Neat idea but it seem really impractical. A sensor on every plant? What happens when a leaf falls off? Are you going to have a sensor that is saying "water me now!" until it is recovered? Is there really that much different between plants of the same species planted right next to each other that they'd each need a sensor?

It would seem better to have a small cluster of sensors every 100m or so. That way you could water the areas that need it and have a bit of fault-tolerance (in case one sensor in the group falls to the ground you have the others in the group reporting correctly).

Having said that though don't modern farmers already have technology that tells them when to water and how much? I vaguely recalled reading something about that kind of technology years ago.