Yes, this is normal for a DC drive configuration. You're effectively setting up an electrolysis experiment where the copper atoms on the positive electrode are being ionised, transported via the soil's water content across to the negative electrode where they are being deposited and returning to being copper atoms. This explains why the negative electrode looks surprisingly clean for having been buried - the top layer of atoms are recently deposited and likely very pure.
To get around this, there are a few things you can do. Gold plating is a good start, but it will need to be thick and consistent (even an atomic hole will allow access to the underlying copper and eventually this will get eroded). Most ENIG plating on PCBs is to ensure SMD pad flatness and minimise corrosion during storage - you would need a "hard gold" plating for long term use and even then eventually it will fail.
The best approach is to use an AC drive. Here, the electrodes swap from being positive to negative frequently during use. Because of this, ions that get transported and deposited in one half-cycle will be returned across and redeposited in the next half-cycle (where the polarity will be reversed). Net result is no overall electrolytic corrosion (and actually a partial self-cleaning function). Most capacitive sensing schemes are net-zero DC and so doing cap sensing as opposed to resistive sensing on the electrodes will likely help, as others have suggested.
This EE Stackexchange Q&A goes into some detail on drive schemes and a discussion of AC circuits. The way I have done it in the past is to use an astable multivibrator to drive the two electrodes with an AC waveform and then measure the DC input current to the multivibrator and calibrate that against moisture - but I am sure there are more elegant solutions out there if you Google enough.
One last point - if you're using an AC scheme, you should be powering the electrodes continuously to maintain the self-cleaning function (unpowered copper will eventually corrode in soil). With a DC scheme, powering only when needed will reduce the speed of corrosion (as electrolytic corrosion will be faster than that of unpowered copper) but it will not prevent it in the long-run.