Determination of bioavailable phosphorus by organic acid extraction

Abstract

The aim of this work was to test if the use of an organic acid as an extractant of phosphorus from sediment could be developed as a new method of measuring bioavailable phosphorus. Based upon work done previously, the organic acid, glycolic acid, was chosen. This acid is one of a number known to be implicated in phosphorus solubilization by biological action, and was chosen because of its lack of interference (except at relatively high concentrations) with the molybdenum blue colorimetric method of determining phosphate in solution.It has been shown that glycolic acid extracts increasing amounts of phosphate from sediment with increasing acid concentration up to a maximum extraction at a concentration of the acid of 1.25 M. It has also been shown that when compared to various inorganic acid extractions of the sediment, glycolic acid extracts appreciably less phosphate, and appears to extract different forms of phosphorus to those removed by the inorganic acids; it also correlates relatively poorly with these inorganic acid extractions in terms of phosphorus extracted.The glycolic acid extraction method has been shown to correlate badly with the very commonly used inorganic extraction method of assessing bioavailable phosphorus, viz. the NH4F / HC1 extraction method. However, an unexpected result was obtained, when it was found that 1.00 M sodium glycolate both extracts more phosphorus from sediment than does the NH4F / HCl extractant, and correlates very well with this method of assessing bioavailable phosphorus. The sodium glycolate method correlates relatively poorly and very badly with the glycolic acid and inorganic acid extraction methods respectively, in terms of phosphorus extraction.Sediment sample pre-treatment has been shown to be of great importance in determining phosphorus extraction, almost regardless of the extractant and method of extraction used.