equation 3:
Equation 3
φ =
ρ
b
ρ
s
= 1−
1.5
2.65
= 0.43
The saturation extract EC can be calculated as illustrated in Equation 4:
Equation 4
Solution EC =
σ
p
Θ + σ
p
φ −Θ
( )
φ
=
0.7 0.7 + 0
( )
0.43
= 0.162 dS/m
where

is the porosity,

b
is soil dry bulk density,

s
is density of the minerals (assumed to be 2.65 Mg/m
3
),

d
is the EC of distilled water (0 dS/m), and

is VWC.
In practice, solution EC calculated from this method and solution EC taken from a laboratory
soil test may not agree well because wetting soil to a saturated paste is very imprecise.