I'm interested in automated nutrient monitoring, because I think it would be necessary to get the most out of hydroponic systems. It's also true that I just don't know much about farming, so some things related to soil nutrients that one would learn from experience are knowledge I simply lack. Even so, the "soil" that I work with in my grow tent is coconut coir pith and perlite, which starts out essentially sterile, i.e., zero nutrients. I do this because I don't want bugs in my apartment. In future projects I may build a fogponic system, in which nutrient monitoring would be even more critical, because there is even less intermediation between the chemicals and the roots.

Real-time and/or automated nutrient monitoring is not common, however, I've found some promising research:

(2017) Detection of Nitrogen, Phosphorus, and Potassium (NPK) nutrients of soil using Optical Transducer

(2018) Vision System for Soil Nutrient Detection Using Fuzzy Logic

(2019) Optical Sensing of Nitrogen, Phosphorus and Potassium: A Spectrophotometrical Approach toward Smart Nutrient Deployment

(2020) Decision support system for NPK fertilization: a solution method for minimizing the impact on human health, climate change, ecosystem quality and resources

(2021) Determination of soil nutrients (NPK) using optical methods: a mini review