8 THEORY: WATER POTENTIAL WP4C
that are near saturation (like many soil samples). A sample that is
close to 0.00 MPa and is only slightly warmer than the sensor block
will condense water within the block. This will cause errors in the
measurement, and in subsequent measurements until the condensa-
tion disappears. The Ts-Tb function helps the user ensure that the
sample will not condense on the sensor block.
8.4 Estimating Osmotic Potential
The WP4C measures the sum of osmotic and matric potential. An
approximate value for the osmotic potential can be found by mea-
suring the electrical conductivity (EC) of the saturation extract of
the soil. The osmotic potential of the saturation extract is computed
from:
Ψ
os
(MP a) = −0.036EC(dS/m) (2)
The osmotic component of the water potential is then computed
from:
Ψ = Ψ
os
(
θ
s
θ
) (3)
where θ is the volumetric water content of the sample and θ
s
is the
volumetric water content at saturation. The matric potential is the
total potential minus the osmotic.
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