In the June 2012 issue of Nuts & Volts, they did a spread about making a soil moisture sensor with an iPhone interface:
And when I heard my girlfriend say that she has trouble remembering to water her plants, I put two and two together.
Most of the N&V article discussed the code required to interface with the iPhone, but the important part for me was that their soil moisture sensor was nothing more than a simple conductivity sensor. The more water in the soil, the lower its resistance.
Such a sensor doesn’t measure soil moisture in any kind of universal unit, but for a single sensor in a particular soil sample, the measured conductivity is repeatable and proportional to the moisture level. I call these conductivity units “AMU”s or “Arbitrary Moisture Units”.