A

I used Raspberry Pi 3, Sparkfun soil moisture sensor, solenoid valve, 12 v external power and a bottle of water to implement this system. The soil moisture sensor sends signals (low or high) to the raspberry pi repeatedly. As soon as receiving the first low signal Raspberry pi sends a request to Nexmo to send a message to my cell phone and let me know that the plant needs water. 

Then it sets the solenoid pin to high and water can pass to the pot to water the plan till the moisture sensor sends the high signal again to Raspberry Pi.