1) Yes the server and App support soil moisture sensors. In fact, we have a test system set up here in the office using the AdaFruit SHT10 but it's pretty pricey ($49.95). The SHT10 can co-exist on the I2C bus, but requires a different protocol to read. There is a library for the Arduino on github SHT1x. We just have a routine strobing the lines to read the device. We are developing a product based on on the much less expensive SEN0114 from DFRobot ($4.80). That sensor has an analog output which needs to be run into an A/D converter (we are using and I2C A/D converter for development (the AdaFruit ADS1015).
2) You can set up your controls the RasPiConnect App to read any values that you wish from the Raspberry Pi and our server. In the above case of the ADS1015, it would be very similar to the BMP085 (included in the example I2C code in the server).
3) Yes, you can turn values on and off with RasPiConnect App. Use an Action Button control to turn the valve on and off on your Raspberry Pi. Do you have a valve in mind?
Thanks. I'm going to order one of each and see what I can make work. I see from the Action Button code in the server that I can set a button to toggle a GPIO line for a valve or some kind of an I2C actuator.