it is a pretty crude circuit.
Analog devices makes a number of impedance analyzer chips for use in things like capacitance meters, but they are all around 1 MHz signal frequency.
Maybe someone makes an IC that does the same at 100 MHz?
Or you could make your own?
Or maybe you take the analog devices chip, like a CN0217, UPCONVERT the test signal to 100 MHz, pass it thru the soil, then DOWNCONVERT the signal back to 1 MHz and feed it back into the CN0217 chip? You would have a VERY stable and repeatable measurement then. One benefit, you could get TWO outputs, the AMPLITUDE thru the soil, and the PHASE SHIFT thru the soil. Two signals are better than one!