Agriculture Reference
In-Depth Information
removing the salinity through site-specific irrigation and leaching out of salts in
combination with adequate drainage. In case the salinity is mainly based on high lev-
els of exchangeable
sodium
, it is not only crop growth that is impaired, since in this
case soil tilth deteriorates as well. This is because sodium ions induce the soil particles
to deflocculate or disperse and thus promote soil crusts. The aim is to replace the
sodium with calcium and then to leach the sodium out. This can be enhanced by
applying calcium-sulfate, sulfur or sulfuric acid. The latter two chemicals help if free
lime is present in the soil, which then reacts with sulfuric acid to calcium-sulfate.
Irrespective of the particular situation, the first step is sensing the salinity in gen-
eral. The traditional method for this has been to measure the electrical conductivity
of a current, which passes through a
soil solution
that was extracted from a satu-
rated soil sample. This method is precise since it focuses on salinity and eliminates
or neutralizes the effects of texture or moisture within the soil. It is still used as a
reference method. Yet up to now this method cannot be applied in an on-the-go
manner, it is just used for soil samples in the laboratory. Hence for practical farm-
ing, this method can be ruled out when it comes to site-specific sensing with a high
spatial resolution on larger fields.
However, it has been shown at several places (Hendrickx et al.
1992
; Lesch et al.
2005
; McKenzie et al.
1997
; Rhoades et al.
1997
) that for practical purposes the
sensing of electrical conductivities of soil volumes is a suitable surrogate of solution
sensing in laboratories. This volume sensing can be done either by methods that use
contact electrodes (Sect.
5.2.1.1
) or via systems that employ electromagnetic induc-
tion (Sect.
5.2.1.2
). These techniques allow for on-the-go sensing with high spatial
resolutions. Correlations with varying texture and with changing moisture of soils
exist, but for many situations these do not alter much the indicated results in terms
of general salinity (Hendrickx et al.
1992
). This is because in most cases the salinity
has an overriding influence on the respective electrical conductivity (Table
5.2
).
Whether crops suffer from soil salinity depends on the respective species.
The yields of most crops are not much affected when salt levels in terms of
electrical conductivities are below 200 mS/m. Levels above 400 mS/m hurt
many crops and above 800 mS/m all but the very tolerant plants are affected
(Cardon et al.
2010
). This means that the field shown in Fig
5.10
presents seri-
ous problems of salinity for most crops, although the distinct local differences
call for site-specific ameliorations.
It must be expected that differentiating between the effects of salinity, texture
and water content on electrical conductivity gets more difficult with low salinity
levels. And even with higher salinity levels the signals needed for site-specific ame-
liorations might be more precise if
separating the effects
of soil constituents were
possible. A concept in this direction has been developed by Zhang et al. (
2004
) as
well as by Lee et al. (
2007
) and Lee and Zhang (
2007
). It is based on the fact that
the amount of total current flowing in a soil can depend on conductive- as well as on
capacitative behaviours of the soil. The methods of sensing by electrical conductiv-
ity as used hitherto use either direct current or alternating current with frequencies
well below 40 kHz. Electric current with these properties ensures that the conduc-
tive behaviour of soils dominates.
