The orthophosphate content of bicarbonate soil extracts

The Olsen P and Colwell P bicarbonate extraction procedures are empirically derived tests that provide an estimate of the soil P that is available for plant uptake. This paper examines each procedure using high performance liquid chromatography with flow injection analysis detection (HPLC-FIA) to specifically measure orthophosphate in bicarbonate extracts. Extract solutions from 3 soils of contrasting plant-available soil P contents were analysed for orthophosphate, total filtered ( < 15-45 μm) P (TFP), and either Olsen P or Colwell P. The amounts of P extracted by the Olsen procedure were not statistically different from orthophosphate (HPLC-FIA) (P > 0.05), suggesting the Olsen P test was a good measure of orthophosphate, the most immediately plant-available form of P. However, the average amount of P extracted by the Colwell procedure was 14% higher (P < 0.01) than the corresponding average orthophosphate concentration, presumably due to the presence of labile organic/condensed P, colloidal associated orthophosphate, or high molecular weight reactive P. These results suggest that there is a pool of non-orthophosphate P present in the molybdate reactive Colwell P extract that is potentially plant available. Future work should focus on specific identification of P compounds extracted from soils and soil solutions to examine their role as a source of P to plants.