If your watering zones dry out proportionately then you can get by with one sensor– this would be the case for an outdoor lawn (full sun areas will be dry before shaded areas, but the relationship between the areas is roughly constant). Probably works well enough for lawns, but a greenhouse with plants at different stages of growth might need more detailed monitoring.
There are cheaper moisture meters for maybe £2 or £3 each
My understanding is that there are a few ways to do this. The resistive sensors that are most popular for DIY tend to degrade and/or malfunction over time but are really easy to build (two nails/conductors, and perhaps some gypsum for encapsulation) and simple to understand. I suspect the commercial Toro unit above and 3+6″ commercial versions seen at golf courses use this method (with stainless steel conductors).
The capacitive approach is probably longer-lived, but slightly harder to manufacture. I wonder about the longevity of FR4 PCBs in wet soil, but am curious enough that I might make one and see how it does.
Adafruit is selling a normal atmospheric (air) sensor PCB in some sort of metal housing as a hobbyist soil sensor that looks like a microphone. I wonder if it works/lasts (if it did work/last, why wouldn’t everyone in the industry be burying cheap atmospheric sensor ICs?)
Others have found solutions by using moisture sensors to accept or cancel a command sent by a timer. If the command isn’t cancelled (ie when the moisture content is below a low threshold), the timer activates a solenoid valve and irrigation commences.
That’s what the Toro unit does with its relay, it effectively disconnects the zones or actuates the rain sensor input (normally open or normally closed are both supported)
I think there are some interesting demand-driven/wireless sensor-based watering solutions that are now do-able for hobbyist/DIY. The real question is how much savings there is to be found compared to schedule based solutions (and variations: schedule+rain/freeze sensor and schedule+weather forecast) that already manage to deliver enough water. If installed base is a guide, then golf courses (big lawns) are more efficient with sensors. Don’t know much about greenhouses.