Copper Sulfate — Frequently Asked Questions

The copper ion binds to thiol groups in enzymes and disrupts protein function, which is lethal to algae at low concentration. The same mechanism works against fungi and molluscs, which is why one compound serves as algaecide, fungicide and molluscicide. It is not selective, which is the source of both its effectiveness and its environmental risk.

A suspension of copper sulfate and hydrated lime, used against vine and potato blight since the nineteenth century and still in use, including in organic production. The lime neutralises the acidity and forms a sparingly soluble copper compound that adheres to foliage and releases copper slowly, which extends the protective period and reduces phytotoxicity compared with copper sulfate alone.

It is highly toxic to fish and aquatic invertebrates, so dosing in water bodies requires care over concentration and over the oxygen depletion caused by decaying algae after treatment. Copper also accumulates in soil and sediment rather than degrading, so repeated agricultural application builds residues, and several jurisdictions cap cumulative copper application.

Because the water coordinates to the copper ion and changes how it absorbs light. The anhydrous salt is white; adding water forms the pentahydrate, which is the familiar bright blue. That colour change is sharp and reliable enough that anhydrous copper sulfate has long been used as a simple qualitative test for the presence of water.