Arduino based soil moisture sensors for sprinkler zone override


I have an automatic drip/soaker irrigation system for my vegetable garden, and I am starting to see a problem that I think I can solve with Arduino. I am a tinkerer and can follow directions, but I am NOT a programmer. The issue is I have a Toro ECT XTRA controller with 10 zones. It has the external wireless weather/rain sensor (which automatically increases/decreases watering time based on temperature and sun exposure), but I have vegetable beds and containers in various places that get varying amounts of sun.

I am finding that the beds that are shaded for part of the day are getting way overwatered, even after cutting down their watering times - and our weather has been kind of wonky, 96 degrees yesterday, 73 today with some rain, 75 tomorrow, 101 Tuesday.. The automatic compensator averages over a week, so right now it's watering at 120% - even though it's not as hot as it's been.. So I can't just set them on a different program with alternating days, because if it gets really hot they will dry out. The rain sensor is set to trip at 1/4", but I find that it dries out too fast and resumes the schedule while the ground is still wet. I also can't really set a manual 24 hour rain delay, because that overrides all zones and the containers will start to dry out.

So here's what I'd like to do. I don't want to build an entire new controller or use OpenSprinkler just yet. And all the Arduino projects I've seen using moisture sensors are made to activate pumps and such. What I want to do is use a moisture sensor (or two averaged) per bed to simply activate a relay to bypass that zone while it is too wet. The timer will still run its schedule but any zone with a sensor above threshold will not open its valve.

Ideally each sensor would be wireless (I could do battery or solar), with a single receiver on the Arduino that would be placed near the timer/relay board. It would not be a huge issue to wire everything though, I have outdoor Cat5, and the longest run would be about 100ft. A pair of Arduinos (one outside that connects to all the sensors, and one inside that connects to the relay board) connected by a single cable would be great too if that's possible.

Is this a realistic project? The hardware part is absolutely no problem for me but I would need some hand holding to write the program..

Thanks in advance!

The unrealistic part is expecting consistent results from soil moisture sensors. The simple sensors will be corroded away to nothing in a week, the medium sensors won't last a year and complex ones, well now you are talking real dollars.

Find a sensor with the endurance you require, then work out how to drive that with an Arduino. Determine your power requirements for the sensor, Arduino and radio, then pick a solar cell and battery. Then work out how to put it all into a box that will survive out in the elements.

The unrealistic part is expecting consistent results from soil moisture sensors. The simple sensors will be corroded away to nothing in a week,

That's if you put a voltage on them constantly. If you only measure for a few ms every 10 mins or so also the cheap ones can last for a while, too.
Still they're not all too reliable, so you have to check on them regularly.
Otherwise, I don't see any real obstacles in building a project like this.