Led will flash when the switch is open. The switch must be replaced by two parallel metal pins (about 1 cm distance) that are put in the soil of a plant pot.
The potmeter is for the sensitivity / threshold.
Explanation: When the soil is getting dry, the resistance is increasing. The voltage on the comparator will at same time get lower than the reference voltage through the potmeter. In this case (simulated by opening the switch) the comparator will give a high signal to the 555 which will take care of the blink of the led.
