The challenge isn’t just getting the sensor to work, but rather if the sensor is actually measuring anything useful. Soil electrical conductivity varies with the salts in the soil more than the moisture. Fertilizers, salt buildup, etc. all have a huge effect on the reading. And the salinity also affects the ability of the plants’ roots to extract water. So, a higher conductivity might mean that the plants are less able to use the water in the soil… exactly the opposite of what we are trying to measure.
And, assuming we had a valid sensor, it would be hard to extrapolate from that sensor to the soil around it. Imagine a tree with a line of drip sprinklers spaced at 18″ intervals. If the sensor happened to be directly under a drip outlet, it would show a drastically different reading than one 9 inches away. Check out http://www.soilmoisture.com/ for more information.
An alternative might be to measure micro climate factors, such as shade and soil temperature. Lawns might be shaded in the winter in yet full sun in summer, or trees might be shading the lawn in one season and not another, etc. A simple soil temp probe or maybe solar radiation could be used to modify the zone’s ETo numbers. Then the problem becomes how to change the sprinkler zone’s behavior as its micro climate changes.