Soil moisture sensor has three connections GND, Power 
and Signal/Analog Voltage.

Volatge need to be >= 3.3V for the version we're currently
testing. (There are other version) The only option to
get >= 3.3V is to use a Lithium Tionyl battery. Which holds
3.6V. Also note that maximum voltage for the AtMega128rfa1
is just 3.6V.

The senors has a didicated analog input for devices like
this. It named  CON_ADC1 and has a 3 pol terminal block:

GND/Prommable Pwr/AD Signal

The programmable power is avoid idle current to drain the
battery. 

The programmable power is enabled 1 sec before reading the 
AD signal then disabled.

There are firmware options to enable power for test purposes.
also there are debugging option to monitor power on/off.