Plan was to use one gram to 100 grams of waste bread. Since the Iron Phosphate is not water soluble I planned to dissolve it in acid. Not wanting too strong a acid I thought I would use Coke a Cola which has a pH of about 4. Looks like this recipe will be for next year.
I may be misunderstanding what you're up to, but kinda looks like you're pursuing overkill, overpay, and just generally too much of a good thing. Slugs have to eat the iron phosphate to be effected by it. It causes them to slime their own intestines so though it's a slow nasty death for them, they can't munch on the garden while they're dying. It's properly non-water soluable so that one seasonal application is usually enough, unlike other baits that work far less well and have to be re-applied practically aftwer every watering. Slugs won't eat iron phosphate on purpose, but will if it is imbedded in a bait that in Sluggo's case will also stand up to rain and watering unlike bread that you seem to believe would be a good substitute.
A product like Sluggo is 1% Iron phosphate in pellets slugs like to eat. It would be just about impossible to apply a harmful amount of iron to the soil with this product, but there IS a limit to how much iron phosphate is healthy. One pound of Sluggo takes care of about 1,000 square feet of garden. It should be used when slugs are breeding or eggs are hatching to interrupt the generations. It is best used in spring and autumn; if timed right the effect will so utterly decimate the slug population that the bait won't be needed again for two years.
If you plan to somehow get it to dissolve, then it will wash deeper into the soil with the first watering or rain, having no effect. If you're mixing it with bread, unlike the granular product which holds up to wet, the bread will just dissolve into the soil or be eaten by other garden critters mainly insects (which won't hurt them but won't be getting slugs either).
So it sounds to me like you've tried to make your bait which will have an inferior effect over a proven product. It sounds like what you're making is at best an organic fertilizer that that won't be solid long enough for any snails to chomp on it, thus requiring so many extra applications one actually risks plant food overdose with still only a mediocre effect on slugs and snails.
Iron phosphates are safe human food supplements so products like Sluggo can even be used around plants to be eaten without risk. But that's in very tiny amounts. And while at 1% doses iron phosphate is non-toxic in the garden except to molluscs, if it accumulates from misuse it can be dangerous to mammals.
There are more deaths of children under six from products containing ferric and ferous phosphates than from any other potential poison in family households, and could harm pets if your home mix of slug bait has maybe zero percent in one helping and fifty percent in another helping from not having mixed evenly, as it won't. Side-effects from ingestion or inhalation are intoxication, fever, abdominal distention, dehydration, spikes in blood pressure, and heart failure. I wouldn't personally play with the pure chemical when sensible safe products that work so well do exist.
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