The approach used in this research, where a base solution is
used for both referencing the baseline and rinsing electrodes and
two solutions are contained in buckets for the two-point normalization,
could be used in an automated sensing system for hydroponic
nutrients in greenhouses. Modification of the existing
system for automated sample introduction, including periodic
sampling of hydroponic solution from the flow line of the recirculating
system as well as the addition of temperature measurement
and compensation, would minimize the problems with electrode
drift and sensitivity changes reported for previous sensing systems
(Bailey et al., 1988; Cloutier et al., 1997). However, one major
drawback to this ISE system is the lack of acceptable Ca and Mg
membranes. Future research should be directed toward identifying
detection elements for Ca and Mg ions in hydroponic solutions. In
addition, the use of a buffer solution should be investigated to obtain
more accurate concentration estimates of NO3 and K.