EVO Tail Pin usage

I want to add a soil moisture sensor to the weather station setup. It looks like Pin 7 is not used. Can I use this to meas a voltage ?

Yes pin 7 can do Analog to Digital Conversion (ADC). Here’s a handy list of what the IO pins can be used for https://electricimp.com/docs/hardware/imp/pinmux/

The imp can talk I2C to this nice soil moisture sensor:
https://tindie.com/products/miceuz/i2c-soil-moisture-sensor/