Just before I retired I was involved in a redesign of a direct soil probe type of EC meter and I think I'll re work the heart of it for my Arduino, It shouldn't be that hard and even PH can be done accurately and inexpensively with other sensors. We, when I left were developing a PH meter with one of those fet sensors, it 4 years the technology should be more advanced and at $600.00 for a device albeit of lab grade is much too rich for my tastes.
Bob