A typical soil sensor has three probes which individually measure real time soil temperature, humidity and salinity.  The sensor is directly burried in the soil and made of anti-corrosion material. 

The sensor which has the following sensitivity is considered as adequate for agriculture IOT:

 +- 0.5 deg for temperature, +-3% for 0 - 50% humidity and +-5% for 51 - 100% humidity, +-3% for salinity

The sensor has an out of RS485 and is supplied by 12 - 24 VDC

The typical use of this sensor is in Precision Agriculture for drip irrigation, water-saving irrigation, weather monitoring and other applications where soil humidity is required.