has anyone "insulated" under their above ground pool? 3" layer of mulch?

I am trying to make my quickset pool warm enough for my wife.
In addition to my solar heater I thought a layer of mulch under the pool would insulate from cold transfer from ground.
Has anyone done this?

I would think that no one has ever done this......................ever.

I think the mulch (straw) being crushed flat would render it ineffective, but I might be wrong about that. I have occasionally thought about rigid styrofoam under my pool, and that is what I once put under a concrete floor with embedded heating wire.

By far the biggest heat loss out of a swimming pool is from evaporation at the surface, not conducted heat out the bottom or sides. (Every gallon of evaporated water takes ~8000 BTU of heat with it.) Most soils are at least somewhat insulating when dry, so as long as there's good drainage under the pool, adding insulation down there won't do much, especially if you don't do anything about blocking the orders-of-magnitude larger heat loss from evaporation.

Using a bubble-pack type pool cover would

A: Block evaporation

B: Put about R2 of insulation between the water & cool nighttime air

C: Add a considerable amount of passive solar heat during the day (even a bright-clouds type day.)

But like any pool cover, you have to put it back on whenever the pool isn't in use to get the benefit.