Agriculture Reference
In-Depth Information
360
Kohl farm,
Coachella valley, California
m
270
average salinities
within 0.0 - 1.2 m depth
180
> 1200 mS/ m
1000 - 1200 mS/ m
800 - 1000 mS/ m
< 800 mS/ m
90
0
0
95
190
285
m 80
distance x on the headlands
Fig. 5.10
Map of soil salinity based on electrical conductivities in the rootzone of a field in
California, USA (From Rhoades et al.
1997
, altered)
The situation is quite different if alternating current with frequencies in the MHz
range is used. Under these circumstances, the capacitative behaviour of soils
becomes more prominent. Hence, if the sensing is done successively by different
frequencies within a sufficiently wide range, signals of both the conductive- and the
capacitative characteristics of soils can be obtained. The electric conductivity pri-
marily provides signals about the salinity, whereas the capacitative behavior of soils
predominantly depends on the respective water content. Thus principally differenti-
ating between salinity and moisture of soils is possible. For further details about soil
moisture sensing see Sect.
5.2.3
.
5.2.2.3
Electric Conductivities and Crop Yields in Humid Areas
The economics of farming depend largely on crop yields. These can be regarded in
a retrospective or in a perspective. Site-specific techniques for retrospective views
on yields are dealt with in Chap.
12
. Both views have to deal with a multitude of
factors that influence the yields, such as the weather in various stages of crop
development, soil and crop properties as well as techniques and practices used for
irrigation, cultivation, sowing, crop protection and harvesting. This itemization
shows that the indication of crop yields by soil electrical conductivities can - at
most - be a partial one. But since maps about soil conductivities are becoming
standard facilities for site-specific farming, their usefulness for predicting yield
potentials should be known. After all, the local yield potential - if it can be derived
a priori
- can help to define the adequate site-specific input of seeds, irrigation
water and agrochemicals.
