[znpy]

I remember my grandfather talking about copper-based fertiliser to be used on live trees (iirc), to help growth, and this was in the early 90ies in south Italy, when i was a child.

Dumb question: could it just be that copper rods in the soil are just providing “copper” to the plants in the same field?

[angst_ridden]

Copper sprays are a common anti-fungal and anti-bacterial used for fruit trees.

I don't think it's an electrical thing in this context, but that doesn't rule out some other effect.

[yetihehe]

Easy to check, just stick some short fragments of rods in soil.

[harvie]

but the electric current may improve dispersion of copper ions through the soil.