Agriculture Reference
In-Depth Information
12
10
8
6
y = 2.31 + 0.10 x − 0.001 (x − 60.31)
2
4
r
2
= 0.71***
(n = 343)
2
20
40
60
80
100
120
electrical conductivity in mS/ m
Fig. 5.11
Electrical conductivity sensed in an induction mode with a depth of about 1.5 m and
yields of wheat in Germany (From Neudecker et al.
2001
, altered)
Traditionally, the yield potential of soils is attributed to its texture, particularly to
its clay content. This holds especially for the topsoil. The clay and its bonds with
organic matter affect the water holding capacity as well as the hydraulic properties
and provide for the cation-exchange-capacity that is needed to store plant minerals.
However, there are limits to the
agronomic benefits of clay
. Very high clay contents
can be detrimental because this can reduce water permeability, inhibit deep drainage
and consequently lead to waterlogged soil conditions. This applies in general for the
whole soil depth that the plants roots penetrate and even some depth below this. Yet
this holds particularly for the subsoil, since the latter hardly is cultivated and in
addition is less penetrated and thus not loosened by the roots of crops.
It should be realized that sensing of the water situation in soils by means of
electrical conductivity is done in a twofold manner,
i.e.
directly since water is a
carrier of ions and indirectly via the effect of clay on the water regime.
When yields and electrical conductivities are compared on a site-specific basis,
rather unambiguous results can be expected provided the conductivity varies distinctly
within the field and the sensed subsoil has no hydromorphic layers or claypans.
Figure
5.11
shows the situation for a field, in which the electrical conductivities
vary along a rather wide range. The diagram shows two typical criteria for the rela-
tion between site-specific electrical conductivites and yields: firstly a rather wide
spread of the data and secondly on the average a change from a positive- to a nega-
tive influence on the yield at a high level of conductivity. The first criterion is the
result of the many factors that affect yields. And the second characteristic can be
explained by the fact that beyond a certain clay level in the soil, its effect on the
yield is reversed (see above).
