All the passive heating ideas for greenhouses are fascinating but you have to be realistic about any gain for the effort/cost in an ordinary single glazed greenhouse which is effectively little more than a wind break in winter. I imagine a few degrees gain is the best you can hope for, potentially extending seasons a little or protecting from a hard frost. With compost I'd have thought burying half rotted compost in a greenhouse bed, with a cold frame of fleece over would be more effective as the heat is around roots, where you want it. It depends how you use your greenhouse...and the plants are still at risk if theres a sustained hard frost. Insulating the beds from outside temperatures, blankets, large thermal mass can all help but it's always small gains while the building itself is 95% glazing.