Important advances have been made recently in identifying the sources of algal‐available P in soils and sediments. Algal bioassays have shown that considerable amounts of the total soil P may be algal‐available under the appropriate environmental conditions. At present, however, the reported research contains no convincing evidence that the bioassay techniques used quantitatively measured all of the algal‐available P present in the soils and sediments tested,. Although chemical fractionation procedures have shown from which fractions algae obtain P, there is no evidence that all of the chemically extracted P in those fractions is algal‐available. Quantitative assessment of algal‐available P in soils and sediments will depend upou the development of long‐term algal assay procedures.

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Eutrophication control programs are based on phosphorus management which, in turn, requires a detailed quantification of the watershed phosphorus budget. This involves an assessment of non-point sources of phosphorus, a key element of which is determining the fraction of soil-P and sediment-P available to support algal growth. This paper presents a review of the state-of-the-art of estimating algal available-P from non-point sources, and an analysis of pertinent data. The need to employ multiinterval algal bioassays in developing phosphorus management systems is stressed.

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