Roy Lewallen wrote:
"The assumption that the skin depth varies inversely as the square root
of the frequency is valid only for good conductors. Most types of soil
fit this category only at low frequencies, with the transition falling
on or near the HF range."

Yes. That is correct. Terman says:
"This depth of penetration depends upon the frequency, dielectric
constant, and conductivity. It ranges from a few feet at the highest
frequencies used in short-wave communication to tens and hundreds of
feet at broadcast and lower frequencies."

The earth resistance and capacitance are in parallel. What happens
around 7 MHz and holds for higher frequencies is that the capacitive
reactance of the soil becomes so low that the earth`s resistance becomes
effectively bypassed and out of the circuit for the high-frequency r-f.

Best regards, Richard Harrison, KB5WZI