Agriculture Reference
In-Depth Information
Table 5.1
Methods of sensing soil properties on volume bases via electricity or via radiation
Frequencies
Wavelengths
a
Sensing objectives
On-the-go use
Electrical conductivity, contact methods
0-1 kHz
Infinite - 300 km
Texture, water, salinity, soil-layers
State of the art
Electrical conductivity, electromagnetic induction methods
0.4-40 kHz
750-7.5 km
Texture, water, salinity, soil-layers
State of the art
Electrical capacitance
40-175 MHz
790-200 cm
Water
Possible
Time domain reflectometry
50-5,000 MHz
600-6 cm
Water
Not yet possible
Soil penetrating radar, surface reflection mode
0.5-30 GHz
0.60-1 cm
Water
Possible
Micro- or radarwaves, mainly satellite based sensing
0.3-30 GHz 100-1 cm Water, roughness of soil surface Does not apply
Compiled from data by Allred et al. (
2008
); Corwin (
2008
); Lesch et al. (
2005
) and Lueck et al. (
2009
)
a
From higher range- to lower range limits, thus corresponding to frequencies
This distinction between volume sensing on the one hand and surface- or area
sensing on the other hand is necessary since not all electromagnetic waves are
able to penetrate the soil. Especially visible and infrared light only provide sig-
nals that are based on the soil surface that was hit. The situation is different for
microwaves, radar waves as well as for electric currents and its electromagnetic
waves. Some criteria that refer to
volume sensing techniques
are presented in
Table
5.1
.
Among the alternatives listed there are two techniques that can operate online as
well as on-the-go and already have been introduced widely into practical farming:
electrical conductivity
- and
electromagnetic induction
sensing. These methods
operate either with direct current or with alternating current on the lowest fre-
quency end and consequently with long waves. The soil properties that can be
derived from the signals of these methods are not without ambiguity. This will be
dealt with later.
Electrical capacitance
sensing operates in medium ranges of
frequencies and wavelengths (Table
5.1
) and depends on the soil water content.
This method can be used online and on-the-go as well, but up to now seldom is
state of the art in practical farming.
Time domain reflectometry
is based on radar- or
radio frequency signals that are guided along a transmission line or cable that is
embedded in the soil. Therefore the denotion often is “cable radar”. The velocity
of wave propagation depends on dielectric soil properties and thus can indicate the
soil water content, but not yet for on-the-go operations. The situation is different if
soil penetrating radar
is used in a surface reflection mode for water sensing. This
method in principle is suited for on-the-go sensing, but despite this up to now
hardly is used in farming. The same applies to soil water sensing by
micro- or
radarwaves from satellites
, a method that at present can be excluded for online
and on-the-go control of farm operations, but might become very useful for tactical
inspections of wide areas.
