"What makes some farmers more productive?  Any clues?"

Can he make this metal superconducting with Prestidigitation and then get an electric current flowing in it persistently using this thing he already magnetized?  Keltham has a detailed qualitative-physics mental model of both conductivity and superconductivity if that counts for anything; he wouldn't know exact numbers on how conductive a piece of impure iron would already be.

"What makes some farmers more productive?  Any clues?"

Can he make a loop of metal superconducting with Prestidigitation, and then use this bit of iron he already magnetized to start an electric current flowing which will then persist?  Keltham has a detailed qualitative-physics mental model of both conductivity and superconductivity if that counts for anything; he wouldn't know exact numbers on how conductive a piece of impure iron would already be.