[heavenlyblue]

I would think a purely technical approach to this is flooding the plant every hour, and then measuring the differential of how much water had flooded the roots vs. how much water was pumped out.

If the differential is tending to 0 then it means that not only the soil is saturated, but the plant is as well. This is _quite_ a basic concept, since you do not want a fully saturated soil all of the time.

A proper biological approach is to estimate how much water exactly the plant needs per unit of time, and then make sure the differential is always exactly that amount. This is not an exact science, especially when you're only working with a single plant and your conditions are probably far from ideal.

This approach would easily take into account other hidden variables, such as the rate of evaporation of water from the soil that depends on the ambient temperature. It also scales to hydroponics.

[exelius]

This is basically hydroponics. And by "basically" I mean "exactly". Replace the soil with an inert, porous medium like hardened clay. And if you aerate the water with say, an aquarium pump/stone, you don't even need to drain it...