Agriculture Reference
In-Depth Information
Table 5.2
Soil types and electrical conductivities
Soil texture class or influence of salt
Electrical conductivity in mS/m
Sand
0.1-1.0
Loamy sand
1.0-5.0
Loam
5.0-12.5
Silt
12.5-25.0
Clay
25.0-100
Saline soil
>100
From Bevan (
1998
), simplified and altered
temporal differences for the situation of soil layers on the one hand and water flow
on the other hand. The soil layers hardly change over time, but the water flow does.
Hence sensing the preferential water flow will depend on
change detection
by
means of repeated recordings over time. And reliable signals probably will need
specific electrical frequencies as well.
Future experience will have to show, how much vertical
horizon sensing
is
needed and for which cases simpler procedures of uniform
volume sensing
with a
depth that approximates the maximal vertical root length is sufficient.
5.2.2
Electrical Conductivities, Soil Properties and Yields
The sensing results in units of electrical conductivity partly depend on the tempera-
ture of the soil and of the measuring devices. To eliminate this effect, an adjustment
according to the prevailing temperature is necessary. This applies especially when
conductivity is sensed via electrical induction. Provided the implements are well
calibrated, properly adjusted and are sensing the same soil depths, the results for the
common systems - the contact method (Fig.
5.3
) as well as the induction method
(Fig.
5.4
) - are very similar (Sudduth et al.
2003
,
2005
). Therefore, the results will
be dealt with
in cumulo
.
The sensed soil properties can be divided up on the one hand in fairly
static
properties
such as texture and organic matter content and on the other hand in
dynamic properties
that vary in time as
e.g.
water content. Generally, soil texture
can be regarded as the most static property. Its influence on the electrical conductiv-
ity is listed in Table
5.2
.
Within the texture classes,
clay
exerts a dominating influence. Contrary to this,
the effect of sand theoretically is close to zero. But in reality it is not since an
autocorrelation exists between the contents of sand and clay in a soil. Because the
higher the content of sand is, the lower the shares of clay and other soil constitu-
ents become.
The pre-eminent influence of clay results from several facts. Firstly, the clay
fraction has the highest ion exchange capacity of all texture classes. In the
absence of electrical conductors like metallic materials with movable electrons, the
