simy1 said:

1) would you put furnace slag in your garden? if you google "slag
chremical composition" the first hit is from our own group and there
are sites linked from there that support the idea of using slag as a
source of micronutrients, specifically Mg, Mn, Fe, and S (there is
also lots of Ca and some K, but given that I already spread wood ash,
those are unneeded)

Some people would worry about heavy metals. Here's something from Penn State:

"Some slags may contain elevated concentrations of trace metals such as iron,
cadmium, chromium, copper, lead, molybdenum, nickel, and zinc. Concentrations
of these metals will vary in slags from different sources. All of these metals occur
naturally in soil, and many are essential plant nutrients. If concentrations in the
slag are similar to soil concentrations, they present no problem. If they are present
at substantially higher concentrations in the slag than in the soil, repeated
application of the slag could significantly increase soil concentrations of the metal
in soil. This possibly could lead to plant toxicity, increased plant uptake and transfer
of metals to animals or humans, or to other environmental problems. Before using
a slag, be sure to obtain several laboratory analyses of the total concentrations of
these trace metals in the slag."

http://agguide.agronomy.psu.edu/CM/Sec9/sec96.htm

2) what is the cheapest source of P, per pound of P?

Cheapest?

Probably concentrated superphosphate (AKA triple superphosphate or triple
phosphate). This concentrated of P2O5 derived from rock phosphate, 0-46-0.
Not organically certified.

Ground rock phosphate would be 'organic' in that it is a natural source. It is
very slowly available (best in slightly acid soils with good organic matter
content). But it is bulky and expensive to ship around. Contains maybe 20%
P (varies by source) but only 4% available P (so it's rated 0-4-0)

You can buy 5 pounds of triple (0-46-0) for $4.95 and 10 pounds of rock phosphate
(0-4-0, *very* long run 0-20-0) for $6.25, using some web prices (and not including
shipping, which can be by weight on bulky items).

--
Pat in Plymouth MI ('someplace.net' is comcast)

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