Soil moisture sensor interface protocol
The output impedance of the module's analog output is in the hundreds of K ohms (it's a voltage divider between a 510K resistor and the electrodes), so it's a relatively high impedance. You could buffer it (at the source) with an op-amp (eg. TSV912) as a voltage follower and add some series resistance (a few hundred ohms is fine) to keep the op-amp stable. So, two parts per module. You're really only using the 510K resistor in the module and a couple of capacitors if you ignore the DO. 

That crude module puts DC (albeit limited to &lt; 10uA with a 3.3V or 5V supply) through the electrodes, which isn't great. Higher moisture = lower voltage out. I wouldn't put too much faith in voltages read from this particular piece of kit.