Are you sure you don't mean iron sulfate? Iron sulfide (not sulfite) is extremely reactive and not a compound most homeowners would have any use for. OTOH, iron sulfate is commonly used to supplement iron availability and lower the pH of higher pH soils.

concur. Iron sulphate is commonly used to increase iron availability. Greens up lawns and other plants that show yellowing. It is essential to development of chlorophyll. https://homeguides.sfgate.com/iron-sulfate-safe-home-gardens-91858.html

I hope the bottle of "Liquid Iron" is also what we're talking about because I just gave the Kumquat a dose of it. The label was too messed up to read if it is Iron Sulfate. That part of the garden was previously inhabited bby three large Sequoias that may have left that ground more acid.