Agriculture Reference
In-Depth Information
layer (Sharma and Gupta
2010
). Irrigation water that ends up in deeper soil lay-
ers remains largely unconsumed by crops and might simply seep down, taking
with it dissolved agrochemicals that pollute underground water. Consequently,
a continuous monitoring of moisture in the top soil layer combined with con-
trolled irrigation can save water and avoid unwanted leaching of minerals and
pesticides.
Presently feasible methods for site-specific and on-the-go sensing of the water
situation in bare fields rely either on electrical capacitance, on electrical permittiv-
ity or on infrared radiation. The latter method is based on
soil surface
sensing
whereas electrical capacitance and -permittivity are measured within
soil volumes
.
These volumes may be restricted, yet principally provide information from three
dimensions instead of only from the two dimensions of surfaces. Capacitance
methods use signals obtained from electric current flow, hence from electrons. And
permittivity methods rely on recording of electromagnetic radiation - thus photons
- from microwaves or radar waves.
Principally,
electrical capacitance
is the ability of a body to hold an electric
charge It is a measure of the amount of electrical energy stored or separated for a
given electric potential,
e.g.
in a parallel-plate capacitor or in a given soil volume.
Since electrical charges are expressed in units of coulombs and electric potentials in
units of volts, the capacitance has the SI unit of a coulomb per volt, which is defined
as a unit of farad (F).
In contrast to capacitance, the
electrical permittivity
is the ability to resist the
formation of an electromagnetic field, in this case in the soil. In other words, it is a
measure of how an electromagnetic field affects the surrounding - and is affected by
it. Thus permittivity relates to the ability of a material to “permit” an electromag-
netic field. Important is that the permittivity ε is the sum of a real part and an imagi-
nary part.
The
real part of the permittivity
is associated with storage of electrical energy
and thus with the capacitance of a material when an alternating electrical field is
applied. In fact, the real part of the permittivity can be obtained from the capaci-
tance in farad by dividing it by the overlap area of the capacitor plates and by mul-
tiplying it by the separating distance of these plates. Consequently, from the
dimensions involved, it follows that permittivity is expressed in farad per m.
However, this is the absolute permittivity of a material. In most cases, this absolute
permittivity is replaced by the relative permittivity. This relative permittivity repre-
sents the absolute permittivity divided by the permittivity of a vacuum or of air,
which equals 1 (one). This means that the numerical values for the absolute- and
relative permittivities are identical. The difference is that the relative permittivity is
dimensionless and because of this, it often is denoted as the
dielectric constant,
although it is not a real “constant”.
The
imaginary part of the permittivity
is associated with energy dissipa-
tion, it is therefore often denoted as dielectric loss. There are applications where
this energy dissipation is the main objective,
e.g.
when foods are heated in a
microwave oven.
