I've looked into exactly that sort of setup for a previous job a few years ago and the problem is that these probes don't exist for every nutrient and the ones

I've looked into exactly that sort of setup for a previous job a few years ago and the problem is that these probes don't exist for every nutrient and the ones that do exist tend to be very expensive. It wouldn't be a cost-effective apparatus unless it was set up with one probe station metering the outflow to the whole of an industrial-scale greenhouse.