Two measurements on a lakebed sediment sample: total phosphorus, which is the whole inventory, and orthophosphate as P, the dissolved inorganic form that is immediately available to algae. Orthophosphate by EPA 9056A, MDL 3.3 and RL 4.43 mg/kg; total phosphorus by EPA 6010D, MDL 151.13 and RL 296.35 mg/kg.

Note the reporting limit on total phosphorus. At 296 mg/kg it is roughly 150 times coarser than the 2 mg/kg limit on the Phosphorus Fractionation Sediment Test. Most fine depositional sediment will return a number comfortably above it, but sandy or low-phosphorus sediment can report a non-detect that means "below 296 mg/kg," not "no phosphorus."

This is the screen. Fractionation is the decision tool.