I have found a very accurate soil moisture sensor which works on capillary reaction and the vacume created. There not as cheap as a basic soil sensor. Note instead of the mechanical vacuum gauge you can get one with a digital presure gauge so you can plug straight into a microcontroller. Hers the link
http://www.irrometer.com/pdf/irrometers/101%20IRROMETER-Family-brochure.pdf
There called irrometers or tensiometer
If you ever go in a business reception and they have a big potted plant look to see if there is a small plastic tube on the soil surface it has a red ball floating on top of the water draw through the capillary tube. I really want to know what they are called but if you find out they should be very cheap then just use whatever to messure the position of the red ball.
Hope that all helps.