I want to measure soil moisture using THIS sensor. I do not want to use ADC. In the sensor there is a digital output port and an analogue output port. I can use the digital output of the sensor without using adc? Is there any other way to get soil moisture with this sensor without using ADC
Nope, to measure analog value you need some kind of ADC.
Maybe smth like that: http://www.doctormonk.com/2013/12/analog-sensors-without-analog-inputs-on.html
I have not tested it but should work. On the other hand it might be simpler just to use
The digital output will tell you when the set threshold has been exceeded. You set the threshold with a screwdriver and the blue pot on the board.
It's simplest if you power the device from 3V3. If you need to power from 5V you will need to add a voltage divider on the digital output to shift it from 5V to 3V3.
This module here can add ADC to the Raspberry Pi. It also has Analogue outputs. http://www.sf-innovations.co.uk/custard-pi-2.html