Agriculture Reference
In-Depth Information
H
+
or NO
3
−
or K
+
etc
. ions. The main objective in the development of ion-selective
electrodes has been to find membranes that effectively prevent the passage of
unwanted ions and just let pass specific ions of mineral nutrients or of water for the
recording of soil pH.
Presently, ion-selective electrodes are used in numerous laboratories for analysing
soils, foods, water and for clinical chemistry. They are widely employed for measur-
ing the pH of soils either via portable handheld instruments or in a stationary mode
in laboratories. Despite this, the ability to precisely just let pass ions of the nutrient
that is to be sensed and to retain all other ions with other chemical formulas still is
a topic of efforts and research. The question is how well membranes - which consist
of various materials - are able to separate ions that have different formulas so that
interferences are avoided.
The
voltage output
of ion-selective electrodes generally is proportional to the
logarithm of the measured specific ion activity. This relation must be included in the
data processing program if it is targeted at the active ions.
As for site-specific fertilizing, the challenge is online and on-the-go sensing by
ion-selective electrodes. Up to now, these instruments cannot record in a continuous
mode in a similar way as implements that sense soil properties via electrical con-
ductivity do. The present procedure is to use on-the go operations, during which
periodically small samples of soil or mixtures of soil and water are prepared and
brought into contact with ion-selective electrodes. In order to avoid incorrect mea-
surements, the electrodes must be cleaned by rinsing with water following each
contact. The cleaning process and sometimes also the sample preparation necessi-
tate an
intermittent sensing operation
.
Selecting the best membrane for the respective ion is a crucial point. Due to the
many applications in various laboratories, ion-selective electrodes with a variety of
different membranes are commercially available. Some recommendations are to use
(Adamchuk et al.
2005
; Kim et al.
2007a
,
b
; Lund et al.
2005
):
•
electrodes with glass- or antimony membranes for sensing pH
•
electrodes with polyvinyl-chloride membranes for potassium and nitrate
•
electrodes consisting of cobalt rods for phosphates.
The polyvinyl-chloride membranes may be treated with special chemicals in
order to improve the performance. Electrode aging and mechanical wear due to
abrasion by soil still are problems, especially with membranes based on glass or
polyvinyl-chloride. For sensing of pH, the wear problem can be reduced by using
antimony instead of glass for the membranes (Adamchuk
2008
).
9.2.2
Sensing pH and Nutrients in Naturally Moist Soils
An important point is the preparation of the soil samples. The traditional procedure
of stationary sensing via ion-selective electrodes in laboratories has been to do this
based on
solutions
or
slurries
of soil samples. On-the-go operation from farm vehi-
cles is facilitated if
naturally moist soil
samples can directly be sensed. However,
