Why must the device be so cheap? Is the intent to have one serve each plant?
Why can’t one serve many plants?
Right now it costs about $750-$3000 to set up each monitoring site, so growers can only afford to install a low number of devices per acre [1], thus a high number of plants per device. In that case you’re not optimizing to anywhere the specific requirements of each plant, you’re assuming (often falsely) that all the plants in that area have the same needs as the one you’re monitoring.
The lower the cost per device, the higher your resolution and precision of monitoring can be, and the more you can accurately provide for the specific needs of each plant.
[1] Very often that number is zero.
Wouldn't a device that travels like a snail along a cable or something, while less glamorous than a self propelled (and navigating) device, deliver a reading for a whole line of plants every day without all of the eroding parts per plant?
Having machines moving around taking readings “each day” is insufficient, as during hot conditions the soil/plant moisture changes too rapidly. You need to take readings at least once every hour or even every 30 or 15 minutes in order to be able to start irrigating immediately when the moisture level starts dropping rapidly.
I can imagine that a lot of small devices becomes economically viable.. I find that unfortunate since I think it will be a lot of permanent ewaste lost in fields relative to very temporary water optimization benefits.