The suggestion has been made on various ham-related websites that improving the conductivity of the Earth around a ground rod produces a worthwhile improvement in the e-m radiation from a vertical monopole using that ground rod. How effective is that?
Below for some perspective on this topic is a NEC4.2 analysis showing the difference in performance of a 40m, 1/4-wavelength, unloaded, base-fed vertical monopole antenna system when driven against either a 5/8" OD or a 12" OD, copper-clad, 8-ft long ground rod buried in poor Earth (1 mS/m, d.c. 5).
Increasing the ground rod OD up to 12" is the equivalent of modifying the Earth in that radius area around a 5/8" OD ground rod to be nearly a perfect conductor of the radial r-f currents in the Earth around the monopole, that flow toward the ground rod.
An improvement in system performance results, but not a very significant improvement.
If by improving conductivity, you mean flooding several acres or square miles with ocean (or saltier) sea water (or via natural intrusion near a low lying shoreline or wetlands), or installing a solid copper floor several square wavelengths in size in the parking lot, yes.
But some people have reported improved antenna performance by putting a metal mesh of gopher wire (not too deep) under their lawn, with the vertical antenna grounded in the middle. Probably soldering or crimping adjacent mesh sheets is desirable.