U-blox C030-U201 humidity sensor

Hello everyone,
I am a beginner in IoT
I have to build an irrigation project by using u-blox C030-U201 and mbed Application Shield (available at https://os.mbed.com/components/mbed-Application-Shield/)
What is the appropriate sensor to measure humidity in the soil and how can I get it?
and if you have any useful similar project or tutorial that u recommend it for me , where I can start learn from it, please add it … I’ll be grateful…