Selective Measurement of Phosphate in 0.5 M Sodium Bicarbonate Soil Extracts

Solutions of 0.5 M sodium bicarbonate (NaHCO3) are widely used to extract phosphorus (P) from soils as an index of its availability to plants and of environmental risk particularly using the method of Olsen et al. (1954). Extracts are traditionally made in the presence of activated carbon to minimize the presence of colloids that are not efficiently removed by centrifugation or filtration. Some extraction methods omit activated carbon, and a variety of post-extraction protocols have emerged for the P colorimetry. These variations have the potential to affect relations of test values with both P-fertilizer requirement and environmental risk for a variety of reasons. For example, the colloids include humic acid that precipitates under the acidic conditions required for the molybdate reaction, and humic acid contains P-esters and orthophosphate (PO4-P), at least some of which react with molybdate. We report experiments which show that (1) PO4-P in 0.5 M NaHCO3 soil extracts containing colloids can be measured selectively and efficiently using on-line dialysis preceding colorimetry in a continuous-flow analyzer, and (2) these PO4-P values closely match those obtained in a blind interlaboratory comparison using 98 soil samples (R2 = 0.99), where the independent data set was obtained following the original Olsen protocol using activated carbon, filtration, and manual colorimetry. The modified protocol is an efficient implementation of the Olsen-P test for high-throughput laboratories. Furthermore, extracting without activated carbon also opens the possibility of measuring total extracted-P, and estimating P in colloids by difference.