Review on Delmhorst GB 1 Gypsum Sensor Moisture by Ryan Olsen

Most moisture sensors measure electrical conductivity and are approximately 6 inches to 30 cm long. These are inexpensive, well-made plaster blocks that measure soil moisture at any depth from a few inches to four feet. You bury the block and leave the wire open. The blocks have a resistance of several hundred MΩ when dry and several hundred kΩ when wet. The company makes a rugged field meter, or you can make or buy one if you're interested in electronics. Those are great little things.