Hi Eric -

Here in northern Colorado, sprinklers are used by about 50% of the population. Rain is seldom received and when it occurs it is frequently short in duration and locally concentrated.

All that being said, it's now reasonable to consider that more than one soil dampness sensor is desired. Short duration rain does not effectively reach the ground under a tree or near a vertical object, like the side of a house. Nevertheless, if one is committed to planning a sprinkler system using sensors as a means for using soil dampness to schedule (or delay) sprinkling, the sensor should be placed to account for the location of plants and sprinkler heads. Consider the plants - Deep root plants should use sensors that are placed at about the same depth as the plant roots will be drawing water. Sensors for shallow root plants should be nearer the surface to account for the faster evaporation occuring there. Consider head placement - Sensors placed too close to a head receive a denser water rate than sensors paced farther from a head. Also, if rain comes principally fom the west, a sensor placed close to the east side of a house will indicate a lower than average amount of soil dampness ... And that indicates a that placement of sensors and sprinkler head zones should be planned to work together.

I'm guessing the foregoing would not be the way a botanist would schedule bulk watering in a greenhouse since the greenhouse has a more even distribution of water and sunlight than a lawn.

As to sensors - You can try most anything that gives you a variation of signal with soil dampness. Sensors using variation of resistance or capacitance can be made to work. I've etched pcbs with interdigitated fingers and covered with soder mask material to act as capacitors. A friend has purchased some carbon rods from Digikey and buried them while controlling the spacing of the parallel rods to measure soil resistance. I suggest you use ac sensor excitation to minimize electrolysis of the sensor elements when a resistance sensor is planned.

Cheers,

Bruce