Field Study Correlating Nutrient Absorption and Transpiration in Lettuce Hydroponics Using an IoT-Interfaced Solid-State Ion Sensor Array

The letter uses Internet-of-Things (IoT)-interfaced miniaturized ion sensors to monitor element-specific nutrient information during hydroponic cultivation in a greenhouse. The sensor provides insights into crop nutrient absorption efficiency and alterations in specific ion concentrations in response to vapor pressure deficit (VPD) fluctuations. Solid-contact ion-selective electrodes were fabricated into an ion sensor array and interfaced with wireless embedded systems to construct an IoT nutrient sensor system (IoNSS). The IoNSS framework included two different microcontrollers for nutrient solution sampling, sensing module controlling, automated procedure setting, and data recording: a private cloud server for data management and MQTT webpage-based interactive interfaces. The IoNSS enables real-time assessment of changes in electrical conductivity values and specific element concentrations (
$\mathrm{N}{{\mathrm{O}}_{3}}^{\rm{ - }}$
,
$\mathrm{N}{{\mathrm{H}}_{4}}^{\rm{ + }}$
, Ca
²⁺
, and K
⁺
,) in nutrient solutions within a commercial hydroponic greenhouse. It also facilitates the determination of the
$\mathrm{N}{{\mathrm{O}}_{3}}^{\rm{ - }}$
/
$\mathrm{N}{{\mathrm{H}}_{4}}^{\rm{ + }}$
ratio, aiding in tracking nitrogen fertilization absorbed by plants. Decreases in K
⁺
concentration measured by the IoNSS correlate with deviations from normal VPD levels, indicating leaf transpiration efficiency. Measurement of other ions reflects physiological disorders resulting from abnormal VPD. In summary, the IoNSS provides specific-element information crucial for adjusting environmental parameters and optimizing greenhouse environmental control strategies, thus advancing precision agriculture in hydroponic cultivation.