In the linked article about passive glasshouses here https://solar.lowtechmagazine.com/2015/12/reinventing-the-gr...
It says:
"Another issue with a solar powered greenhouse is the lack of a CO2-source. In modern greenhouses, operators aim to have a CO2-level at least three times the level outdoors to increase crop yield. This CO2 is produced as a byproduct of the fossil fuel based heating systems inside the greenhouses. However, when no fossil fuels are used, another source of CO2 has to be found. This is not only an issue for solar greenhouses. It’s also one of the main reasons why geothermal energy and electric heat pumps are not advancing in the modern glasshouse industry."