Here's a picture of my setup. http://imgur.com/a/BV188
I have a enclosure (that I recently made waterproof) that sits out in my garden that has the ESP8266 wireless chip in there, which works very similar to an Arduino with built in WiFi. I have it reading data in from a soil humidity / temp sensor, an air humidity sensor, a light sensor, and a air temperature sensor.
That data gets sent back to a simple django webserver that I have running (indoors) off of a raspberry pi. It records all the sensor readings every 10 minutes and registers them to various plots in my garden. And then, if there are any big issues (no light for 2 days, lower than average soil humidity or soil temperature, etc), it texts me.
Eventually I'll connect it to my irrigation system, but I don't trust it enough yet!
I have the exact same problem with soil humidity sensors that you mentioned. I even sprung for some fancy ones (http://bit.ly/2sMNRnD) that claim to be waterproof. I cannot make them read useful information and, once it rains or I water outdoors, the sensors read 99% for the next few days. It's very frustrating and the missing piece to make all of this work.
Like you, this started as a quick, month-long project and now it's become something a lot bigger :)
I think eventually I'd like to build this out to be a vegetable garden planner, so I can plan my vegetable garden at the start of the season, monitor what's happening with them, and automatically trigger my irrigation system if needed.
Anyway - it was great to read this! I'd love to hear how this project evolves and would be happy to share any of my experiences as I've put this together.
P.S. And, it's a long shot, but if you (or anyone is reading this) figures how to accurately measure soil humidity temperature in a waterproof environment, I would be forever grateful!