Spectroscopic Sensing for Hydroponic Crop Monitoring: From Analytical Validation to Closed-Loop Management

Hydroponic production enables precise control of nutrient supply, irrigation, lighting and root-zone conditions, creating a need for rapid, reliable and biologically meaningful monitoring. Optical and spectroscopic approaches can non-destructively assess nutrient solutions, roots, plant physiology and crop quality. This review critically evaluates VIS–NIR and NIR spectroscopy, multispectral and hyperspectral imaging, chlorophyll fluorescence, Raman spectroscopy, FTIR/MIR spectroscopy, laser-induced breakdown spectroscopy (LIBS) and optical sensing of nutrient solutions. Rather than ranking technologies by predictive accuracy, we classify them by the information they provide: direct or near-direct measurement, calibrated quantitative prediction, physiological proxy sensing, and classification or anomaly detection. Despite substantial proof-of-concept progress, evidence remains constrained by homogeneous datasets, information leakage, limited external validation, calibration transfer barriers and scarce commercial evaluation. Reliable deployment requires representative calibration, validated reference methods, biologically independent validation, uncertainty assessment and continuous quality assurance. Progress will depend less on increasingly sophisticated algorithms than on stronger analytical and biological foundations and integration of complementary solution, environmental and physiological measurements. Spectroscopic sensing should therefore progress from isolated sensor-to-trait models towards validated decision support systems linking measurement, state estimation, diagnosis, intervention and outcome verification, evaluated for analytical validity, transferability, biological interpretability, deployment readiness and demonstrated management value.