Evaluation on the determination of available phosphorus using blue molybdate in Organosols

Abstract

Bray and Olsen methods have widely been used for measuring available phosphorous (P) in soil. Those methods have been known resulting good correlation between available P in soil and crops, especially for corn. Correlation between P uptake by plant and P concentration in Bray extractant ranges from 0.74 to 0.94; the method is, therefore, recommended for neutral and acid soils (pH ≤ 7.0). Meanwhile for Olsen method, correlation could span from 0.73 to 0.96 for alkaline soils. Both, however, have been observed to potentially result in false-high available P in Organosols. This might be due to dark color of Bray and Olsen extracts that reacting with blue molybdate and resulting in high values of absorbance during spectrophotometric measurement. In the case for Organosols, dissolved organic substance originates dark color of the extract. Therefore, organic substance should be eliminated prior to the measurement. This research explored an improvement for available P measurement in Organosols by treating the extract with H2O2 and an activated charcoal. This treatment the Olsen extracts has resulted in a notable available P gap, i.e. 102.7 ppm of treated extracts compared to 1207.7 ppm of untreated one.