Here is another tool that lets you keep a close eye on the health of your garden. The ECOWITT WH51 Soil Moisture Sensor can get a reading in 72 seconds. It combines with GW1000 WiFi gateway to bring you live data on your smartphone. You can get readings from up to 8 soil moisture sensors with one gateway.
This sensor is IP66 waterproof rated, so you can use it indoors or outside. You can view past readings and graphs online. This soil sensor is available for under $20 at this time. More information is available here.