I have seen regular pH probes that had been ruggedised to use in soil. Those should be able to connect to a regular pH probe amplifier, which in turn can be read by an Arduino.
and not really the *soil pH* as gardeners tend to want it reported
Yeah there's some measure that gardeners use and call pH that is more of a measure of how much hydroxide salts are present in the soil. I don't understand it very well as I'm not a gardener either. But I am a chemist who has been asked about it a few times and figured out that they have something other than solution concentration of hydronium that they all pH.