Electronic Plant Watering System Circuit
If I were you, I'd simply trash the article you found. It might be me, but I cannot wrap my head around the circuit nor the explanation.



Some random thoughts:


You can approximate your garden to be exactly the same humidity everywhere at any given time. That way, one pair of probes (properly placed) can be enough.
Given that sensors are unreliable (they break, oxidize, etc.), you could get false reading. Redundancy in the number of sensors may work in this case. Distribute any number of sensors throughout your area of interest and implement an average/mean/median filter.
If however there's a lot of spatial variation (eg. damp on one side and dry on the other), you could implement a system of "local watering" - say divide your garden to 4 squares and irrigate each part separately.


I'll try describing the first option:


Use an off-the-shelf comparator IC. Add some hysteresis to ensure reliable operation.
Feed that to your relay. Use a N-FET or an NPN to provide the relay with enough current, if needed.








simulate this circuit &ndash; Schematic created using CircuitLab

Please excuse the crudeness of the schematic. I hope I haven't messed anything up.

Also, note that the comparator is meant to have Push-Pull output. Open-drain piece can also be used - it would require minor changes and the usage of a MOSFET instead of a BJT.

The resistor values are all empirically determined. The one unknown is R4 - should match the mean soil resistance.