"David E. Ross" wrote:

Some 20-25 years ago, I bought a 5 lb bag of zinc sulfate. It was
packed by Bandini, which is no longer in business. I use only a little,
on my four dwarf citrus and on my gardenia. Now that the bag is almost
empty, none of the nurseries around me carry it.
[snip]

From Wikipedia:
http://en.wikipedia.org/wiki/Zinc
Toxicity

Even though zinc is a very essential requirement for a healthy body,
excess zinc can be harmful. Excessive absorption of zinc can also
suppress copper and iron absorption. The free zinc ion in solution is
highly toxic to plants, invertebrates, and even vertebrate fish. The
Free Ion Activity Model (FIAM) is well-established in the literature,
and shows that just micromolar amounts of the free ion kills some
organisms. A recent example showed 6 micromolar killing 93% of all
Daphnia in water.[84]

Zinc sulphate is ZnSO4·7H2O which has a molecular weight of 287.
Zn has a molecular weight of 65, so zinc sulpate is 22.6% Zinc.
So a 5lb bag has 1lb of zinc in it. That seems an awful lot of Zinc
to be adding to your land. How can you be sure you're not poisoning
your land, or the groundwater?

(I am not any kind of expert.)

--
Sleepalot aa #1385