It seems iron, in its magnetic form, is a little more common in soil than I
thought.

As just described, our 1-metre cube of space containing garden soil has
higher values of conductivity and permittivity than an empty space. When
associated with a grounding system the effect of increased permittivity is
to much reduce the velocity of propagation of em waves through the soil and
along any wires buried in it.

A quarter-wave of buried wire may have a physical length only 1/6th of a
quarter-wave of wire in free space. The velocity along a radial wire just
lying on the surface of good soil is reduced to only 1/2 of the free-space
value and should be pruned accordingly. Old-wives and Handbook editors
please note.

A high content of magnetic iron in the soil would reduce propagation
velocity further still in the proportion 1/Sqrt( Mu ). How this would change
the loss-distributing effectiveness of radial wires I have no idea. My back
yard contains no iron and I have no intention to dig in 10 or 20 tons of
iron filings just to see what happens on the S-meter.
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Reg, G4FGQ