simy1 said:

I worked briefly in the past for a mining company, and chemical
composition of the rock was an issue. I was amazed at the difference
in heavy metals concentration between iron ore and granite rock. Iron
ore typically has 100 times less lead, mercury, or uranium than
granite. It is a very pure rock, as far as toxic stuff is concerned.

I understand that if the foundry is used to smelt lead all bets are
off, and of course I had not thought of Cd and was originally worried
about Al (the other elements will not be a problem for a light
application). On the other hand, anyone eating local foods in red clay
tropical areas intake far more aluminum than I would intake. In a
Michigan acid sandy soil, my guess is that slag mainly serves a
purpose of providing Mg and perhaps Ca.

Dolomitic limestone will provide Mg and Ca, but no other trace minerals.

Langbienite is a mineral which will provide Mg and K. This naturally
occurring mineral is the source for 'Sul-Po-Mag' also sold as 'K-Mag'
(sulfate of potash magnesia). It is 22% K and 22% Mg by weight.

Kelp is the gold standard for trace mineral composition. The most cost
effective way of buying it and applying IMO is Maxicrop powder mixed
into foliar sprays. Kelp meal is expensive, but when we were a little
more flush (cash-wise) I bought and used it when planting corn. The
effects seemed obvious. The seed germinated more quickly and grew
more robustly. I wish it were more *affordable* to as a soil amendment!
--
Pat in Plymouth MI ('someplace.net' is comcast)

Any technology distinguishable from magic is insufficiently advanced.
(attributed to Don Marti)