I am attempting to keep plants warm in a greenhouse. Therefore, I assume that heating the soil should be most efficient, as that's what plants seem to care about the most (i.e. the temperature of the roots appears to be more important than the temperature of the leaves, though I can't seem to find much evidence around this). If my assumption is true, this is convenient because greenhouses have lousy insulation (and are somewhat difficult to insulate retrospectively), so much of the heat loss happens via the air.
So, which is more efficient for heating the soil?
A) An electric wire element buried in the soil (i.e. a long wire connected directly to A/C power that generates heat).
B) A tank of water containing a heat resistive element and circulation pump with a hose running through the soil.
Intuition may suggest that the pumps are wasting energy, and some heat is lost via evaporation if the tank is not air-tight and insulated, but perhaps the water system is perhaps a little more versatile because you can take energy from other sources (such as the sun) to heat the water. Then again, you could use PV solar energy to power the wire element (though it could take a large array).
I'm attempting to prove which approach is more efficient, and a greenhouse is a difficult environment to control, so I'm struggling to get scientific results. Any insight into which approach generates the most energy loss would be appreciated.
Edit: From the comments, a heat pump sounds like the most efficient option. Caveat: This seems like by far the most expensive in terms of equipment and I'd probably have to buy off the shelf since it's beyond my skill level.