replying to Rob Morley, DStocker wrote:
Go back to the idea of using Potasium Nitrate, which can be purchased on
line for $20 for 10 pounds. Theres a reason they use it for stump remover.
Drill holes in the remaining post as far down as possible. I used a 22inch
long, 1/2 inch machine bit from HF. The wood was soft so I probably got a
good 15" down before friction kicked in. Mix a 1/2 pound or so of Sodium or
Potasium Nitrate with water. It should be thin enough to soak into the wood.
Keep the sprinklers off. Then fill the holes with the P. N. powder. Can you
wait 6 months? Probably not. So speed up the process. Wait a couple days to
dry out. A blower of fan would help. Then, light it up! Soak 4 or 5
charcoals, soaked in mineral spirits, on top of the post and light it up.
There is not enough oxygen past 15 inches to support combustion with out the
P.N. that absorbed P N will smould for up to two days. I've had stumps where
the PN gets absorbed into the roots and it smolders 6 feet following the roots.

--
for full context, visit https://www.homeownershub.com/uk-diy...ut-211062-.htm