What’s the typical range of density for compost? My gut tells me it’s a lot less than the density of water, but that’s it.
Also, how much non-water mass is lost during the process of making compost from plant materials?
While the non-water mass lost from an input of strictly plant material is unclear to me from this source the author contends that, in general, 50% of mass is lost (from non-specific input) via evaporation of water. He further states the percentage remaining after composting as 30% of the initial mass, hence one infers that 20% of non-water mass is lost during the process.
See the paragraph under Figure 4.4.
There could be other mass loss besides that from water. For example, any process that generates CO2 or methane or ammonia would lose things other than water, and elements other than oxygen and hydrogen.