I don't want to spam.
The point is, that the conducting based sensors are poison.
The electrolytic processes destroy the sensor and giving ions into the
earth !!!

Look at this site:
All Technologies
There are compared all possible technology's.
Giesomat wins.

Why not using this sensor?
Giesomat

It didn't cost much and work like an angel.

And if the frequency is to high, or you need a other logic level, you can use this one:
frequency divider and level shifter

Don't have any troubles with this.
I only count the pulses. That all.

You should check out this side (Use Google Side Translator):
https://github.com/Zentris/erdfeuchtemessung/wiki

Theres also the sensor from here:
Giesomat

The problem is, that your design is based on conductivity.
This means, that the electrodes will be destroyed over time
and the produced metal salts are in your flower,garden,...

I don't will have metal salts in my tomatoes !!! ;-)

This sort of sensor is not good, because the electrolyticeffect.
You always should use soil sensors, which are based on the capacitive effect.

Like this one:
https://forum.mysensors.org/topic/4474/capacitive-soilsensor-for-measurement-the-humidity-of-flowers

Best regards,
NetRap