I have a paper showing measurements of soil conductivity made by the
accepted method of using a buried open wire line, which sounds similar
to your method. The authors found that the conductivity varies
considerably with frequency. Have your measurements shown this also?

Roy Lewallen, W7EL

Reg Edwards wrote:
It is fairly easy to measure soil conductivity at DC and power
frequencies. See program EARTHRES. Such measurement values apply up to
a few hundred kHz.

A few years back I became curious about what happens to conductivity
at HF. And at what frequencies does soil permittivity begin to matter.
Most amateur activity is at HF and above. Yet, without even thimking
about it, we persist in plugging in power frequency values into
formulae and computer programs.

A volume of soil between a pair of electrodes behaves as a resistance
in parallel with a capacitor.

So this is what I did -
. . .