John Rumm wrote:

From the measurements I have taken from my installation (heavy clay
soil) a 4' rod will give a resoanably consistent 11 - 13 ohms year round.

I'd say that's exceptionally low for a single 4 ft. 50 - 100 ohms is
more common, so you've clearly got very good soil.

For my workshop & shed installations I've got two 8 footers in parallel
(one of them deliberately near a soakaway) and find a consistent 7.5 to
8 ohms.

--
Andy