Moderately dense, moist soils easily transfer heat and therefore make the best soil type for geothermal greenhouses. Very loose or dry soils such as sand or gravel conduct heat so poorly that they may make geothermal heat impractical, unless better soil can be found at greater depths.
Some kinds of soil may also be less suitable. Clay, despite its density, may not conduct heat well, where a moist soil even with some organic matter might do a better job.
