Improving the Accuracy of Soil Phosphorus Testing Under Organic-Rich Conditions: Evaluating Pretreatments to Mitigate Optical Interference from Colored Organic Compounds

Accurate determination of soil phosphorus (P) is critical for fertilizer management and environmental monitoring. In organic-rich soils, dissolved organic compounds interfere with colorimetric measurements, leading to inaccurate results. This study aimed to evaluate P determination in colored model solutions and soil extracts containing soluble organic matter. Model solutions were prepared in deionized water at a fixed P concentration with humic substances added to create varying color intensities. Soil extracts were obtained using Olsen and Bray extractants. P was measured using three colorimetric methods – ascorbic acid molybdenum blue (AAM), stannous chloride molybdenum blue (CSM), and vanadomolybdophosphoric acid (VAM) – along with inductively coupled plasma spectrometry (ICP). To mitigate optical interference, four pretreatments – activated carbon (CA), acid digestion (AD), standard addition (SA), and untreated control (NT) – were tested. Analysis of variance and cluster analysis evaluated method performance under varying color intensities. Color intensity significantly affected measured P concentrations. In model solutions, AAM with AD or SA, and CSM with AD, produced deviations < 5% from the reference. ICP showed the highest accuracy and precision, followed by AAM, CSM, and VAM. VAM with CA or NT exhibited the lowest reliability. In soil extracts, VAM performed poorly in Olsen extracts regardless of pretreatment, whereas in Bray extracts, ICP and all colorimetric methods with SA yielded comparable results. Overall, ICP is preferred for colored soil extracts where available. When using colorimetric methods, SA is recommended as a reliable pretreatment.