What is the best oscillator to be used in a capacitive soil moisture sensor?
There are lots of Oscillator circuits that could work .Circuits that have a grounded cap will be more noise immune .RC oscillator circuits are generally not as stable as LC circuits .These days there are lots of SMD inductors to chose from on the internet so going LC is not a cost penalty .LC circuits generally make sinewaves so they are better from an EMC viewpoint .Chose an LC oscillator circuit that uses a single inductor to keep purchasing easy .I have used a   modified dynatron osc for capacitive liquid sensing .The output level of this circuit is about 600mV peak sinewave .I used a comparetor to square it up for counting by a micro .