 Dry mass

Dry mass is the mass remaining after a specimen has been dried under a stated procedure. Removing water makes comparisons less sensitive to hydration, but the result remains operational: drying conditions can also remove volatile compounds or alter the material. A reproducible measurement specifies specimen preparation, drying conditions and the criterion for stopping.

For example, a rice-biomass protocol separates leaves, stems and reproductive structures, dries the components at $70{}^{\circ }\mathrm{C}$, and checks for constant mass. Its stopping criterion is no change in the measured mass over 24 hours.^[ref] This is a protocol for those samples, rather than a temperature prescribed for every biological material.

For a synthetic calculation, a specimen has fresh mass 12 g and dry mass 3 g. The loss on drying is 9 g, or $9/12=75\%$ of fresh mass. The same loss is $9/3=300\%$ of dry mass. Both ratios describe the stipulated measurements, but use different denominators. Interpreting the loss specifically as water requires the assumption that other losses are negligible. Dry mass also includes retained inorganic material; it is not synonymous with the mass of organic nutrients.