Interesting application Mike!
If I understand correctely you need to measure ground moisture far from your home network and control an OpenSprinkler unit connected to your home network.
Keep in mind that with a cheap external repeater you can connect an ESP8266 at 300ft to 1000ft from your home and that for powering it you can simply use one of the several phone USB solar power supply available in the market.
The ESP8266 is consuming only power when is awaike , during sleep consumption is negligeable, and I presume your measurement can be done only few time a day.
Total cost cost be in the range 20 to 40 $ ;this is much less than the cost of a couple of XBee transmitter + MCU + solarpower supply.
In addition once you are connected to the nework used by OS units it is very simple to send API commands to conrol those units.
I am preparing in the github wiki a description of few example of the OpenSensor possible application:
one is a unit located inside a glass house that read internal temperature and humidity and ground moisture sensors and control en Open Sprinkler for spraying and operation of a blower and of a heater.
If you give me more detail on your application I can try to study better your case and develop a example, detailing HW and SW, that I will put in the wiki pages