| Soil moisture measurements can be scientific if you have enough money. Look up Time Domain Reflectometry and Soil Moisture Tension. I don't understand TDR well enough to explain it, so I will let you search for your own info. The cheapest usable TDR sensor is about $350. Tension has a simple analogy; a Slurpee (is that trademarked?) is easy to drink through a straw in the beginning, but it gets harder to pull the liquid up the straw as you drink more of it. That's tension and plants also struggle to pull the water from the soil depending on lots of factors including the composition of the soil, the amount of water present, drainage, etc. There are several sensors that can be used to measure tension but none are accurate in all conditions, so people who want to measure tension tend to use multiple sensors of different types and triangulate on a useful number. Large scale farming (in dirt across thousands of acres) is where the complex sensors are needed because the soil attributes are not uniform. When you're gardening in a pot indoors or in a greenhouse with the same soil everywhere (because you bought bags of it), you can use those $0.43 resistance probes and just calibrate your watering amounts and intervals over time. Pump on for X units of time, ignore for Y units to let the water move through the soil, then sample every Z units until it needs more. |
Then one needs a/two metal piece of defined length, and a high-frequency oscilloscope, that can also give a pulsed signal (ideally of high frequency).
The biggest issue I see is that you'd need something like at least 0.5 GHz resolution in the oscilloscope with a metal piece of 25cm (quick mental math and some guesses, don't hold me to it). Probably better to have at least 2 GHz. I don't think computers and software alone is up to it after a quick search. Maybe one can increase the travel time in the probe (metal stick) somehow to lower the needed resolution, but not by orders of magnitude.
https://electronics.stackexchange.com/questions/268333/ghz-c...