I'm looking to build a quick and simple soil moisture sensor. A few minutes of googling suggests that the quickest and easiest technique is to measure the resistance of the soil using a pair of metal probes and reference resistor to make a simple voltage divider circuit.
Several posts mention the problem of electrolytic corrosion of the probes, they also mention that a possible solution is to only to turn the sensor on once every 10 minutes or so when a reading is required.
The easiest way to switch the sensor(s) on/off seems to be power them from a digital output on an arduino, according to the spec they can source 10ma per pin.
Will this be enough to power a moisture sensor? and does this seem like a sensible way of turning them on and off?