The Teros-10 sensor claims to be measuring permittivity at 70MHz, not simple conductance or resistance.
For what you need, a simple AC-fed resistance measuring circuit should be OK.
In principle, use an oscillator to generate a signal with a known amplitude and frequency, eg. a few KHz.
Feed that through a series resistor and capacitor to the "live" probe, with the other probe connected to circuit ground/common.
Connect another capacitor to the live probe, which feeds an active rectifier circuit.
The output of that (which you can connect to an ADC) will be proportional to the ratio of the resistance between the probes to that plus the fixed series resistor - they form a potentiometer-type divider.
The capacitors need to be non-polarised, eg. polyester, and a value large enough to have relatively low impedance at the frequency in use, compared to the circuit and test probe sample resistances.
With an appropriate value of series resistor, dry soil will cause little reduction of the rectified signal, saturated soil will reduce it a lot.
The AC source ensures there are minimal or zero electrolytic effects, but you still need to use stainless steel probes to avoid corrosion.
We have used that basic setup in a few devices for liquid level sensing.
You could use an MCU PWM output for the signal source, then a dual opamp for the active rectifier.