Determination of biologically available phosphorus using a radiobioassay technique

SUMMARY. A kinetic approach in which the uptake rate of simultaneous additions of orthophosphate and carrier‐free phosphorus‐ 32 by a standard culture of the green alga Selenastrum capricornutum under near neutral conditions was used to obtain an estimate of the readily available components of the total phosphorus pool in natural freshwater samples. A comparison was made between the values of the biologically available phosphorus obtained using this radiobioassay technique and those obtained by the soluble reactive phosphorus procedure for thirty‐one freshwater samples. Only about three‐quarters of the phosphorus measured as soluble reactive phosphorus was readily available to algae. The results agreed with those obtained in an earlier study in which an enzymatic procedure was used to determine orthophosphate. The results of tests carried out on the water samples used in this study were consistent with the conclusion that the presence of a hydrous ferric oxide‐orthophosphale complex in the samples caused the soluble reactive phosphorus value to be greater than the corresponding biologically available phosphorus value.