K1TTT Technical Reference

The following chemicals, listed in order of preferred use, are 
recommended for good results and less corrosion on the grounding system:

1. Magnesium sulphate (Epsom salts).
2. Copper sulphate (blue vitrol).
3. Calcium chloride.
4. Sodium chloride (common table salt).
5. Potassium nitrate (saltpeter).

From: Training Circular No. 11-6, HQ Department of the Army.

73 de N3UN Wayne.
additional note from rice@world.std.com:
I was looking around your amazing site and saw a recommendation
from an army training circular to use copper sulfate for earth
ground doping.  /~robbins/technote/doping.html
That stuff's pretty toxic. LD50 for a 50lb kid would be about 
700 mg, assuming animal tox results apply.   (30mg/kg is the
rule of thumb LD50 given for test animals, rats, e.g.

It's probably not a big deal, but I thought you might want
to know about it if you didn't already.