Using 555 timer with soil moisture sensor
Your circuit is a little different to the "normal" 555 circuit. The normal 555 circuit gives a duty cycle of &lt; 50%. your circuit gives exactly 50% duty cycle.

Take a look at the 555 datasheet, you can see how TRIG and THRS work to turn the OUT pin high and low. The basic idea of this circuit is that it's charging and discharging C1 through the combination of R1 with R3-C3-R4-C2. As the value of R4 changes, the charge and discharge rate of C1 changes, so the period of the waveform changes. 

The DIS pin is almost the same as the OUT pin, but it is open-drain (Pull down only). OUT pin will drive both high and low so it will source current to charge C1, and sink current to discharge C1. It looks like the junction between R1 and R5 is the place to read the output from the circuit.