Agriculture Reference
In-Depth Information
Table 5.3
Correlation between soil properties and electrical conductivities
Corr. coeff. squared (r
2
)
to soil profile-averages
Soil moisture 0.50 0.24
Clay 0.66 0.72
Silt 0.30 0.28
Cation-exch. cap. 0.70 0.70
The horizontal surface-layers and the vertical soil profiles denote the places where
the reference samples were taken (averages for 12 fields in the North-Central USA,
extracted from Sudduth et al.
2005
)
Corr. coeff. squared (r
2
)
to surface-layer soil
Soil properties
clay- and silt content are defined, at least partial autocorrelation for the silt still
applies, when the effect of a high clay content is known.
It is this partial autocorrelation to the effect of clay that probably explains the
contradictory results about the influence of the silt content on the electrical conduc-
tivities (Fig.
5.8
, bottom). The depth weighted clay contents in the field Frankenforst
are much higher than those in the field Dikopshof. The misleading effect of autocor-
relation thus probably was much higher in Frankenforst than in Dikopshof. In short,
the results shown in Fig.
5.8
, bottom, might be uncertain.
An important question is the respective impact of clay on tbe one hand and water
on the other hand on electrical conductivities. Table
5.3
shows summarized results of
extensive sensing with the presently dominating systems either by contact methods
(Fig.
5.3
) or by induction (Fig.
5.4
). The depth of sensing corresponded to the
deep
response curves
in Fig.
5.6
. Since with well adjusted implements the records for the
contact- or induction methods are similar if the depth that is sensed is about the same,
such results were pooled. Effects of salinities on the signals probably can be ruled
out, since these are based on areas with humid climates. The results are presented
separately for reference samples taken from surface-layers (topsoils) and for samples
that came from vertical soil profiles that include the effects of respective subsoils.
For both cases, the influence of the soil moisture on the electrical conductivity
is lower than the effect of the clay content (Table
5.3
). The influence of the silt too
is rather low. The correlation to the site-specific cation-exchange capacity (CEC)
is on a similar level with the clay content. This is in line with basic expectations
since it is the clay particles in combination with organic matter bonded to them that
mainly provide the cation-exchange capacity. Moreover, the ions are the carriers of
the conductivity.
Yet the
ions need water
to function as carriers. Whereas the texture and
organic matter in the soil might vary spatially but remain temporally constant,
the soil moisture changes on a time basis as well. Hence the question arises,
should the sensing be done when the moisture is at a low-, at a medium- or at a
high level. Whereas a low level might reduce the temporary variations of the
signals since it decreases the influence of a transient factor, a high level princi-
pally promotes the current flow.
The electrical conductivities in Fig.
5.9
are based on sensing of several loamy
soils in a vertical induction mode, thus with a depth of approximately 1.5 m (Fig.
5.6
).
