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where SAND and CLAY are percents of sand and clay in the tested soils. The
index WP stands for wilting point as Wang and Schmugge [99] correlated the
transition water content with the wilting point water content.
Fig. 11.
Determination of the dielectric constant of soil solid phase using extrapolation and
three-phase soil dielectric mixing model α.
Further discussion concerning 4-phase models assumes:
− If θ > θWP
, the soil dielectric constant is represented by the 3-phase
models, that is the formula (27) for the α-model, and the formula (30) for
the de Loor model at θ bw =0.
− If θ ≤ θWP
, the soil dielectric constant is represented by the 4-phase
models, that is the formula (28) for the α-model, and the formula (30) for
θbw
the de Loor model, at θ bw = θ and ε bw = ε s + ( 80 , 2 − ε s
) , where ε s is
θ
the soil solid phase dielectric constant, according to (32) for α = 0,5.
5.2. Results and discussion
The relation between the soil dielectric constant, ε, and soil water content, θ,
is presented in Fig. 12. It comes that for water content below 0.3 the values
calculated from the empirical model (31) are higher than the measured ones. This
is particularly evident for samples with high clay content (Table 6, no. 10, 11 and
16), ie for clay content more than 60%. High clay content is positively correlated
with the amount of bound water surrounding soil solid particles (Or and Wraith
WP
37