If you want to store solar heat, you cannot beat water @4000J/kg.K. Black barrels of water is probably the best solution, yet a very simple solution. I've checked the thermal capacity tables, only ammonia would do a slightly better job than water !

I also toy with the idea of building a greenhouse myself, that's why I have done some research. If you want to harness the thermal capacity of earth for a greenhouse, I know of two tricks:

- Excavate one or two meters of soil to build a “pit” greenhouse. You will get earth walls connected to an immense thermal storage capacity. This is the so called Walipini greenhouse concept https://en.wikipedia.org/wiki/Walipini

- The diurnal and seasonal temperature oscillations dampen quickly with depth in the ground. Below some meters of soil, the temperature will be stable at the average annual temperature at your location (Ta). If you can dig a trench to lay underground pipes to create a ground/air heat exchange connected to your greenhouse, you will get a free source of air heated at (Ta) in winter or cooled down to the same (Ta) in summer. Fans to force air circulation will help.