Agriculture Reference
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But the plant available orthophosphate ions do. These anions stand at the far end
of the Hofmeister series, they are strongly hydrophilic. Hence these anions are
heavily hydrated. And in turn, the consequence of this is that their electric potential
largely has been taken away by hydrogen ions, and thus their free energy is rather
small. This makes the sensing of phosphate ions more difficult compared to those
from hydrogen-, potassium- and nitrogen, however, not impossible (Kim 2006 ).
A second prerequisite are suitable ion-selective electrodes. The general design
is as outlined in Sect. 9.2.1 , but differences exist in the treatment of the membranes
with special chemicals in order to improve the ability of separating specific ions
(Kim et al. 2006 , 2007a , b , 2009 ). The need for frequent calibrations and the wear
of some membranes must be considered.
A third prerequisite are appropriate extractants that provide for the targeted
ions in slurries or solutions from the soil sample. The simplest situation is when
water can be the extractant and the content within a naturally moist soil suffices.
This situation holds for sensing of the soil water pH. For nitrate, the situation is that
water too can be used as an extractant, however, some is added to naturally moist
soil to create a slurry. Kim et al. ( 2007b ) searched for an extractant that can be
applied to simultaneously sense the ions of the three macronutrients nitrate-N,
plant available phosphate (orthophosphate) and plant available potassium. The
Kelowna extractant - a mixture of acetic acid and ammonium fluoride - can be used
simultaneously for all three macronutrients. This solution is employed as a multi-
ion extractant in laboratories in British Columbia. It should be noted that the simul-
taneous sensing of different ions implies that the electrochemical cell is provided
with a separate ion-selective electrode for every nutrient.
A fourth prerequisite is that there should be no interference between different
ions. Theoretically, the composition of the ion-selective electrodes or its membranes
aims at excluding any interference. However, there still are limits in this respect.
Figure 9.9 shows the situation for simultaneous sensing of the three macronutrients.
The respective ion-selective electrodes that were used complied with those men-
tioned in Sect. 9.2.1 and their membranes - when necessary - were prepared with
suitable ligands. The Kelowna solution (see above) was used for extracting the ions.
The results are based on 37 different soils located in Illinois and Missouri, USA.
For each of the three graphs in Fig. 9.9 , the respective primary ion that is to be
sensed is on the axis along the bottom. In case of cation sensing (Fig. 9.9 , bottom),
the voltage rises when the concentration increases - as must be expected. Contrary
to this, for the anions, the voltage mainly goes down (Fig. 9.9 , top). For the elec-
trodes that pick up nitrogen and potassium as primary ions (top left- and bottom
graph), there was no interference by phosphate ions. Furthermore, the nitrate and
potash electrodes were not sensitive to potassium and nitrate ions respectively (these
graphs are not shown). But the responses of the phosphate electrode were influ-
enced by concentrations of nitrate ions (graph top right) as demonstrated by the
changing colour. Even more important probably is the fact that the voltage induced
by the phosphate ions did not change unidirectionally when the concentration
increased. Simultaneous sensing of several macronutrients in electrochemical cells
in an on-the-go mode without doubt is worth to strive for, but it is not yet state of the
art in precision farming.
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