Salt is less expensive, but I prefer Roundup because
I don't want the sodium introduced into my soil. For
$1 worth of Roundup, I know that the stump will die
and that the Roundup which leeches into the soil will
be harmless. (I buy Roundup in the huge containers
and the price per ounce is very economical that way.)
For many stumps in an area in which I'm not concerned
about the sodium, I would go with the salt. And, of
course, inexpensive sodium nitrate provides a double
benefit: It helps perform a somewhat quick kill on the
stump (still not as fast as Roundup), and it provides the
first step in a very, very good method of burning a stump
and its root structure.
Interesting question for which I don't have an immediate
answer: Does the use of Roundup or sodium impede the
absorbtion and distribution of nitrates throughout the stump
and its root system? In other words, is the distribution of
the nitrate optimized by a still living stump and root system?
I also drill vertical holes into the stump, but instead
of roundup I fill them with table salt.